Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
LS3 boat engine with GM E67 ECM, so few and simple circuits unlike a car. Getting CKP erratic or shorted low DTC and ECT sensor shorted low DTCs, and occasionally MAP and "reference circuit B shorted low". I have unplugged all other sensors and other harness loads but still get same DTC's. The DTCs appear almost always immediately when ignition is turned on/before starting engine. When the ECT faults are active, the temp displayed is often correct but sometimes displays -40 or other default. I have noticed that fuel pres sometimes reads almost twice what it should but no fuel pres DTC displays but 5v ref and good ground is at connector is present when back probing with DMM.
Although the DTCs are consistent, occasionally the DTC's do not set and I can run engine and it functions and runs normal for periods of time at all RPMs, but DTCs will always return if I start moving the harness around, and the engine begins to run rough and struggle. DTCs clear when reset, but immediately come back with key cycled off/on. Because the engine is capable of running normal, I expect all sensors including CKP and ECT are good and problem is in harness.....BUT, I have load tested all ref wires and inspected areas of harness that seem to cause the fuel pump and injectors to activate when moved without finding any damaged wires. BUT here's the strange thing i cant figure out...
If I move the CKP sense wire at different parts of harness the the pump & injectors cycles (essentially ECM is seeing RPM and turns pump on for 4 seconds which is same as prime when key turned on and injectors operate as if engine is running).... this can be just barely moving the sense wire slightly closer to another wire OR closer to a grounded surface WITHOUT MAKING ANY CONTACT.... it seems the wire is hunting for any nearby ground. If I ground the sense wire or use a test lamp on it, it will do same. It will also cycle pumps if a ground other 5v wire from ECM unused terminal. I believed the sense or ref wires
When I look at freeze info when DTC sets (when ignition is turned on only), it will often show about 1250 ish RPM and a ECT temp well above legit ot a default but often display correct temp and zero RPM. So, at IGN power up, the common circuits to both sensors are sending bad info to ECM enough to set DTCs but display correctly after engine is running (both RPM and temp).
I know what everyone is thinking.... a bad ground is culprit and at IGN power its not sinking the circuit(s) so the CPK sees RPM. But I have scoped the ECM grounds at ECM connector (reading volts between bat neg term and the back probing the ECM terminal) with starter and pump loads and it barely moved from .005 to .02V. I also read about same at the 2 wire ECT connector on low ref side.
I have never had such a problem finding cause fault and running out of methods. I have done the following
-conducted continuity and volt drop testing with bat neg term to all areas of engine block (under load as well)...all less than .02 Ohms and .5V
-checked same between bat neg and ECM neg terminals
-checked grounds at CKP and ECT...same as above methods
- load tested CKP and ECT between ECM and connector
-disconnected all other sensors and loads without affect
Have following questions:
1. What can be causing the CKP wires to cycle pumps and injectors (essentially causing a RPM/running signal to ECM)? Again, where this happens all wires are in good condition and don't have to make contact with the grounded surface or other wires... essentially wire is acting as the CKP magnetic field and triggering as it would with reluctor wheel. This issue is constant and likely root of all other DTC's
2. on 3 wire ECT sensor I have 5v ref unplugged, but it drops to about 2 volts when plugged in but displays correctly and resistance matched displayed temp........ is this drop norm? I expect both the high and low ref sinks and increases per sensor resistance other than the 5v ref wire remaining at 5v but manual doesn't say. Should the CKP also maintain 5v ref at all times?
3. The GM manual states the ECM has 2 reference banks ....the occasional DTC says a fault in the B bank but doesn't list all circuits on that bank (I have reached out to GM tech for list but don't expect much)....As a fault in a single sensor can effect others on that bank, is there a way to figure out what else is on same ref 5v bank......I tried grounding 2 ref wires expecting to see others drop below 5v but didn't happen (ECM likely shuts off power to grounded wire to maintain 5v to others) . there is a better way to find this out?
