Do PCMs fail?
 

Hi,

So, my 95 Z28 6 speed with about 175k started throwing some random codes from time to time, like the low coolant would flicker (though there was plenty of coolant) and the brake light would come on and go off for no apparent reason. Also, seemed to be running a bit rough (vibration at low rpms) with bad mpgs. During all of this my coolant temp sensor clip broke, so I didn't have a temp reading. When I got around to putting in a new temp sensor, it read that I was running at about 235 (luckily I hadn't been driving it much since the old sensor was disconnected). In light of all of this, I brought it to the shop and they noted that my fans weren't coming on though they're working, they replaced the temp sensor by the waterpump and finally told me they traced the problem back to the PCM. However, having searched the forums a bit it seems the PCMs don't go bad that often. Is there something I should be looking for or is the shops diagnosis seem right? The Z28 had been sitting for quite a while before all these things started happening, for what that's worth. Thanks!

Re: Do PCMs fail?
 

doubtful the PCM is bad. All your sensors need to be working correctly for the car to run right. The temp sensor on WP needs to be working. if you unplug it both fans should turn on

your coolant reservoir has a low fluid float in it that is likely stuck giving you a low coolant light. You can try taping the reservoir or removing it and trying to clean it out. Old dead coolant, especially Dexcool, can crud up the float

bad grounds can cause all kinds of weird problems. look carefully at the connections on DS head where the coil is for any lose/broken connections

Re: Do PCMs fail?
 

Yes, they fail, but very rarely.

Don't see how you got codes for low coolant and the brake system. Neither of those is connected to the PCM and cannot set codes. Typical problem with the low coolant sensor is either a dirty or corroded sensor. The "BRAKE" light is turned on by the switch in the e-brake lever, or by the differential pressure switch on the side of the ABS unit, via the electronic brake control module (EBCM). Did you check for low fluid, air in lines, or leaking brakes?

Appears you personally replaced the gauge coolant temp sensor, and found the engine was running at 235*F. That sensor has nothing to do with the PCM.

The shop appears to have replaced the coolant temp sensor in the water pump, for the PCM. If that sensor is faulty or not working, it may set a code, and could make the engine run rough, with poor gas mileage. If the PCM thinks the engine is cold, it richens the A/F mixture.

Did they check the coolant temp sensor harness connector for the required 5V reference voltage (connector off the sensor)? Did they check the accuracy of the sensor with the standard GM table for sensor resistance vs. temperature? Did they check the fan relays? Did they check the fusible link that supplies 12V power to the load side of the fan relays? Did he check the calibration of the coolant temp senor in the head, for the gauge, to insure that was accurate? Do the fans come on when the HVAC selector is set to A/C?

What codes have you actually pulled from the PCM?

Have you ever tried to do a data log of the PCM? That will tell you what the PCM sees from the coolant temp sensor, and whether it is reaching the temps required to turn the fans on (226*F fans on low/235*F fans on high)?

Blaming the PCM is typical, when they can't figure the actual problem out. Not saying the PCM isn't a problem, but unless you/they have checked all the points I mentioned, and a few more, you can't know either.

Re: Do PCMs fail?
 

Thanks for the replies!

So my main concern when I took it to the shop was the over heating. They first replaced the temp sensor by the waterpump to make it that wasn't the cause. The fans do turn on when I unplugged the sensor by the waterpump, so that fans work, but just aren't turning on when they should.

As far as the brakes, I haven't checked for air in the system, but the fluid level is good and there are no obvious leaks.

The shop claims that they checked for bad grounds, the fan relays and the temp sensor harness. The rest I'm not sure about, but I'll check.

I picked up a remaned PCM today, but I'd sure like to make sure that's the problem before it's put in and reprogrammed, and thus can't be returned.

Is there any reason the new temp sensor in the block (the one under the manifold) would be giving an inaccurate reading? Also, should I be worried about any internal engine damage from having run the car at 235-240 for a while (warped heads/blown gaskets). It never seems to get any hotter than that.

Thanks!

Re: Do PCMs fail?
 

There is more than one fan and/or fan speed involved. It is possible that the 226F threshold does not turn on any fans while the 235F one does. When the fans are running after unplugging a sensor (like the ECT sensor or the TPS) do they both run? If only one is running, the problem is with the wiring, relays or the fan that doesn't run.

It is also possible that the gauge sensor you replaced is faulty and reading incorrectly. Like Fred said, get it scanned. You can get scan software and cable for less than half the cost of a remanned PCM.

By the way, there is no float in the coolant level sensor; it uses electronic sensing. It is the oil level sensor that usually has a float.

Re: Do PCMs fail?
 

Thanks for noting Gary. I know it is a "electrical" sensor. I thought it had a small check ball in it essentially acting like a switch (float) when it settles due to low coolant and that sometimes when crud gets in it, does not work.

Didn't know it was entirely electronic

https://www.rockauto.com/dbphp/x,cat..._19299321.html

Re: Do PCMs fail?
 

Plus, the low coolant sensor is not in the reservoir so taping the reservoir (I assume you mean tapping) will not do anything.

Re: Do PCMs fail?
 

I'll check to see if both fans are turning out. It seems like it might make sense that only one is turning on at 235 - I thought it was odd that it would run at that temp but not really get any higher. I'm inclined to think that the new sensor itself is okay because it seems to read in accord with the car warming up.

Being kind of new to LT1s I am curious as to whether you guys think running it at and around 235 could have caused damage or serious wear.

Re: Do PCMs fail?
 

GM wouldn't have set the fans not to come on at high speeds until 235° if they thought that temperature would cause engine damage.

Re: Do PCMs fail?
 

Thank you - that's reassuring. I'd hate to think that I ended up doing further damage.

I'll post an update once I figure out what's going on. Again thanks for the help.