Hi! I have a weird issue with my GTA (L98/350 TPI). Car has A/C and the stock dual fan setup. My Primary cooling fan (the drivers side, ECM controlled one) is running almost all the time when engine is running, even at normal operating temp. After diagnosis I suspect faulty ECM but would like more input.

I did some diagnosing today, and this is what i know:

*Replaced the CTS for the ECM yesterday. Old one was probably not faulty though, no change and old/new have equal resistance at room temperature.

*Replaced both fan relays, primary and auxiliary.

*Fan does NOT run with ignition key on, engine off. When I cold start the car and it's been sitting overnight the fan doesn't start running immediatly so it's not hard-wired to always run or something like that.

*After the very short drive from my parking garage to the shop where I work (close by, and car not even close to operating temp) the fan started running.

*If pulling CH/FAN fuse as Factory Manual states to do a short circuit check when fan is running, fan stops.

Then I tried this after a little drive:

With the car idling, I used an infrared thermometer to measure temp right at CTS. It was 185 degrees Fahrenheit (85 Celsius) but fan was running anyway (should start when CTS is at 223 degrees F). I turned the car off, and started it again. Fan didn't run with ignition key only, but started running immediatly when I started the car, as usual.

Turned engine off and disconnected plug for ECM CTS, connected the plug to my loose old known-good-CTS which was currently at ambient temperature (68 deg. F / 20 deg. C). Started car again and the fan started when I started the car, even though the connected CTS now only was at ambient temp!

According to the Factory Manual, the only thing that is supposed to make the primary fan start is the ECM sending ground to the Cooling fan relay. The only reason the ECM should ground this relay is when it's told by the CTS that the coolant temp is above 223 degrees F.

Therefore I suspect the ECM is faulty and that it's grounding the relay all the time when the engine is running.

Has anyone ever experienced this problem before? What do you think?

 

To be clear, you are replacing the CTS in the front of the intake, correct?

 

Do you have your AC on?

 

Yes, the 2-pin at front of intake under throttle body.

No, tested with both VENT and OFF, same behaviour

 

Does the ECM still have the stock MEM-CAL in it?

 

There is a high pressure switch in the A/C line (small diameter one) that runs from the condenser to the evaporator. The switch is located near the pass side strut tower. I suspect it might be shorted. If it is, the fan will run even if the A/C is not on. Unplug it and see what happens.

 

I had nearly the exact issue sounds like and it was driving me insane trying it figure it out.
My car is 89 iroc 5.0 tpi. But as soon as the engine fired up the driver side fan would turn on and stay running until the engine was shut off. Not with the key in the on position but when the engine started.
This car is bone stock, wiring all 100% factory intact. I had previously changed out the cts sensor while replacing fuel injector o rings because the black plastic end was broken and the electrical lead wouldn't stay plugged in. It was a part I picked up at carquest.
Anyhow I put it in, finished up the o ring replacement ,fired car up had no more o ring leak so on my way I went. Then I finally noticed the fan running all the time and had a ses code. Code was 14 I think wich is cts. But i had just put new cts so figured wtf. Swapped relays still same , only was to stop fan was either pull relay,the fuse or shut engine off.
For the hell of it I went and got a new ac delco cts , installed it and when fired the engine up what ya know the fan stays off until it's suppose to run. And no more ses light.
Looking visually at the carquest sensor vs the delco they appear identical. Only diff was the delco had a factory style thread sealer coating the threads and carquest did not, it was bare and I had put thread tape on it.
I dunno if it was just a bad sensor, or the thread tape caused it not to ground properly but all is still good. Lesson learned to me, use ac delco when all possible.

 

Unfortunately I don't know, only owned the car for a year. Everything else on the car was stock though when i got it, no hacked up harnesses or such. Original injectors still there when i got it so i doubt some PO changed the memcal. If you suspect it's reprogrammed to go on at a lower than stock temp, i did too but connecting a cts which was 20 degrees Celsius, and fan still running ruled that one out...

I have no codes though . Old sensor was delphi (PO replaced it once too), new one is "cub"/STM.

Will try this, I tried unplugging the small one next to the drive belt with no change, but i Think that was the secondary fan pressure switch. You are referring to another switch closer to the drier, right?

 

i know what you mean, i would have bet the farm it was something other than that cts sensor. I went thru every wiring schematic and even another ecm, thinking that must be it, as well at the memcal chip. no matter what until I changed that cts sensor it still came on when engine was started. No code 14 confuses me though.
theres something grounding out somewhere to make it run.

 

I had a similar situation a few years back. Does you car have headers?
Mine does, the cause was due to the wires connected to the fan switch on the engine block on the passenger side had burnt on the headers.

 

I would search for the resistance values vs temperature for the CTS, IIRC, my old Chitons manual had them...let the car warm up until the fan comes on, check the resistance across the CTS..will probably need a laser temp probe to know where it's the temp actually is.

This way you can see what the ECM is seeing

 

Yes, the A/C pressure switch you should check is "near" the drier, but it's in the small diameter high pressure line, not the one "in" the drier.

The thread sealer on the CTS in unrelated to this problem because the CTS does not need ground through the threads to function, it simply returns a resistance based on temperature. See attached chart. BTW, this chart works for both the CTS and the air temperature sensor in the TPI plunum.

 

X2 on checking for a burned wire on the passenger side near the exhaust. Look to see if the loom has fallen down against the header.

 

One more thought...does the passenger side fan come on when you turn on the AC?
It should.

I wonder if the 2ndry fan switch was replaced with a lower temp one or faulty, when the 2ndry fan switch is closed, both fans should turn on.

In your case, the 2ndry fan may be bad and not coming on, while the fan switch is closed.

 

Yesterday I drove around in a cruising event in town for a couple of hours, and at first the primary fan ran all the time. After driving around a while it stopped though, and the engine got hotter than normal operating temp of course. I think it returned to the way "it should be" at this point.

I tried turning on the A/C, and both fans started after a couple of compressor cyclings. I drove around and did a couple of burnouts too and it got up to slightly beneath the point between 220 and 260 but never over that, and didn't overheat. So I think both the AC pressure switch and the secondary fan switch works as it should. This also means the fault is intermittent since the primary fan suddenly stopped...

I think I'm going to replace the secondary fan switch as I have already purchased a new AC Delco switch which is lying in the car.

Maybe I will install a couple of manual switches inside the car, one grounding switch for each fan relay ground wire so i can turn them on and off individually. Doesn't seem necessary really but if i wanna do a burnout in hot summer weather I would want the engine to be cold first...

It would be really useful with an aftermarket temp gauge with Celsius scale (since I'm european and used to counting temp in celsius) but I really don't like the idea of molesting my almost stock interior with an aftermarket gauge...

 

I can now finally update this thread with a solution (probably)!

Today I got connection with my ALDL Cable, the coolant temp sensor reported +40 degrees C with KOEO, but the ambient temp in the garage was only 5 degrees C. Keep in mind that the CTS is brand new and checks out okay with an ohmmeter.

TPS reported 0.55 Volts with pedal at idle, but only 20% (around 2-2.6 volts) with pedal at WOT.

With car warmed up, the CTS reported around 96 degrees C, but if i disconnected the CTS there was no change!

If i shorted the CTS connector 150 degrees C was reported though.

I started to suspect faulty ECM when I measured voltage and it was only 4 Volts (should be closer to 5 V).

I followed a couple of procedures in the FSM, and it states that CTS should report -40 C if connector is removed, which it did not.

I removed the ecu connectors and checked the CTS and TPS wires for shorts to ground, but no shorts were found.

The final test that nailed that the ECU is faulty was that I did the tests on the TPS. Two tests was supposed to show 100% throttle when you jumpered the signal cable and -5V ground cable, or signal cable to B-.

Both of the tests showed only about 21% throttle opening or around 2 Volts. Ruled out the TPS, and showed me that the ECU is faulty.

I just ordered a new ECU on rockauto.com, and will report back when It's been replaced!

 

Yes! My diagnosis was correct! I replaced the ECU today with a new (remanuractured) from Rockauto, and holy lord... It's like a completely different car. Crazy throttle response compared to before, runs like a bat out of hell. Started in like half a starter turn when cold, fans run when they should, no idling up and down like before. Everything is like it should be!