97 Jeep Grand Cherokee 4.0
Intermittently cutout and stall, lights flickering during cutout
My own mindset goes to crankshaft position sensor problems when I see an engine cutout and stall on these jeeps. Here though we have the lights flickering as well. That could happen just because the rpms are so erratic during the event that the voltage is surging but the lights could be indicating an electrical power problem that is causing the cutout. The initial approach will be the typical, *verify the complaint* method.
The customer had said the truck hasn't failed to restart after stalling but this morning it would not start at all after some lengthy cranking attempts. I decided it was a good time to check for the problem but before I could test for no-start, it fired right up on the next try. After a short drive with no problems I tried a couple hot restarts. The engine did begin to have an intermittent cutout that could be related to crankshaft position signal. It was time to do some testing.
I wanted to start by testing the ASD (automatic shutdown relay), or more specifically to monitor the ASD power output. If the engine computer sees a good crank position signal it will ground the ASD relay, applying power to the ignition coil.
An easy way to test is to use a relay test jumper. You can put these in between the relay and the power box to create easily accessed test points.
If I lose ASD output then I can continue on my theory of crankshaft position signal problems, if the output is good then I can move on to something else and not have wasted any time. Monitoring the ASD showed steady output voltage, even during the engine cutout so there was no problem with that part of the system. I had to this point ignored the check engine light but since my suspicions of a bad crank sensor were proven wrong I needed another point to consider for further testing. The stored engine code was P0351 which indicates a problem with the primary circuit of the ignition coil. Now I had just finished testing that circuit feed at the ASD and it was fine but was the circuit actually fine all the way to the coil? I decided to see. I clamped the meter to the coil primary circuit but this time near the coil connector. The power was good to that point so if this was a circuit problem it was on the negative side of the coil since the code indicated the coil not firing. I kept the engine running and clamped a current probe on the coil trigger wire. The computer provides the coil trigger and is monitoring that circuit so I removed the coolant recovery tank to get clear access to the engine computer connectors. I was suspecting a bad wire or connection so began pulling slightly on the harness. It was immediately apparent that moving any computer connector would cause the cutout and/or stalling symptom.At this point I would expect that I need a computer but an inspection of the connectors was necessary. I noticed that the two metal screws in the faceplate of the computer were loose. I tightened those. There was a lot of what I assume was dielectric grease on the connectors and the terminals seemed good. I sprayed contact cleaner on the pins and terminals and reattached the connectors. Guess what? I couldn't make it stall or cutout now no matter how hard I worked those connectors. A test drive looked good as well.
Kenneth Hayes
AKA Deranger
