This is weird. 5.7L Formula running great, all of a sudden, engine light comes on, runs like crap. Turns out this corresponds to the driver side cooling fan coming on, and when it goes off, engine light off, runs good. Also the passenger side fan comes on with the key on and engine off always (been like that for a while, no ill effects) and now the driver side on comes on too with key only and engine hot.This second one coming on creates the problem. No codes are generated when this happens, and I cant check the codes when the second on is on, engine light just stays on with jumper in there, but flashes when second on goes off. Whew!

Im thinking coolant fan relay, but why is that effecting the ECM like that, its like the ECM goes off completely when the second fan comes on. totally wierd

 

Wow this board is dead. I may actually have to take my car to a mechanic (hear the sound of giant suction on my wallet).

 

The Heavy duty coolant fan switch was bad, causing the fan to run all the time. REplaced the switch, then the regular coolant fan began coming on with just the ignition switch in run position and motor off. I disconnected both the AC pressure switch and coolant sensor switch that supposedly turn this fan on, and it still turns on with just the ignition switch on. Both relays check out ok. If I start the car, the driver side fan comes on and the engine light comes on, the car runs like crap. Also I cant check any codes, the engine light just stays on. There arent' any codes anyway. I checked them several times before and there werent any. I tried disconnecting the relay to the driver side fan, and of course it doesnt start with the ignition on, but if you start the car you get the same thing, engine light on, runs like crap.

I hate this. It still seems like the ECM is bad. Auto zone has an eCm for $94. You turn you old one in an put the old chip in the new ECM. Could it be the chip too? They have a hypetech thermomaster chip for $164, is this any good? whats better?

 

Pay close attention to the junction in the engine bay that ties the alternator, battery, coolant fans, fuel pump and ecm power leads together.

 

I followed the procedure in the gm shop manual for what to do if the check engine light comes on and stays on with the jumper in place. The only step im not sure of is when you remove the jumper, and "back probe ecm ckt 451 with a test light to ground" then turn on the switch and see if it flashed code 12. I located ckt 451, its on e12. on the yellow connector, its a white wire with black stripe. stuck it with the lamp probe and put the clip on a ground. Nothing happened. If there is a short it supposed to flash code 12, if nothing happened it says its the ecm or the mem-cal, which I guess is the chip. I pulled out the mem-cal and reseated it, no improvement. Im thinking of pulling the ecm out of my 91 camaro, sticking the mem-cal for the firebird in there and hooking it up. If it works, its the ecm, if it doesnt, ???? have to go through the wiring harness like you said. It seems like a short in a way because the fan comes on and off for nor reason. It could however be a bad ecm just turning it on and off for no reason too i guess. I'll let you know how it comes out tomorrow, thanks for the response.

 

wht/blk wire on circuit 451 is the wire that runs from the ECM to terminal "B" on the diagnostic connector (ALDL) under the drivers side dash. to have the ecm flash out the codes, one normally shorts terminal "A" [wht/blk wire] to terminal "B" [blk/wht wire (system ground)] on the aldl port. shop manual procedure is having the technician trying to diagnose whether there is electrical contact between the aldl port & the ecm. that's why the troublesome part of pulling the ecm connectors out & backprobing them.

since you've got a spare ecm around anyways, you could try swapping them. pinout should be the same. if you have a no spark/no fuel condition after the swap, don't forget about the chip in key (VATS) theft deterrent setup. autozone remanned ecms have a bad reputation, you would probably be better off with a used one from a salvage yard or off of ebay.

If you're probing around looking at the check engine light wiring, don't overlook that the ecm uses a pulsed ground to control the light. Light bulb always has +12volts applied whenever the igntion key is in the run position, bulb is illuminated when ecm provides a ground path.

Cooling fan relays are controlled in a similar way by the ecm. Relay coil always has +12 volts applied, relay is engaged when a ground is provided on the DK GRN/WHT wire (primary fan) or the GRY wire (secondary fan).

 

I borrowed a friends mem-cal chip and it didn't help. Replaced the ECM and the fan quit running with just the key on and the code 12 flashed with the jumper. This indicated the ecm was bad. However, it still didn't start and run right with my old chip. PUt the borrowed chip in and it ran like a bat out of hell in. conclusion, both the ecm and chip were bad. Bad luck I guess. Im going to buy a fastchip standard chip for 91 firebird 350 a/t. Its only $160 and thats only a little more than stock. When I have time Im going to learn how to burn my own.

 

It seems to run fine with the reconditoned ECM and my friends borrowed chip. So im going to buy a new chip to put in, but, Im wondering if I do an electrical test to be sure that I don't have a wiring problem that will cause the new ones to burn out.

Any suggestions?