Mha8649

Alright, let me start by saying "Hi," and this will be my first post. Well, I just bought a 96 z28 and am having a few problems. The one that bothers me the most is the cooling fans not working at all. I used my genisys and checked the fans and they both come on when commanded. I checked all the relays and they work fine. I changed out the ect that goes to the pcm and still a no go. I ran down the trouble shooting chart from ondemand5 and they all tell me relay and it's not the relays. I hooked the genisys up and did a live data record and the fan was never commanded on not even when I just let it sit there and run and had it almost to 260. I am at a loss. I was thinking pcm but I was told that it might not be the problem but I don't know what else it could be. The car has had its air compressor removed along with the high and low pressure lines to the compressor. I'm not sure if the genisys is bypassing the pcm to activate the fans or if it is telling the pcm to activate them. Oh and the pcm is reading the accurate temp as the gauge according to the scan tool. Any help would be greatly appreciated.

GaryDoug

Your scantool is "asking" the PCM to turn on the fans, and it's evidently doing it. Check all the other parameters to see if anything looks way off, like VSS, MAP, IAT, etc. Maybe something is so far off it is blocking the PCM algorithm for the fans. I just tried this on my benchtop 94 PCM and it would not turn on the fans if the "vehicle" was doing 50mph, but then later it did, so not sure what to think about that. I assume there are no trouble codes of any kind.

Mha8649

The only trouble code I have is secondary air pump not working. I am going to replace it but I don't think that would have anything to do with the fans. I went back over the live data and everything else was looking great. iat was reading like 120, vss was right on, map was reading within parameters. I am not sure. I went over the wires and they are in good shape. Like I said in the previous post is I think the pcm is just not telling them to come on for some reason but I don't want to replace it and still have the problem plus pay for the pcm to be programmed to my car. Pulling hairs here.

Injuneer

Is your PCM programming stock, or has it been altered?

Mha8649

I'm not 100% sure. I just bought the car. It has a rebuilt title. The guy put a new engine in it but he never said anything about the pcm. I think I am going to fork out the 200 and get a new pcm and have it flashed to my car and see if that fixes the problem.

GaryDoug

Does your scantool have 3 fan actuators like the Tech II? On my 98 there are three actuators (1, 2 , and both). One of those is never used by the PCM in normal operation. Typically it uses 1 and both. If yours has the three actuators, do they all work?

Mha8649

The genisys has 1, 2 and 3, and 1-2-3. when I put on 1 both fans come on low speed. when I put on 2 and 3 the left fan comes on high and the right fan shuts off. when I put on 1 2 and 3 none come on. The genisys never tells the fans to come on it doesn't send the command. I have to pay for a pcm and wait a week and then when it comes in flash it to my car. It's going to be a fun two weeks.

GaryDoug

So the actuator for relays 1,2, and 3 does NOT work! This seems to indicate an overload issue, since the current is, in that case, doubled for the fan power circuit and is increased by 50% for the fan relay circuit. I would try removing the fan relays one at a time and see if the fan(s) run at any time or temp in normal operation. It is possible that fusible link #C is intermittent and opening under load. The conditions may be different if this is tested with the battery voltage lower than normal (using the scantool in the garage with the engine off maybe?) rather than with the engine running and charging the battery as in normal driving. Another possibility is that one of the relays is drawing excessive current and causing the PCM driver to overload when it is activated, but only when the voltage is normally higher while running.

It just seems so unlikely to be a fault with the PCM's internal circuits.

Mha8649

I'm sorry, the pcm never sends the command on. The genisys will turn them on when commanded on. I use it while driving to avoid over heating in town. But I don't like to do that because when I goto work I leave the scan tool in the car in the case and will that might be enough of an incentive to break in to my car. While driving and using a live diagnostic the pcm never sends the command to the fans to come on neighter high or low. The pcm is getting the same temp as the gauge. the wires and the relays are good. I ordered the new pcm and will let everyone know if that fixes the problem.

Mha8649

Alright, update. I replaced the pcm and reprogramed it with the tech. The fans kick on now and using actuators 1, 1and2. and 1and2and3 all make the fans come on. But the temp goes up to the white line before the red before the fans kick on and then it drops the temp down to halfway mark. I was told the command on doesn't happen until around 230 for my car. That to me seems a little high. I guess it's better than going up to the red and beyond . I don't know what temp it requires before it turns both fans on high though haven't seen it happen yet.

Injuneer

Stock programming turns the fans on low speed at 226*F and high speed at 235*F. GM designed the engine to run at 210*F. The engine should typically run near the middle of the gauge, unless you are stopped in traffic in hot weather.

The gauge runs off the sensor in the head, the PCM reads the sensor on the water pump housing. I would check both sensors.

http://shbox.com/1/4th_gen_tech2.html#ect