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Avionics failure during flight

Jonathan Alvord

Well Known Member
Went for fuel filling trip today and lost all avionics including transponder, EIS, GRT displays. Thankfully had steam gauges for backup. That is the short and sweet. Here comes all the details leading up to it. The plane has roughly 460 hours on it and I am the second owner. Two months ago we replaced the alternator when the displays turn off when coming in to land. No real issues otherwise up until today. Run up normal, flew to Oregon for gas and avionics shut down while on final and rebooted. Filled up, run up good, Voltage 12.9 amps 10 I believe all in the green on the EIS and displays. Half way back in VFR all the avionics turned off, no contact with ATC and I was on flight following, called Flight services on cell to ask them to relay to ATC that I had lost comms and would continue VFR direct to my local airport.

Now why did it fail? It appears the PTT on joystick was pulling power from the avionics, that's my theory. I and two others have looked through the plane most of the day. All the solenoids in the engine compartment appear to working. WE have power at the master switch, we no longer have power to Avionics (GRT, Radio stack) and Fuel pump. I have checked all the fuses and can't find a single one that is bad. We were able to jump the power to the avionics and it still works , just not from the switch. When testing the panel for endurance fuses (fuel pump, manifold pressure, avionics, EIS, and inst lights) we could only get 2.7v, All of the Main fuses were working (nave lights, auto pilot, pitot, strobe, trim ignition, trim switch, landing, pwr source 1, interestingly Power source 2 had no fuse in it).

I am unable to find a wiring diagram, although I know it would help we drew out one and I have a picture but can't post here. I did not pull the floor panels off and have had no problems with the fuel pump in previous 250 hours.

Any ideas? There are no mech or avionic specialist at the airport, this is new territory for me as to how to get it repaired. Any Advice would be greatly appreciated. BTW I am near Yakima/Tri Cities Washington.
 
If volts were 12.9 in cruise flight, it's likely the alt was dead again, or the regulator was off line. It's unlikely that the PTT line could cause signifcant drain on the system, unless there's a catastrophic wiring fault somewhere. The PTT circuit is by nature very current-limited.

As you know, you really do need a wiring diagram. But to start, if it's wired conventionally, and the battery is fully charged, you should see ~12.5V at the 'top' of the master switch *when it's off*. When it's *on*, it completes the master contactor coil's circuit *to ground*, so you'll see zero volts on both switch terminals when it's *on*.

From that point, troubleshooting is checking for 12+V, starting at the load side of the master solenoid, then the various buses.

Hopefully, that will get you started.

Charlie
 
If all of your avionics (i.e. your avionics buss) are drawing current through a single "avionics master" switch, I be curious about how its rated. Or, if its some kind of switch/relay set up, how that is wired.

If you were at Arlington, I'd offer to take a look next week. But, being in Yakima, I don't know when I could get over there.
 
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+1 Ken

Saw a repetitive failure like this some yrs back - owner kept replacing an avionics relay switch with incorrect unit. Would work long enough to get him off the ground....
With low voltage, check your avionics switching.
 
12.9 volts on runup means your charging circuit was not working and you were operating on the battery. Sounds like as the battery voltage dropped you went below the minimums to operate the glass. What voltage does the battery show now. Voltage on runup should be 13.8 or better. Usually about 14.2.
 
If there is an avionics master switch I would try bypassing it and see that gets power back to avionics. (could be the builder put the fuel pump on the same bus with the idea that it is an essential item).
 
My first guess would be failed charging system, either alternator or something else. In flight, voltage should be around 14 volts to charge 12 volt battery. Anything less, and the battery will not take a charge. Start by putting a volt meter straight across your battery. If it?s less than 11 or 12 volts, it is likely the charging system. If low voltage, charge battery and recheck. Once at 12v, turn on system and verify radios are working. With recharged battery, fly to somewhere they can trouble shoot.

If the battery is reading the normal 12 to 12.5, I would focus on a loss of circuit continuity. With the system turned on, check the voltage from your essential bus or after the fuse going to your radio. On the volt meter put the positive on the fuse, and the negative on any exposed aircraft metal (no paint). If it reads 12 volts, then turn on the radio, and it should work. If no voltage, start working upstream with the volt meter until you find the issue. Check at any switches or solenoids and work all the way back to the battery. At some point, you should find either a loose connection, broken switch, or broken solenoid (or potentially a switch that is driving a solenoid).

I?d be very surprised if it was the PTT switch. The current generated by your system would likely overheat and start a fire if that wire was really drawing all that power.

Aaron
 
What everyone else said about your charging circuit.

14.3V is what your batt should be seeing. Below that your batt is providing all the power. I'd bet that wrt the PTT switch, since thats the transmit side of the radio, the radio pulls a lot of amps to do that, and when you transmit, if the batt voltage is already on the edge, it prob pulls down the batt voltage just enough to make your master solenoid field to collapse, and thus you lose all power.
 
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The PTT switch is a simple radio transmit key that when pushed, grounds the circuit allowing comm radio transmission through Mic circuit. Normally very small gauge wire that would not support the amperage load of all the avionics.
 
Charging

I went through a similiar issue. Tracked wires for days trying to first, determine what went where. Next, the problem was absolutely intermittent. Also had just installed a new alternator. Plane Power with me all the way and completely backed their product. Every time we thought we’d found the problem, (suspect crimps, etc.), we’d fly it and all would seem good until it failed again. We checked every single wire by checking continuity while tugging and pulling. We didn’t pull and tug on the alternator output wire as afterall, we had just reolaced the alternator. The wires were all really well supported by adel clamps and we didn’t take every clamp loose. Yes, we did replace the field/sense connector. I finally placed one voltmeter on the field wire (to see if I was losing field voltage) and another on the output side. This way, we could determine if we were losing field voltage or if the output voltage was failing (internal regulator). After two hours of ground runs with no failure, I was just about to give up when it failed! The field voltage was good at 12.4 volts but the output went to 15.1 volts! I quickly shut off the alternator. Now, we knew the alternator was working as was the voltage regulator. We were losing bus voltage somewhere downstream of the alternator. I decided to go back to the basics and start with the wiring at the output side. and check every connection to the bus. The first was the output connector which
Was well secred with an adel clamp. I took the adel clamp off and once disconnected, I tugged on it and the connector came off in my hand. Put a new connector on, recrimped it and 20 hours of flying later, everything is, once again, perfect. Moral of this story.........start with checking the very basics and don’t assume anything,
 
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charging

Depending on whee your system is measuring the voltage, anything from 12.9 to 14.5 volts could be OK, as long as the voltage remains stable. If it is 12.9 then slowly drops, Your charging system is the culprit, as others have said. If the voltage is higher and stable, and the electrons are not getting to the efis, radio, etc, then you have a break in the line from the batt/alt to the electronics.

With the failure duplicated, the electronics on but not working, check for a large voltage drop across each connection.

If you show 12-13 volts across the master then that switch is bad. There should be no voltage drop across a switch or CB. Or measured another way, the voltage from either side of the sw to gnd should be the system voltage. (12-13v)

Let us know what you find.
 
Cascade failure.

The PTT switch is a simple radio transmit key that when pushed, grounds the circuit allowing comm radio transmission through Mic circuit. Normally very small gauge wire that would not support the amperage load of all the avionics.

While true, when transmitting the radios will draw more amps, and with a battery voltage on the edge of dead, it simply fell below the operable voltage.

Back to the charging system.

1. charge the battery on the ground. Keep a power supply on it.
2. Check each system for operation while monitoring voltage on the battery with a DVOM.
3. repair charging system or what did not work.
 
I suspect an intermittent open wire, under shrink wrap....at the alternator. Works when touching, doesn?t when not. Hard to find an intermittent open. Pull every connection and wiggle it...bet you find one broken. A friend of mine (lifelong A&P at a major) just chased something just like this on his plane. It?s frustrating to be sure, but he found the alternator line broken under the shrink wrap. Ohm?d out fine on the ground...but put some vibration on it and the electrons stopped electroning.
 
Charging

Exactly what mine was. The connector used on the alternator output had a hard plastic insulating cover that went back on the wire about an inch. This hard covering gave the appearance of a good crimp but the underlying metal connector wasn’t fully crimped. The covering held it in place (along with the adel clamp) until I disturbed it when installing the new alternator. When I finally took it back off and gave it a tug, it came off in my hand.
 
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These are photos that Jon sent about this.

Dave


YneDrAw.jpg


Piwlu6A.jpg
 
The small box in the upper right we have not tested yet, but we suspect that may be where the problem lies, Anyone know what this is? I believe on the opposite side it says Alternator and has a 5a fuse. There is low voltage 2.7 to the Endurance buss. All the Mains Buss fuses appear to be working as we have power to all those and none of the fuses are blown, I don't recall testing that Main power supply. It will be several days before I get back to the hanger as my real job beckons, but thanks to all of you for your thoughts and Ideas. I will def check the alternator wire crimps, the alternator is new but the wire is not. First priority is to get power to avionics and fuel pump!
 
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The small box in the upper right we have not tested yet, but we suspect that may be where the problem lies, Anyone know what this is? I believe on the opposite side it says Alternator and has a 5a fuse. There is low voltage 2.7 to the Endurance buss. All the Mains Buss fuses appear to be working as we have power to all those and none of the fuses are blown, I don't recall testing that Main power supply. It will be several days before I get back to the hanger as my real job beckons, but thanks to all of you for your thoughts and Ideas. I will def check the alternator wire crimps, the alternator is new but the wire is not. First priority is to get power to avionics and fuel pump!


That is a diode that prevents the Endurance bus from backfeeding the "normal" bus when it is taken offline. I doubt that is your problem--most likely you will find, as others have stated, you have an alternator issue.

Download the following publication:

Aeroelectric Connection

Go to Appendix Z and look at the Z-11 wiring diagram. I suspect your aircraft is wired in a similar manner, you can see how the diode is used to separate the two buses.
 
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Jon,

Are there more than the 2 buses shown in the pics? The ones in the pic are labeled main & endurance, but the drawing is labeled fuse block 1 and avionics.

The gap in the drawing labeled <?> *should* be your master contactor, which would be controlled by the master switch. See my 1st response on troubleshooting. Sam's tip to download the Aeroelectric Connection is excellent advice; you can then see the most common and reliable ways to wire a plane, which should help you diagram your system.

As I and others have said, you almost certainly have an alternator issue, but you likely have additional issues. If you have only ~2V at the 1st fuse block, but have 12+V at the battery (with the master switch on), there's obviously a high resistance somewhere between the battery and the fuse block. It *could* be right at the battery post. When tracing voltage, remember to check both sides of every segment. That means, for instance, a check directly on the battery post, then on the terminal that connects to the post, then (next segment) the terminal on the other end of that wire, then (next segment) the mating terminal or post for *that* terminal, etc.

There are faster ways to do what I described, but you need to have a clear idea in your head of what you're doing, and what to expect to see in the measurements, before moving to time saving 'tricks'. It's easy even for those of us with decades of troubleshooting experience to get lost when trying too hard to save time.

Key principles to troubleshooting: Do stuff in an orderly sequence, make clear notes to yourself on what you've checked, and in what order (including results of each check/measurement), and never change more than one thing at a time, once you think you've found a problem.

Charlie
 
Two comments. You posted

"We were able to jump the power to the avionics and it still works , just not from the switch."

which implies you pulled power from somewhere and bypassed the avionics switch. Where did you pull power from to feed the avionics during your test? Was that just a wire from the input of the main fuse panel to the main input of the endurance panel?

I noticed you have 5A fuses for your power sources. Is this for the field of your alternator? You could have experienced a nuisance fault. If they are not "slow to blow" fuses I would replace them. I replaced mine with a 7.5A fuse and then replaced that with a circuit breaker. That may be your charging issue.
 
Voltage 12.9 amps 10 I believe all in the green on the EIS and displays. Half way back in VFR all the avionics turned off...

We were able to jump the power to the avionics and it still works , just not from the switch. When testing the panel for endurance fuses (fuel pump, manifold pressure, avionics, EIS, and inst lights) we could only get 2.7v, All of the Main fuses were working (nave lights, auto pilot, pitot, strobe, trim ignition, trim switch, landing, pwr source 1, interestingly Power source 2 had no fuse in it).

Sure sounds like you have a problem between the main bus and the endurance bus - either a connection gone bad or the avionics/endurance bus switch itself.

The alternator could be a red herring. You say you believe voltage was 12.9 - any chance it was 13.9? "All in the green" implies that there was normal voltage (unless the alarm limit was set incorrectly).
 
Update

Thanks everyone for you help and opinions, they are much appreciated. After some more testing and downloading a wiring diagram that closely resembles N122EF we determined that rectifier (diode) had 12.7v going in and only 2.7V out which also matched the E-Buss power in. Its also the only thing that could have gone bad that wasn't easily diagnosed/replaced. When bypassed everything works great. New rectifier is ordered, actually 2 have been so it will never happen again. Hopefully this will solve the problem of the charging system as well as I have checked all the lines for continuity, ground, etc. and all appear to be operative but we will chase that squirrel once the EFIS/EIS have power.
 
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Questions

Does your plane have a Ebuss switch? The picture looks as if it does not. Therefore no dedicated/alternate power for the Ebuss. Therefore no need in the diode. The diode prevents you alternate power from powering the main bus. Glad you tracked it down
 
Does your plane have a Ebuss switch? The picture looks as if it does not. Therefore no dedicated/alternate power for the Ebuss. Therefore no need in the diode. The diode prevents you alternate power from powering the main bus. Glad you tracked it down

It does have an ebuss switch.
 
Thanks everyone for you help and opinions, they are much appreciated. After some more testing and downloading a wiring diagram that closely resembles N122EF we determined that rectifier (diode) had 12.7v going in and only 2.7V out which also matched the E-Buss power in. Its also the only thing that could have gone bad that wasn't easily diagnosed/replaced. When bypassed everything works great. New rectifier is ordered, actually 2 have been so it will never happen again. Hopefully this will solve the problem of the charging system as well as I have checked all the lines for continuity, ground, etc. and all appear to be operative but we will chase that squirrel once the EFIS/EIS have power.

Sounds like you have found your problem! However looking at how the rectifier is wired you may be able to make an improvement quite easily. As wired you are only using 1 diode in the rectifier, but there are of course 4 in there. You could solve your immediate problem by simply moving the white wire diagonally across to the bottom right contact, which should use a different diode in the rectifier. When you do get a new rectifier, I believe it is possible to run wires to both the top left and bottom right (as per your photo) thus using 2 of the diodes instead of just 1.

Perhaps someone more experienced could confirm I am correct?
 
Ed,
You're right about using a pair of the diodes in the block, but he does need to be sure that the diode block is sized properly for the load, and is properly heat sinked if it's operated at any significant percentage of its rated capacity. And rather than guess at terminals due to orientation in a pic, the pair of source wires should go to the two '~' terminals, and the load wire should go to the '+' terminal, with the '-' terminal having no connection.

Jon, hopefully, you'll create an accurate diagram of the system. You may well have found your problem, but I spent over 40 years working around that type of diode block, and while I've seen them fail shorted, and fail open, I don't recall ever seeing one fail to simply high resistance (which is what your description of symptoms implies).

Charlie
 
Some thoughts:
- The old block silicon bridge rectifiers where nice things to play with back a few decades ago, but for aircraft use I offer there are hundreds of better 20-30 amp dual package Schottky Diode to consider. I buy mine from Allied Electronics - they run $2-$6 a piece.
- I would not consider a vital buss being fed only via an isolation diode as acceptable. The only place I use isolation diodes is the output of the standby alternator going through a set of diodes to the left and right avionics buss. This prevents a fault on one buss taking down both. Each buss is normally powered via a dedicated power relay from separate batteries (two PC-625 batteries). The standby alternator provides power to each buss automatically if the primary alternator fails.

Carl
 
There are always "better" options. But what is unacceptable about the original solution?

The orginal designed failed and the OP lost all avionics. What else need be said?

Blind adherence to Knuckes or any other design without understanding pros and cons is not what builders should strive for.

Carl
 
Note 12, page Z-9 of "The Aeroelectric Connection" addresses this problem. I'm guessing the E buss was pulling more current than the isolation diode could handle without a heat sink.

Carl, what are the specs on the Schottky Diodes you picked out?
 
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Still have questions

I wonder does the aircraft have a E buss switch or a Avionics switch? If it truly is a E buss switch what difference does the diode make? The drawing posted earlier shows a avionics switch. E buss is to be powerd directly from a battery. The diode is used to stop flow to the main buss, If it failed and there is no power to the E buss then I would guess there is no E buss switch?
 
I wonder does the aircraft have a E buss switch or a Avionics switch? If it truly is a E buss switch what difference does the diode make? The drawing posted earlier shows a avionics switch. E buss is to be powerd directly from a battery. The diode is used to stop flow to the main buss, If it failed and there is no power to the E buss then I would guess there is no E buss switch?

I misread your post first time. I was thinking about this also. I wonder if the OP had this switch closed during normal operation, thinking it was an "avionics master" when its really intended to be "normally open" and used as an alternate means to power the E buss.
 
Yep

That?s what I think also. If the OP had an E buss/alternate switch he could have flipped said switch and had power to the panel?
 
SNIP

Carl, what are the specs on the Schottky Diodes you picked out?

The only place I use diodes is on the output of the standby alternator. I use a total of four, 20 amp dual packages mounted on a nice heat sink. One set of two dual packages feeds the left avionics buss, the other set the right. The left and right avionics busses are powered via a set of 30 amp relays, two on each battery (primary and backup to cross connect - e.g. left avionics to right battery, etc). Relays are connected directly to the battery. The two master solenoids provide power to only the non-avionic loads and starter.

Four dual 20 amp packages are way overkill for a 20 amp alternator, but they are the same price, size and weight of smaller units - so why not?

The point - the diodes are installed for the sole reason to isolate a fault on one buss from taking down both busses. No avionics buss gets power just via a diode.

Of note - the new project will not have a standby alternator, just the two PC-625 batteries. I may add one later if mission creep sets in.

Most IFR setups now have dual screen glass panels, more than one GPS receiver and two radios. Having everything on one avionics buss (essential buss or whatever you call it) builds in a single fault risk. Split the panel into two busses - power them as you see fit.

As I posted in the past, anyone interested how I do power distribuition should PM me your email address.

Carl
 
The orginal designed failed and the OP lost all avionics. What else need be said?

Blind adherence to Knuckes or any other design without understanding pros and cons is not what builders should strive for.

Carl

Seems like the failure was not the design, but the implementation, as noted below (inadequate heat sink), perhaps. There must be hundreds or even thousands of planes flying around with this design, with no problems, and no need to go to two batteries and two alternators to mitigate a fault.
 
That?s what I think also. If the OP had an E buss/alternate switch he could have flipped said switch and had power to the panel?

Yes the plane has a Master Switch (dual pole), Avionics switch, Nav lights, Strobe, Ebus, and Fuel Switch.

I would be interested in isolating the navigation instruments from the radios. In my mind the Radio is the most important piece to have, so that contact with ATC can be made in the event of emergency, or to cont Flight Following. In this case I was on flight following and then abruptly had no more contact. I did however have Foreflight and a way to navigate safely to home Airport.
 
In my mind the Radio is the most important piece to have, so that contact with ATC can be made in the event of emergency, or to cont Flight Following.

I disagree. The *flight instruments* are most important (if you're in IMC), followed by the navigation. ATC will see you and get people out of your way, and you can deal with the lost comm later.

If you're VFR, navigate to the nearest airport and land.

Aviate
Navigate
Communicate
 
Seems like the failure was not the design, but the implementation, as noted below (inadequate heat sink), perhaps. There must be hundreds or even thousands of planes flying around with this design, with no problems, and no need to go to two batteries and two alternators to mitigate a fault.

Sorry but disagree. All designs should be examined for the result of any component be it a switch, relay, terminal or equipment failure. If the result is not acceptable for the mission, then the builder needs to address the design. In this case a single failure resulted in the loss of all avionics. The cause of the failure is not the focus. Everything you put in your airplane can fail, design to mitigate such failures if you cannot live with the result.

For a day VFR airplane less rigor is required.
Carl
 
Same thing

I had similar symptoms and it was due to a faulty EXP Bus that was intermittent. It drove me crazy for almost a year. I blamed every single other electrical connection in the plane before we figured it out. Now, with breaker switches, it all works.

Good luck. These kind of things make you lose confidence in airworthiness and certainly take the fun out of it.

John
 
AeroElecric

Jonathan. The Aero book will be a great start on getting your system as you want. If the design of your wiring is well thought out you should never be without power to whatever you want to call the buss, with what ever you feel is most important. If designed properly that diode failing could have been overcome by a single flip of a switch.
 
Sorry but disagree. All designs should be examined for the result of any component be it a switch, relay, terminal or equipment failure. If the result is not acceptable for the mission, then the builder needs to address the design. In this case a single failure resulted in the loss of all avionics. The cause of the failure is not the focus. Everything you put in your airplane can fail, design to mitigate such failures if you cannot live with the result.

For a day VFR airplane less rigor is required.
Carl

Agreed. So something about *this* design allowed a Single Point Failure at the diode which took everything out. But I don't think the Z-whatever design from Nuckolls has that flaw...failure of the diode wouldn't cause the system-wide failure. I could be wrong, but if my understanding is right, diode failure would simply result in inability to isolate and power the E-buss.

I did a full FTA on my system, to ensure that no single failure could leave me without sufficient capability in IMC to get down safely, as you note. Given the backup batteries for primary and redundant EFIS, plus an E-buss wired basically as per the Z drawing (specific items on E-buss chosen to mitigate an alternator failure), I ended up with no SPFs that would result in a safety of flight issue...and no dual battery/dual alternator configuration.
 
I disagree. The *flight instruments* are most important (if you're in IMC), followed by the navigation. ATC will see you and get people out of your way, and you can deal with the lost comm later.

If you're VFR, navigate to the nearest airport and land.

Aviate
Navigate
Communicate

I do have back up airspeed, artificial horizon, altimeter, turn and bank indicator, heading indicator in steam gauges and these continued to work and I am thankful and yes they are absolutely important/critical. I do find I really enjoy the EFIS and the navigation aspects and I do carry paper charts (good for blocking sun most of the time) in addition to Fore Flight on IPAD and Phone, so there was a Primary, secondary, and backup for navigation, but not for comms. I will address this issue and see about making it bullet proof!
Lots of ideas out there and my plane will be better in the near future! Yay
 
A handheld and convenient access to your external comm antenna cable would go a long way toward mitigating comm loss.

Even a handheld with its rubber ducky can work pretty well in an RV.
 
SNIP Given the backup batteries for primary and redundant EFIS, plus an E-buss wired basically as per the Z drawing (specific items on E-buss chosen to mitigate an alternator failure), I ended up with no SPFs that would result in a safety of flight issue...and no dual battery/dual alternator configuration.

Agree - having a bunch of equipment specific backup batteries mitigates the single battery shortfall for IFR and seems to be is a popular approach for builders using Van?s wire kit and/or Knuckles? designs. I have not noticed a similar wide adoption of such backup batteries for communications or transponder.

I also note the fixation of alternator failure as the prime mitigation target falls short of all the other stuff that will bite you (like listed by the OP).

In my RVs I use two PC-625 batteries and a thoughtful power distribution scheme, and no backup batteries. I believe this provides for a more robust system and makes for a simpler, as light or lighter and more easly maintained install.

But as with all such things, builders should build as they want, not how people tell them to build.

Carl
 
Update from Original poster

I have replaced the Rectifier diode and put in a back up line on the load side, and now have 13.5v at idle on the ground. I have all my instruments and radios back, and it all appears to working as designed. Makes me think the rectifier has been slowly failing over the last 4 months and led to a possible unneeded replacement of the alternator (but also a good learning experience).
Best of all I feel much more confident diagnosing an electrical issue, have spare rectifiers and fuses. Now if the weather will just clear up!

Thanks you to all those who helped with their ideas.
 
I have replaced the Rectifier diode and put in a back up line on the load side, and now have 13.5v at idle on the ground. I have all my instruments and radios back, and it all appears to working as designed.....

Did you mount the new diode package on a heat-sink?
 
My vfr rv4 shows ~14.4v at idle, right after cranking, using a Denso 55A automotive alternator. 13.5v will take forever to charge a battery. (Edit) Sounds like you either have an alternator problem, or a measurement problem.
 
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