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Slow cranking speed

Reilly

Active Member
Hi

I would appreciate some views here for a potential cause.

Some background:

Purchased a RV9a last year with 555 hours on. 0320 carb engine.
No problem starting.
Had PIO in Jan which resulted in a nose over and substantial damage. (Beautifully repaired now:))
The engine was removed and tested and fitted back to the plane.

The only problem that I have with it is that the first start of the day it is a struggle to start.
The one thing that I noticed is that the engine cranks over very slow. Almost like the battery is nearly flat. It has a new battery fitted.

Before I got it back the Company that fixed it said that my battery was on the way out and wanted to fit a new one which they did.

It has one magneto and one e-mag. My understanding is that if the power from the battery is too low then the e-mag turns itself off which means its trying to start only on the one magneto.
It sounds like it has a bit of spark and wants to fire but just does not catch.
Eventually it will start after about 8 or 10 attempts and then it starts ok for the rest of the day.

The engine has new rings and bearings but I doubt it has increased compression to such an extent to as cause such slow cranking.

Battery mounted on the firewall.

I don't think my old battery was on the way out. I think that after the re-installation something changed which caused the slow cranking and they assumed it was a weak battery. Battery shows good strength on the EMS.

On start I only have fuel pump switch on and Dynons come on with master.

Will check that all cables are tightly fastened and good earths.

Would really appreciate some views from those with more technical knowledge than me please.
 
I had the same issue with my -6. Replace the starter solenoid. They are prone to pitting on the contacts and will cause slow cranking.
 
Try cleaning and tightening all power and ground connections from the battery to the starter. That fixed the slow cranking issue I had on our aircraft.
 
After experiencing what appeared to be a very weak battery, I found the battery ground strap attach point to the airframe to be extremely loose. I torqued the bolt down and the slow, weak cranking hasn't happened again.
 
Cables

My -9A had starting troubles that were eventually traced to bad crimps on the supplied heavy wires. As others have said, check those connections. Put them in a vice and pull hard.
 
Previous replies have pretty much covered the common areas to look at.

Keep in mind that an electrical circuit is like a circle-------whatever comes out of the battery must get back to the battery. A small amount of electrical energy gets used/converted to heat, but most will need a good solid supply side path, and return path.

Good luck.
 
Check the wires and the whole system the easy way. Put a DC volt meter on the battery negative post and the positive lead on the engine case. When cranking you want to see less than .1~.2 volts, if more than that start moving the positive lead back towards the battery one connection at a time until you find the
what's developing the voltage.
Check the positive side the same way, measure between the positive battery post and the starter terminal. This side typically has more connections plus a couple of relays so let's give it .3 volts or so.
As a previous post mentioned, a good chance you'll find the starter relay generating a lot of voltage drop.
Tim Andres
 
Sky Tec has a pretty good troubleshooting diagram that backs up what has already been said...... just in pictures. I like pictures.
 
Hi

I would appreciate some views here for a potential cause.
Some background:
The engine has new rings and bearings but I doubt it has increased compression to such an extent to as cause such slow cranking.

Would really appreciate some views from those with more technical knowledge than me please.

....One check that would be worthwhile, just to eliminate the possibility, would be this; When engine is cold, take the prop and move it back and forth slowly. Take note of the force required to rotate it (not on compression) with the engine cold. Repeat the process with the engine after it has been warmed up and compare the results. There should be very little difference in required effort to rotate the engine hot or cold. Sometimes if there is an unforeseen issue in the engine, bent crank, tight main bearing, damaged thrust etc. this can be an indicator. These problems seem to go away after engine warm-up and the case expands and makes for more clearance. These problems usually are related to the last things that were worked on or touched. Easy check just for elimination purposes. Thanks, Allan:D
 
Last edited:
Still not solved. Need help interpreting readings

Hi Guys

So 3 years on I still have slow cranking issues. Its more of an issue in Winter and some times does not get past the first compression stroke.

I have read the various threads on this and the info re starter replacement and testing the system.

I have an Odyssey battery and a Sky-Tec 149-12HT starter.

Tested the system with a multi-meter yesterday and got the following readings. (I am a novice with this thing in hand)

Battery under no load at the terminals - 13v
Battery under load while cranking - 10v
Charge at the starter under load - 9v
charge at the solenoid - 9v

I tested the differential on the positive from battery to starter and got a difference of 0.35v
Same on the ground got a difference of 0.25v

So I am not sure what all this means.

Is a 3v drop at the battery under load acceptable/normal? Sky-Tec checks say it should not be less than 11.
Could this drop be caused by something other than a problem battery?
My gut feel is that the battery is ok.

Sky-Tec say the charge at the starter should be at least 10v and mine is 9?

Not sure why I have a 1v drop from the battery to the solenoid which sits just under the battery. One cable of about a foot long. I took it off and cleaned it up and it made no difference.

Any advice would be appreciated.

Thanks
 
Troubleshoot

One way to rule out all the components between the battery and the starter is to test with a direct connection. Some Caution is required, but I would power the starter with your battery and see how well it cranks. The battery negative needs to be connected to the starter as well. I?ve done this numerous times with auto jumper cables or spare heavy battery cables. You not going for a start, just 6-8 blades with the ignition off. If it cranks slow try another know good battery (like the one in your car) If it still cranks slow sounds like starter issue. If it cranks like it should then I would say you have a power or ground issue between the battery and the starter. I would suspect high resistance in one of the big wire crimps or thru the starter solenoid. All the usual WARNINGS apply. Use long enough cables to stay away from the turning prop. You will make a spark with the temporary connection so plan for that. Get help if you need it. Old mechanics will be familiar with the process.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
The voltage drop while cranking is too low. This implies a bad positive or ground connection to the starter.

For a simple test, add a #2 or #4 wire from the starter to the battery ground on the firewall. See if that helps. If so, rework your current ground connection.

Things to look at:
- Do you have a common ground on the firewall that is directly connected to the battery ground - or do you ground the battery on the airframe?
- Engine ground - confidence that this actuall provided a good ground for the starter?
- If the ground proves good, look at bad master or starter solenoids, and associated wire runs.

Carl
 
I encountered the same slow crank with my magnaflite starter. My problem ended up being with the starter brushes. There are four on the magnaflite and two were worn out. The starter tech said I was only getting half the capacity of the starter. He replaced the four brushes and it made all the difference. Works perfectly now.
 
Here's your clue:
Battery under load while cranking - 10v
Charge at the starter under load - 9v
charge at the solenoid - 9v

As you realized, a 1V drop across a 1 foot cable is a big red flag.

But: You need to measure every 'joint'.

Battery positive>cable>master+>master-load>wire>starter solenoid+>starter solenoid-load>wire>starter+>starter case>engine case>wire>airframe ground>wire>battery negative

And don't forget that each *cable* has two joints: the connection between each wire-end and its terminal.

If you have a 1V drop between battery+ & 'solenoid', you have a problem between those two points. Now, where in the chain (detailed above) is 'solenoid'? Which solenoid, and which terminal on that solenoid? Where ever you made that measurement, look 'upstream' (toward the battery positive) for your problem.

BTW, 10V at the battery positive might not be great, but it isn't unreasonable or out of the ordinary. An old flooded cell battery would likely be even lower than that (higher internal impedance in the battery).
 
Thanks for the response so far. I will only get back to the airfield on Friday to test these out.

Cajunwings I used to jumpstart my microlight direct to the starter as it was easier to access than the battery so will give that a try.

A question. I assumed that if I had 9 volts at the power in side of the solenoid and the same at the starter then there was nothing wrong with the circuit from solenoid to starter?
I did see that the connection of the power cable to the starter had some green color and a bit of rust but have not yet cleaned it up as the voltage there was the same as at the solenoid.
If this connection is bad, could it be the cause of the load pulling the battery down to 10v?

I also replaced the P10 cable from battery to solenoid with another and got the same 1 volt drop. This cable is attached to the battery together with 3 other smaller cables and lugs all squashed together through a rubber hood. I suspect this connection to the battery might be the 1v drop problem.

Reilly
 
Phone me when you can

Phone me when you can Reilly, had the same problem on our RV10 for years. Ended up being the starter
 
Another slow cranker fixed. The Sky-Tec NL starter was falling apart, so to speak, as others have pointed out. The two #10 screws holding the motor to the drive were loose, the miserable bent-over tabs intended to prevent screw rotation simply do not work. You must remove the starter to access the upper one. Don't just tighten them up. Separate the units to clean the joint. Besides being gunked up, there was pitting due to the high current trying to cross the gap. Took quite a bit of scotch-brighting to assure the surface was again completely flat and clean.

Locktite and star washers to finish and I'll be watching the torque seal.

John Siebold
 
Problem solved

Ok it has taken a while but the problem is now sorted out and engine cranks about 10 times faster.

I took advice and took my car battery out and used it to jump direct onto the starter motor. Still had the very slow cranking speed barely making it through the first compression stroke.

My existing starter motor was a Skytec HT. I ordered a new Skytec NL from Spruce and waited 2 weeks for it to arrive. That's when the learning started!!

Fitted the starter, connected the wires and jumped in to crank it with excitement rising. As I flicked the master on it started cranking. Master off and it stopped. I had not even touched the starter button.

So long story short I discovered that my RV had used the internal solenoid in the starter motor for the starter button. I did not have a separate starter solenoid. The new NL starter has a jump wire on it between the two terminals which must remain on a 12 volt system as the small terminal has been "repurposed".

So I had to buy and fit a separate starter solenoid on the firewall and reroute the starter button wire from the starter motor to the new solenoid.

When all done I jumped in again to witness my new handiwork. Master on. No cranking so problem solved. Hit the starter button and got nothing. Problem not solved. ****.

In my search for a local starter solenoid I could only find a body mounted continuous solenoid, not an intermittent one. I figured this would work for a starter just not the other way around so I bought it.

Google saved me in that I found that the starter solenoid grounds itself when bolted to the firewall but the continuous solenoid required a separate ground wire. Wire fitted and it now cranks so fast I can't believe it.

Starting problems over and I am much wiser for it all.

Thanks for all the tips and help and thanks to SA master builder Jan Hanekom for all the phone calls.

PS - I found the lack of info on the Skytec website and in the box with the starter very disappointing.

Reilly
 
Problem solved, but only temporarily. That contactor isn't going to last, and when it fails, there's a good chance that it will fail with the contacts welded closed. There's a reason mfgrs make a separate design just for starting.
 
Start contactors are designed to repeatedly make/break extreme currents; master contactors are only expected to *carry* extreme currents, with the assumption that breaking extreme currents would be a rare, emergency function. Start contactors have much stronger (higher current) coils and much stronger return springs, so opening/closing happens faster to minimize arcing. More:
https://www.google.com/search?q=starter+solenoid+vs+continuous+duty&oq=stearter+solenoid+vs&aqs=chrome.3.69i57j0l3.23591j0j7&client=ms-android-google&sourceid=chrome-mobile&ie=UTF-8
 
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