Need help troubleshooting an ignition/engine controller problem. Engine quit running exiting the freeway. Could not restart at all. Towed the truck home. Checked for spark with a timing light. No spark signal. Tried squirting starter fluid into the throttle body; no start and no hint of fire in the cylinders, not even a cough. Checked for 12v at coil POS terminal with key in ignition and turned to ON. Voltage using a DVM is about 12volts for less than a second, and then drops to 0. Checked for battery voltage and ignition switch voltage at ASD relay. Both voltages present at the relay socket with ignition switch ON. Pulled the ASD and Fuel Pump relays and checked their operation, both are good. Confirmed that by swapping ASD, Fuel Pump and Starter relays around, same result in each case; no start and coil POS voltage drops to zero after a second. When I jumpered the constant 12 battery voltage from ASD relay pin socket 30 to pin socket 87, which feeds to the coil and injectors and checked the coil POS, I found constant approx 12 volts. Checked for switched voltage at ASD relay pin socket 86 and it is good, turning on or off with the ignition switch. Did the key On-OFF procedure to get the following error codes: 12-battery disconnected, 42-ASD relay open or shorted, 31-EVAP solenoid open or shorted, 44-Battery Temp Sensor Volts out of limit, 55-End of DTC messages. AT this point, I am starting to think I need a new engine controller. Truck has over 260k miles with no problems since new.

 

You will see 12 volts at the coil for about three seconds at key on, and then it will drop to zero. The PCM won't turn it back on again until you turn the key off, or, if it sees the engine turning. (even cranking) Trick is, do you get power back at the coil when you crank the engine?

 

Checked again as you suggested. Ignition switch to On, coil POS voltage jumped to about 10 volts and then back down to zero in about 1 second. Turned the ignition switch to crank the engine and coil POS voltage only came up to about 8 volts. No start. Battery voltage is about 12.2 volts, but I may need to recharge it soon.

 

Yeah, need to have a full charge on the battery...... Do you get spark out of the coil?

 

The ECM grounds the coil to get spark so seeing less than full battery voltage during cranking is ok. From the description it appears the coil sees voltage and gets triggered but no spark is detected by the timing light. I'd look closer at the coil, see if you can get a spark from the coil wire to ground. If not I'd replace the coil first.

 

Well, I may have found the strongest candidate for the failure, thanks to you guys. Coil is an aftermarket MSD Blaster 3 coil, P/N 8223. Took it off to check primary and secondary resistance. Both read zero or open. Went from smallest to tallest DVM resistance scales while measuring between POS and NEG terminals on the coil, and from POS to center post. Zero. Checked a similar MOPAR Perfomance coil from my Suburban; primary resistance is 3.9 ohms, secondary resistance is 4.9kohms. So, for now, I need a new coil for the truck before I continue troubleshooting. Will come back with an update.

 

That is likely your entire problem.... Let us know.

 

Well, good news and bad news. Good news - Replaced the dead MSD ignition coil with a new one. New coil showing resistance readings: Primary 2.8 ohms, secondary 5.2 kohms. Installed in the truck and all the computer error codes and the CHECK ENGINE light were cleared. Bad news - still no spark. Replaced the distributor cap and rotor. Old ones looked a bit worn and carboned up so what the heck, let's go with new ones. Forgot what a PITA it is to get to the cap and wires on this truck. All put together, tried to start it. No joy. No spark from the coil, no signals to trigger the timing light. Still see close to battery voltage at the coil POS terminal for about a second, then drops to zero with the ignition switch in ON position. When I turn the ignition switch to start, voltage comes up to about 10-11 volts. I figure a timing light on the wire from the coil to the distributor should at least give me a fast paced flicker, but none noticed. Clipped the pickup to nr 1 spark plug wire, no flash. I am stumped at this point. Taking a break for the night and recharging the battery before I try anymore troubleshooting.

 

Best course of action is to confirm or eliminate the ECM. Ideally the tool for that is an oscilloscope to check if the ECM pulls the negative side of the coil to ground. As that's probably not an option you could try a couple of other things: test light connected between the positive and the negative side of the coil and see if there's flickering, voltmeter on the negative coil terminal and check if voltage fluctuates. Disclaimer: I've only done this on older points ignition systems so it might not work that well with ECM controlled coils.

 

Test light across the coil connector terminals should work just fine. See if the PCM is actually controlling the coil.