Help with finding a ECM ground fault

Long story, but I installed a GM V8 into a Cessna airplane. The engine is controlled by a GM E67 ECM. After 3 years and about 500 flight hours of no issues, the engine suddenly ran rough and several DTC's tripped, including codes the engine does not have components for or are included in the calibration. Some DTC's related to electronic throttle control (engine is manual control with TPS on throttle body; EOP fault even though oip press displayed accurately, for example. Also, the fuel pressure displayed 103 psi (norm 60) which is impossible due to system regulators.

I expected either a sunk ref signal or bad ground which is confusing the ECM and causing the misc DTC's and fuel pres. I started with the normal troubleshooting of placing a test light between battery and ground....it lit up and then started removing fuses and opening breakers, to no effect.

I then tested all continuity between the battery (in the tail section) and engine block, ECM grounding points, ECM ground connector, ect...all less than 2 Ohms resistance. Then repeated same with positive voltage with less than 2 volts drop throughout chassis/airframe and ECM connector. I am confident airframe and engine block grounding is good.

I opened up the harness and began visual inspection without finding anything such as damage, and tested connectors and wiring for good ref and ground power, all again good. Then disconnected harness from both sensors and ECM and tested both ground and ref wires for grounding while tugging on harness (ECM on continuity with probes on wire and engine) but no luck.

What I find is high resistance on ignition wires, almost zero sometimes.... BUT sometimes it is OL and no codes appear at start up and then engine runs norm at least for a short time.... but when the many DTc's appear on key up engine immediately runs real rough. I began measuring resistance at different points starting at battery, all less than 1 ohm. Between the battery and ECM is a master switch and key switch, and have a few questions if someone can help out.

1. When I measure resistance between known ground and power on hot side of master switch, it shows 0 but normal volts. Question is normally I expect to see less than Ohms between ground and positive wire......I also see anywhere from 85-120 Ohms on unpowered POS wires, which drops to OL/infinity when power is applied to circuit if/when the DTC codes do NOT appear and engine starts and initially runs normal. WHAT SHOULD THE RESISTANCE BE BETWEEN GROUND AND POWER WIRE WHEN BOTH POWERED OR UNPOWERED? SHOULD IT ALWAYS BE LESS THAN 2 Ohms? Should powering the ECM circuit cause the resistance to change considerably?

2. Using the test light on the ground or positive circuit, should the lamp always be out on a normal circuit/system? What is the max acceptable voltage when measuring points along the grounded

3. I check continuity between the ECM pin and sensor connectors, all good. I back probed the EXM ground and ref pins, and show good as well. I have 5v and good engine block grounds while ECM is powered...... When testing with the system powered to put load on it, can you still get a reliable continuity measuring between ground and powered wire?

4. One interesting note....the DTC's appear when turning key on. I can start the engine but it takes longer to start, runs poor and idles rough about 150-200 below set idle RPM, and it sluggish and stalls out if you advance throttle too fast. However, if, after turning on IGN before starting, I clear the ECM DTC's with my reader, the engine will start and run normal and DTC doesn't seem to return. So, it seems the faults may only being caused by fault(s) detected on startup. I thought the fuel pumps which prime themselves on key up, but same occurred with pumps circuit breaker pulled so no load would be caused.

5. I get about 85-150 Ohms between ground and positive ignition wire before powering on, which sometimes increases to higher ohm when power is on....what, if any, difference should I see between power on and off?

It seems the fault is not easily seen with normal test lamp or DMM techniques with no power on the system, and wondering if such a fault is usually a poor ground or grounded power.


The engine is pretty simple compared to a modern car. There's no Air conditioner or other accessories. Its more like a boat engine configuration.

Overall, I believe there is either a bad ground somewhere I cant find or a power being shorted to ground I can't find. Any words of wisdom appreciated. I just about your book and will read it tomorrow for possible insight.

skydawg, welcome to the forums! I don't know jack or squat about aviation, just the automotive side, so I'm honestly not sure if anything I have to offer will help. :blush: I'll offer whatever I have.

For clarity, do you recall what the donor vehicle was for your engine swap? The E67 ECM part narrows it down, but knowing the specific donor will allow us to look up YMME specific wiring diagrams and service info.

WHAT SHOULD THE RESISTANCE BE BETWEEN GROUND AND POWER WIRE WHEN BOTH POWERED OR UNPOWERED? SHOULD IT ALWAYS BE LESS THAN 2 Ohms?

This is kinda a tough one to answer, because I don't know of any training material or OE service information that recommends measuring resistance between a positive and negative wire, other than to check for a direct short.

Should powering the ECM circuit cause the resistance to change considerably?

Technically, yes, because resistance cannot be measured on a powered circuit. The presence of any voltage on the circuit being measured for resistance will invalidate the measurement.

I'm not here to tell you how to do your testing. :silly: But I'm concerned that the resistance testing is having you chase your tail. If at all possible, I'd suggest sticking to voltage and voltage drop readings only. Or loaded circuit tests with a test light.

Using the test light on the ground or positive circuit, should the lamp always be out on a normal circuit/system?

Could you add some detail to this? An example of a test you tried to do with the test light, for example. Where is the test light connected on both ends, connector plugged in/unplugged, ignition position.

Not to be Captain Obvious, but if one end of the test light is connected to power, and the other is connected to ground, current will flow and the bulb will light. If there's no voltage difference, then no current will flow.

When testing with the system powered to put load on it, can you still get a reliable continuity measuring between ground and powered wire?

Not with the circuit powered, no. However, I'd still rely on your voltage readings until you've narrowed your focus to a specific circuit. Voltage testing with the circuit loaded is a better overall test, anyway.

I get about 85-150 Ohms between ground and positive ignition wire before powering on, which sometimes increases to higher ohm when power is on....what, if any, difference should I see between power on and off?

Again, I can't say I've ever taken that measurement before, so I have no point of reference. I'd think that measurement would be heavily based on how many and what types of loads exist on those circuits (motors, solenoids, sensors). I would think that zero ohms would represent a dead short. Beyond that, I don't know what your reading of 85-150 ohms says about those circuits.

One interesting note....the DTC's appear when turning key on.

Do you recall what the fault codes are, specifically? If you could take note of all the faults that reset when you turn the key on, that'd be a great place to start.

Thanks for help.

RE test lamp…..I put it between + battery terminal and the cable.

I get continuity is unreliable or likely worthless with a powered circuit but trying to get some additional indication after trying different changes. I’m going back to tracing amps.

RE the codes I get, it changes but always have several electronic throttle, TPS and pedal codes indicating out of range or correlation related codes. The system is manual cable throttle control so there is no way to check such sensors. The TPS functions normal and all readings are correct of return sensor voltage and ground is near zero ohms to block. I currently have code for MAP and coolant sensor…. both seem to be reading correctly and have a good ground with less than .2 ohms to block.

I was going to try back probing and jumping MAP sensor ground to block to increase connection to ground to see if thy makes a difference. Again, the faults appear only when turning on ignition……if I clear the codes prior to starting or after starting the codes don’t return no engine runs normal. It seems the fault/codes are either only detected when powering on ignition……wondering if the ECM detects faults differently compared to after initial power up. The faults do not return after starting engine and resultant starter load……which I am surprised as if the ECM ground was poor the load should likely trigger faults.

I am confident there is a ground fault but can’t figure it out. Other things I have done are:

Back probed ECM grounds terminals and block with DMM on volts and monitored it while turning ignition on and under load of fuel pumps….I don’t have a scope but dmm has hi/low and voltage drop was only about .2 v briefly.

Questions
- would adding ground jumpers to sensor connectors help find the culprit if it is poor ground ?

- what should I expect to see on the ignition circuit in terms of ohms to ground when unpowered? It’s bout 85 ohms now.

- because the battery is maintaining voltage over long periods nd there is no mo load when ignition is off, I don’t suspect a ground fault on the bat side, so it’s likely on ignition power side…..anything else I can check to confirm.

- I cannot figure out why the codes don’t return after clearing them after turning key on. I have disabled the pumps to eliminate load at initial ignition on but still got the codes….. anything else I should try?

- I’m using a threshold of .2 volts between positive circuit points to decide if there is too much resistance…is this bout right?

Thanks gin for the help.