Can an ICM still work even though its bad (Page 2/2) Gall757 MAY 14, 09:13 AM quote Originally posted by Markeeise: mmm, ok if it were throwing codes would the check engine light be on? Thanks Usually, but there are exceptions. When in doubt, check for codes. fierofool MAY 14, 10:52 AM I have used failed ICM's as spares, rotating between them as they fail again. Eventually, they will die a final death and may leave you stranded. One of the circuits in the ICM is for the fuel pump. It can fail or loose contact and you will still have spark but no fuel. Difficult to diagnose. Pins in an old harness end can come loose and slide back, causing a broken circuit. Always look into the end of the harness to see if all contacts are equally inserted. I know of an 88GT that was bought for parts just because of a loose pin on the ICM harness. Markeeise MAY 14, 12:01 PM quote Originally posted by fierofool: I have used failed ICM's as spares, rotating between them as they fail again. Eventually, they will die a final death and may leave you stranded. One of the circuits in the ICM is for the fuel pump. It can fail or loose contact and you will still have spark but no fuel. Difficult to diagnose. Pins in an old harness end can come loose and slide back, causing a broken circuit. Always look into the end of the harness to see if all contacts are equally inserted. I know of an 88GT that was bought for parts just because of a loose pin on the ICM harness. wow, that's crazy is there any definite way to tell if the ICM is bad even though the car runs? Thanks fierofool MAY 14, 02:32 PM The major parts stores can test the modules. They have to load it at least 3 times. This heats up the circuits and board. The common cause of failure is the circuit traces break and separate when the module heats up. Other problems are as described with poor ground through the screws. Markeeise MAY 14, 02:45 PM quote Originally posted by fierofool: The major parts stores can test the modules. They have to load it at least 3 times. This heats up the circuits and board. The common cause of failure is the circuit traces break and separate when the module heats up. Other problems are as described with poor ground through the screws. okay thanks theogre MAY 14, 03:48 PM quote Originally posted by fierofool: One of the circuits in the ICM is for the fuel pump. It can fail or loose contact and you will still have spark but no fuel. Difficult to diagnose. Pins in an old harness end can come loose and slide back, causing a broken circuit. Always look into the end of the harness to see if all contacts are equally inserted. I know of an 88GT that was bought for parts just because of a loose pin on the ICM harness. ICM Does Not control the fuel pump or anything else. It only sends a Timing Reference Signal thru Ref line/pin and "fake ground" to ECM as shown in cave. ECM Will Not turn on Injector(s) and maybe turn off pump relay if ECM actually or just thinks the timing signal is not present. ICM or Wires to ECM can be "bad" causing lost signal. HEI w/ F'd Tach Filter, I-coil, and more will hurt or kill ICM. ICM and I-coil w/ crap mounting hardware can kill ICM, ECM and more because has Ground problems for the whole Ignition and tries any other way to dump Power (I-coil alone uses ~ 7 amps normally) to Ground. ECM can be "bad" and even see Timing signal but have other problem(s) and won't turn on whatever. Many times this issue Won't set "Codes" but may show a function works or command to do X but isn't using a ECM scan tool. Even then they might not show you much. Is why often need Noid lights etc to show some ECM/Wiring problems. Many old ECM has problems w/ board "paint" swelling over time actually popping parts loose. Many ECM/PCM has crap solder joints too that have same results. fierofool MAY 14, 06:02 PM I don't know what the wires do, but I've experienced 2 cars that wouldn't start even though they had spark. It was due to a wire in the 4-wire ICM harness being pushed back. Once the wire was reseated, the car started. That's why I think it had something to do with the fuel pump circuit. I believe it was the 2nd wire from the right as it's plugged into the ICM.