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have a 2002 trans am ls1 with a 460 in my 78 c10 truck. All stock. been running for 2 years no issues. Had the harness rebuilt 2 years ago. everything perfect UNTIL yesterday when my cooling fans will not come on at any temp. hooked up a snap on reader and watched temp go to 217 before I shut it down. I believe my PCM is set at 195 and 205. I am using the dark blue and dark green wires off the harness (PCM)to go to separate relays. I can put ground to the two wires at different times and get the fans to come on. Therefore, I assume the PCM has gone bad. I just replaced the temp sender on the left head and no help. Anyone know if PCMs can go bad for the fans?? One more thing. I just finished hooking up the factory air and I wired a completely.
separate cooling fan harness that gets power off the compressor to trigger the fans (thru a relay). I tied them into the fans after all the relays from the PCM fans. When I currently turn on the a/c the fans come on and cool the engine quickly. someone thinks I back feed the pcm and smoked it. Not sure how that would have happened since they are a neg. feed to the relay.
Your thoughts
I did replace the themo stat from a factory 160 to a 189.
Last edited by kglowacky; Sep 26, 2023 at 05:04 PM.
Yes, he should be able to command the fans on through the software. Very easy to test that a (-) ground output is being output through the PCM once the fans are commanded on.
Draw a wiring diagram of how you tied in the two fan relays and the new A/C relay.
I suspect you supplied 12V to the ECM relay pins which shouldn't blow the board until the first time the ECM tries to ground them when it detects the coolant temp to be high. In a battle between the ECM grounding the pins and 12V compressor clutch supply trying to raise them up, a puff of smoke and 12V usually wins. If my suspicion is right and you try to manually ground the pins while the A/C is on, you will see a spark.
the relays are wired the same as far as the input output triggers. I wrote them down differently but they should show the same. The only difference is pcm triggers off neg signal and a/c off positive signal.
I have only tried to ground the two fan pcm wires with the ignition off. and it turns the fans on. did not try while the engine running and ac on
the relays are wired the same as far as the input output triggers. I wrote them down differently but they should show the same. The only difference is pcm triggers off neg signal and a/c off positive signal.
I have only tried to ground the two fan pcm wires with the ignition off. and it turns the fans on. did not try while the engine running and ac on
According to what I think you did in the diagram, that would not backfeed the PCM. It appears that you isolated the PCM signal. It would be clearer if you had the relay pin numbers (87, 87a, etc) in there but you say that it was running for a while so I have to assume the ones connected to the PCM were right.
Did the problem occur when you added in the A/C setup or did it just fail randomly?
BTW, unrelated to your issues, looks like you don't have it set up as a two speed setup but rather one fan on for low speed and both fans on for high speed. That's a different topic though.
on the a/c setup It turns on both fans when compressor gets power.
On cooling ,per pcm each fan is connected to each pcm wire. so they come on at different temps. now that I think about it I have driven it after a/c connect without overheating. so not sure I can tell you when this start happening other then it isn't working now. (it did work when I first installed the LS1 as I let it run at idle until they came on). As crazy as this sounds had my mechanic change out the thermostat to a 189 from a 160 then these problems started. I changed it out because everyone told me my engine was running to cool at 180. Maybe at 180 the fans haven't been coming on for awhile so this problem has existed for awhile.
I hate to replace the PCM just to smoke another. I may change the thermostat out and sees what happens
Last edited by kglowacky; Sep 27, 2023 at 10:05 AM.
Chances are that you had the right idea, but the actual wiring was not true to your diagram. I would add the relay pin numbers to your diagram and then go back and check that you wired it the way you intended to. I've also made mistakes wiring relays in the past. It happens.
Here's how I would have wired it. This would give you a true two speed fan setup. When A/C comes on, it would give you only the lower speed. When the fans run, they both run at the same time, but they are wired in series in low speed and in parallel in full speed. I like this better because if only 1 fan runs, it pulls air from the other fan opening instead of pulling it through the radiator and A/C condenser.
I haven't tested this diagram. I run brushless PWM fans.
If you want full speed when the A/C comes on, I can draw up that diagram.
Needless to say, it would be better if the A/C request came into the PCM somewhere and the PCM could turn on the fan(s) when it saw the A/C was on. Then you wouldn't need the additional A/C clutch relay circuit.
Needless to say, it would be better if the A/C request came into the PCM somewhere and the PCM could turn on the fan(s) when it saw the A/C was on. Then you wouldn't need the additional A/C clutch relay circuit.
I live in Houston so My concern would be overheating. So I guess I would like the other diagram. I am going to remove my entire wiring on both and rewire per your diagram. Can you recommend a brand of relays I should use. Thanks
I live in Houston so My concern would be overheating. So I guess I would like the other diagram. I am going to remove my entire wiring on both and rewire per your diagram. Can you recommend a brand of relays I should use. Thanks
Before you re-do the whole thing, make sure that was really the problem.
This should run the fans at full when the A/C clutch comes on.
Before you re-do the whole thing, make sure that was really the problem.
This should run the fans at full when the A/C clutch comes on.
I have no suggestions for relay brands.
yes going to put scanner on pcm and see if I can get the fans to run. If not going to replace PCM and then look at rewiring. Never installed diodes before . looking at ebay how do they connect?
Last edited by kglowacky; Sep 27, 2023 at 11:35 AM.
starting my work on solving my fan issue. Any thoughts on scraping the entire relay PCM A/C wiring and just using a aftermarket external fan unit like the street Rodders use? splice into the factory sensor wires and let it read the temp and adjust to what temps I like,
starting my work on solving my fan issue. Any thoughts on scraping the entire relay PCM A/C wiring and just using a aftermarket external fan unit like the street Rodders use? splice into the factory sensor wires and let it read the temp and adjust to what temps I like,
Sure, You can buy a fan controller or make your own, if you're into DIY. Check out My DIY fan thread
Sure, You can buy a fan controller or make your own, if you're into DIY. Check out My DIY fan thread
replaced my pcm today and first disconnected the a/c fan setup (wiring and relays) and magic the fan came on at 195 and went off at some temp. So, my pcm is toast for the fans command. Going to drive awhile without the a/c setup connected and then try to rewire my ac fan system.
replaced my pcm today and first disconnected the a/c fan setup (wiring and relays) and magic the fan came on at 195 and went off at some temp. So, my pcm is toast for the fans command. Going to drive awhile without the a/c setup connected and then try to rewire my ac fan system.
ls swap. you show a chassis ground and a PCM ground what is the difference?
ls swap. you show a chassis ground and a PCM ground what is the difference?
Chassis grounds are for high power dirty devices like fans, A/C clutch, coils. These things pull alot of current create signal noise and can throw off sensors. Most vendors have a separate set of smaller gauge wires for grounding sensors like map, tps, where they don't want noise introduced to throw off the reading by the heavy loads turning on and off.
Chassis grounds are for high power dirty devices like fans, A/C clutch, coils. These things pull alot of current create signal noise and can throw off sensors. Most vendors have a separate set of smaller gauge wires for grounding sensors like map, tps, where they don't want noise introduced to throw off the reading by the heavy loads turning on and off.
makes sense. to clarify use a small gauge wire and ground where?????. It seems all grounds end up in the same battery system on the truck.
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PCM cooling fans 
