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EFIS/EIS RPM erratic in displays

Jonathan Alvord

Well Known Member
Over the last 10-20 hrs I have noticed that my RPM indicator has become more erratic and now is unreliable. Im not sure where to start but everything else working great. I now have 690 hours TT, and run the Pmag/MAG combination. All the Mag checks are good. I have the GRT EIS and EFIS. Not sure if it could be a loose wire bad connection. When I bought the plane there were no wiring diagrams that came with it.

Any ideas on where to start? I thought it might be the electrical sensor in the CS prop but I really hate to have to pull the prop. Hoping it is something simple.

Jon
 
Bad connection...

Jonathan,

I would obtain the wiring schematics from Grand Rapids from here http://grtavionics.com/home/eis-engine-monitor-support/ and for the PMAG from here https://emagair.com/downloads/

Usually, the RPM value is obtained from a wire running between the PMAG and the EIS system. Sometimes there is a Hall Effect sensor also attached to the Magneto and also connected to the EIS...and sometimes there is a mechanical/electrical transducer attached to the RPM drive shaft on the engines accessory case which also runs to the EIS.

Find the EIS box, trace the relevant wires back to their source(s)

B
 
A fouled or misgapped plug or a bad ignition wire can cause these problems. Check this first before digging too deep.

V
 
Jon,
You most likely have a loose wire somewhere. When you do your mag check, if the RPM reading on your EIS goes to zero when selecting the P-mag, then you are getting RPM information from your mag. On the EIS, this is a wire running from your P-lead, through a 27K ohm resistor to a pin on the EIS. If you get RPM indications during a mag check no matter what mag is selected, then you are getting your RPM information from the P-mag. This information is supplied from Pin 6 on the P-mag to the EIS or EFIS, depending on GRT EFIS generation. There may also be an additional ground wire running form the P-mag ground pin (pin #1) to the EIS. I suspect your RPM input is coming from the P-mag. Check pin #6 on the P-mag to make sure it is tight. Also check pin #1 the same way. To check these, just grab the wire and give it a little tug to see if it?s tight. Don?t try to rip it out by the roots - just a tug will do. You should check all these connections whenever you have the cowl off. These connections are simply a wire inserted in the connector and then tightened in place with a very small flat screw driver. These connectors work great on a computer, not so great on a Lycoming that shakes like a wet dog sometimes. The wires in this case, and all cases, are stranded, and when you clamp them in this type connector it tends to squish the wire strands and put undo stress on the strands. Wires used in this type connector should be ?tinned? prior to inserting them in the connector. This means applying a small amount of solder on the wire strands to solidify them and give the clamping mechanism in the connector something to grab on to. Checking the security of these wires will take less than 1 minute when you have your top cowl off. If there are loose wires, loosen the screw, pull the wire out and see if it has been tinned. If not, heat up your soldering tip and get to work - it only takes a few minutes, and you shouldn?t have to worry about it again.
 
I once had a similar problem with the tach on my Vision Micro Systems VM1000C. I went through heck with this. Checked connections on both ends multiple times, cleaned contacts, replaced connectors, replaced Hall effect transducer on mag 3 times, swapped displays, etc. I finally resolved the issue with replacing the 3 wire harness with a new one I made up with new connectors. The old harness tested good with no breaks and no shorts. It was very puzzling and frustrating. I dealt with this off and on for two years.
 
quick update. After checking all electrical lines into EIS and out to EFIS, I then checked the PMAG plug wires which had a greasy like corrosion at the connection to the PMAG. I cleaned up the wires and a quick run up was performed and Showed the RPM to be spot on and not bouncing around. So glad that was a quick/easy fix.
 
quick update. After checking all electrical lines into EIS and out to EFIS, I then checked the PMAG plug wires which had a greasy like corrosion at the connection to the PMAG. I cleaned up the wires and a quick run up was performed and Showed the RPM to be spot on and not bouncing around. So glad that was a quick/easy fix.

Great news!

I like easy fixes!
 
quick update. After checking all electrical lines into EIS and out to EFIS, I then checked the PMAG plug wires which had a greasy like corrosion at the connection to the PMAG. I cleaned up the wires and a quick run up was performed and Showed the RPM to be spot on and not bouncing around. So glad that was a quick/easy fix.

I have several oscilloscope pictures of what happens when plugs or wires misbehave. Unfortunately, after moving houses and hangars and changing computers, I can't find them.

The #1 indicator of these problems is erratic RPM readings using the P-Lead type of tachs. Thanks for posting this problem/solution.

V
 
quick update. After checking all electrical lines into EIS and out to EFIS, I then checked the PMAG plug wires which had a greasy like corrosion at the connection to the PMAG. I cleaned up the wires and a quick run up was performed and Showed the RPM to be spot on and not bouncing around. So glad that was a quick/easy fix.


May have spoke too soon. Still having issues. Plane goes down for Conditional this month, and will try to trouble shoot. ugh. Have replaced all spark plugs, cleaned wires, Will check all the mag connections. Planes are fun! haha
 
May have spoke too soon. Still having issues. Plane goes down for Conditional this month, and will try to trouble shoot. ugh. Have replaced all spark plugs, cleaned wires, Will check all the mag connections. Planes are fun! haha


Also realized I don't have a PMAG, but have a Slick Mag and Light speed Plasma II+ CDI with Direct Crank Sensor.
 
Also realized I don't have a PMAG, but have a Slick Mag and Light speed Plasma II+ CDI with Direct Crank Sensor.

EIS can have one or two different RPM sources. intermittent RPM changes seem to imply you only have one RPM connection or only one is woking. Best to figure that out first. Troubleshooting will vary, depending which ignition is delievering the signal.

Larry
 
EIS can have one or two different RPM sources. intermittent RPM changes seem to imply you only have one RPM connection or only one is woking. Best to figure that out first. Troubleshooting will vary, depending which ignition is delievering the signal.

Larry

I did speak with builder and he stated the RPM sensor was located in the flywheel, so I suspect it is the Lightspeed Crank Sensor. Is it possible that the RPM sensor could go out and not affect the Plasma II+ and timing?
 
I doubt your problem is the Lightspeed crank sensor, or the LS ignition at all. The LS control box outputs a square wave rpm signal and since your engine is running fine, I'll bet that signal is OK too.

You need to find out exactly how your RPM indication system is wired. There are a lot of different way to do it. My guess is that RPM is being pulled off of the mag p-lead, or perhaps the p-lead and the lightspeed tach signal combined. As the mag has aged the method used to derive a tach signal from the p-lead is no longer reliable.

I have a lightspeed/mag combination and the problem you describe sounds similar to what happened to me.

Questions to start with:

What happens when you do your ignition check in run-up?
When you shut off the mag do you still have an RPM indication?
When you shut off the lightspeed do you have an RPM indication?

Did the builder provide any schematic diagram?
 
What happens when you do your ignition check in run-up? No drop in RPM's for either position.
When you shut off the mag do you still have an RPM indication? one position has rpm (erratic) the second position has never shown an RPM. Unfortunately I am not sure which is the Lightspeed or the mag.
When you shut off the lightspeed do you have an RPM indication? I have not been able to determine that. Will have to dive in behind to the fuses to find the fuse for the Lightspeed to remove and then try to fire it up.

Did the builder provide any schematic diagram?
That would have been very nice but unfortunately no.
Jon
 
spark plug wires - shotgun approach

I had some electrical gremlins pop up that were ignition related. Yours may or may not be directly ignition related, but rpm is close enough with EI. Someone recommended to me that the plug wires could have developed a loss of resistance. Prior to diving into the daunting task of chasing wires, checking continuity, calculating and checking resistances, checking grounds, etc etc that is likely needed, a smart 50lb head kinda guy recommended a "shotgun approach" to just replace the plug wires. So, I got Taylor pn 60651 wires from Summit at approx $40. Took about an hour to install and my specific electrical issues were solved.

Quick, inexpensive, and I had a new set of plug wires. Keep in mind this was before any troubleshooting, so I was being a little lazy and got lucky. Not sure if those plug wires are applicable to your system, but damaged or worn plug wires will release EMI/RFI that can cause havoc.
 
I did speak with builder and he stated the RPM sensor was located in the flywheel, so I suspect it is the Lightspeed Crank Sensor. Is it possible that the RPM sensor could go out and not affect the Plasma II+ and timing?

The cranks sensor is an RPM INPUT. RPM indication output is a different function. If the crank sensor were erratic, you would have erratic spark timing and would notice it in the engine performance or feedback. That sensors output is the orchestrator of all igntion output. The LS has an RPM output that alternates between 12 and 0 volts to indicate RPM to an instrument. This is where your issue likely is, assuming the LS is the RPM source for the EIS. Either the LS CPU has an issue with sending this signals reliably, your EIS has a problem reading them or the wiring between the two has issues. There will be one RPM wire between the two devices that you need to check. These don't have a dedicated ground and any grounding issues with either box would also show other intermittent problems.

You need to look at the two RPM pins on the EIS and trace one or both of them to their source. The EIS can have either one or two RPM inputs. Intermittent indication isn't likely with two sources, as the EIS will always pick the higher one. If you still have RPM at all times during ign drops during run-up, you usually have two RPM inputs. There are some mag based RPM pickup tools that will still work during a mag drop though. IF you only have one RPM input, you can deduce which it is driven from based upon RPM feedback during the ign drop tests.

Larry
 
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