Hi all. Like many others this time of year, I'm trying to debug a cooling issue. I'd like to walk through what I've done so far and hope you guys can give me some more things to try.

I have a 1996 Z-28, 6M, bone stock, about 50k miles. Don't drive it much these days because the a/c compressor is apparently shot. But this spring, cooling problems popped up.

Original symptoms was that the engine would overheat at low road speeds, then cool off at speeds over about 35 mph. Coolant level was OK, re-bled the air vents, checked the T-stat to make sure it wasn't upside down or anything.

I unplugged the coolant temp sensor, both fans come on at idle and I seem to get good airflow out the back of them. At this point I figured the coolant temp sensor or the cabling to it was shot. With the iginition key ON I measured the valtage in the connector; 5 vdc if I recall correctly. So i replaced the coolant temp sensor. Before I screwed the new one in, I checked its resistance at ambient using the table at shbox.com; seemed OK. I broke the old coolant temp sensor in the process of removing it, so I didn't bother checking its resistance (the wrench slipped and snapped off the plastic housing).

So that I can use the car for short trips, I've left the coolant temp sensor unplugged. The engine runs at pretty normal temps; temp gauge needle at the 1/4 point at speed, at about the 3/8 point at stoplights.

I've tried resetting the PCM, to no avail.

Yesterday I followed the instructions at shbox.com to put it into Field Service Enable Mode. Used some copper primary wire to jump pins 5 & 6 on the DLC. Here are the dash lights that came on;
Service engine soon
Brake
Check Gauges
(seat belt icon)
I don't know if that proves the PCM was in Field Service Enable Mode or not. Sadly, the radiator fans did not come on.

So at this point, I can only think of 3 things, all bad.
1- new coolant temp sensor is bad
2- wiring to coolant temp sensor is bad
3- PCM is bad

I am thinking that the next best step is to get a USB scan cable and software, to see what the PCM thinks the coolant temp sensor is claiming the coolant temp to be. I have one of those El Cheapo code scanners from Walmart, but the PCM never seems to throw a code complaining about coolant temp issues (it does complain that the coolant temp sensor is unplugged).

If I go the scan tool route, I ran across a Crescent brand USB scan cable w/ software on sale for $33 at Amazon.

What do y'all suggest?

 

"Original symptoms was that the engine would overheat at low road speeds, then cool off at speeds over about 35 mph. Coolant level was OK, re-bled the air vents, checked the T-stat to make sure it wasn't upside down or anything."

Did anything change or is this still the symptom? If unplugging the coolant sensor turns the fans on, the fan circuit is good and it has to be the sensor. If it was a wire issue between the PCM and the sensor unplugging it would not turn the fans on. Are the fans coming on high or low speed? They are two speed. You may have a problem where low speed is not working and high is. Other than that I'm stumped

Al 95 Z28

 

Still the same.

When the coolant temp sensor is connected, I don't believe the fans come on at all. If I let it, the temp will climb up to the "red zone" at idle. With it disconnected, the fans are surely on high as they are quite loud relative to the engine noise.

 

This is a stumper. Low fan speed is called for when the CTS reports 226 degrees, high fan speed at 235 degrees.

When you unplug the CTS, the PCM thinks it's over 235 degrees so it will call for high speed.

Relay one is for low speed. Relay two and three are for high speed. In high speed mode, all three relays must operate so you're unplugging the CTS and observing the fans in high speed confirms all three relays are working AND that the two wires for low and high speed coming from the PCM are good. The fact that the PCM put the fans on high with the CTS unplugged confirms that it can send trigger grounding to low and high relays.

This all also confirms your fuses and grounds are all good.

It really looks like a bad CTS. What are the odds the original and new one was bad?

 

Interesting how in like 1988 I designed a three relay fan controller to allow low speed fans in series or high speed fans in parallel for my Monte. Then GM comes along in 1995 and does the exact same thing.

I used a dual fan setup and shroud from a Toyota Camry station wagon. There was no aftermarket electric fans back then.

 

Is is possible/probable that the PCM has lost its wee little mind, or its configuration parameters got screwed up?

I guess I can pull the CTS, heat it up in a pan of boiling water, then test its resistance. See if its somewhere near the 177 Ohms listed on the table at shbox.com.

 

This is possible. Try a swap with a buddies car of the same year/engine/transmission PCM to confirm this.

This is a good idea.

Keep an eye on ebay. You should be able to find a PCM for your car for less than a hundred bucks. The sellers usually flash it to your VIN & options. This way you'll have a spare PCM that you'll be able to get mail order tuned without losing your ride.

 

A quick update...

I replaced the new CTS with another one from AutoZone. Same symptoms. Still have 5 VDC at the CTS connector. So it seemingly must be the PCM.

I'll take a look at eBay, but other suggestions would be welcome.

 

If you have a way to scan the PCM, see what it thinks the temperature is when it's too hot. I don't think it's a PCM problem. I think it's something in the relay circuit. Are you positive both fans are coming on when you unplug the sensor?

Al

 

I would not do anything further without looking at temps via a scanner.

 

Positive. With the car up on ramps, I got underneath and felt the airflow behind both fans. Yeah, not safe, but had to do it.