Hey guys. Been having issues with the RS but I think it's close to running perfect again. It's a bone stock 305 TBI with a T5 behind it.

My gauge has been saying that I've been overheating at random times quite a bit. Sometimes it seems dead on at 220 and will cool with the fan on and raise back to 220 with the fan off. Other times it'll pretty well peg out the gauge. The coolant temp sensor has been replaced (along with the fan switch, heater core, water pump, hoses). I'm getting a SES light with a code 15 saying that the coolant sensor circuit is low or open. I'd been figuring that my car was overheating and that the gauge was accurate, but yesterday at idle I noticed something. With the fan running and the gauge showing at the first hash mark above 220 right before redline, the fan actually shut off. This tells me that the fan switch in the head has told the fan that the coolant is actually cool enough for the fan to turn off. So, something about my gauge has got to be wrong. I wouldn't put it past the new CTS being bad, but here's my question:

Does the ECM actually use the readings from the CTS to adjust idle speed or any other electronic engine controls?
I have another mechanical gauge that I believe I'd like to use to make sure that I'm not actually overheating. It will thread into the stock CTS location. I'm just afraid that if I leave the plug to the factory CTS unplugged, it will constantly think the car is cold and keep the idle up.

If the CTS does need to be plugged in to keep the ECM happy, are there any other locations to thread the mechanical gauge in? I was wondering if there would be a plug in the driver side head I could put it in, pretty much opposite where the fan switch threads in on the passenger side. Or should I just get an electronic gauge and "T" it in to the factory harness?

Any thoughts/input would be greatly appreciated!

 

ECM does NOT use any temp readings on a TBI setup.

Your temp sender is in driver head between #1 & #3 spark plug, dark green wire going to it. Sounds like either you have a bad connection, bad gauge, or bad wire (my vote on the last) - check it out. Might try replacing the sender as well.

I'd try getting factory gauge to work better before I spent time with mechanical gauges - I just don't like mechanical.

If you think fan should come on at lower speed, swap out the fan switch for a lower temp one.

 

I agree, I'd much rather have the factory gauge working. I'm not really sure where the fan is coming on since I can't trust my gauge currently.

I've put 2 pictures on here, which one of these is the temperature sender? The one on the passenger side head between #6 and #8 has been replaced, it should be the fan switch and it's an OEM. It has the flat blade style connector on it.

Is there a seperate sender on the driver side? I didn't know that. I thought the coolant temperature sensor on top of the intake manifold was also where the gauge got it's temperature reading.
Any ideas on that code 15?

 

left is the fan switch, right is the temp sender

 

You shouldn't have a temp sensor on top of your intake in a 1992 TBI car - do you?

 

Yes. There is a sensor that threads into the intake manifold and has a 2 wire plug on it. That's the coolant temperature sensor. The cooling fan switch is on the passenger side between 6 & 8. If I ground it with a test wire, the fan comes on. So this means the gauge reads off the sender, which is between 1 & 3 on the driver's side head, correct? I didn't know about the sender on the driver side.

 

Yes, gauge reads off driver side sender.

Strange - my 1992 TBI car did not have a CTS, neither did my 1990 TBI car. I assumed the only cars with them were the TPI cars, since the TPI cars had dual fans.

 

Yep here's a pic of the CTS in the intake manifold, and one of the CTS sitting out. Ignore the coolant sitting on top of the intake manifold, I had already taken the CTS out to install my mechanical gauge but put it back in momentarily to take the pic

 

But here's what I've found out. My coolant temperature sender must be bad. The mechanical gauge is plugged into the top of the intake manifold now, so it's reading next to the thermostat and it will obviously be lower than what I'm showing at the head with the gauge in the cluster. I took a pick of what the cluster showed with the car warmed up, fan running (turned on by switch in passenger side head), and sitting at idle. Then took a pic of what the mechanical gauge is showing. In the 10 seconds it took for me to get the pic of the mechanical gauge, the cluster gauge dropped down below 220, as shown in the 3rd pic. I'm going to O'Reilly's to get a new coolant temp sensor and new coolant temp sender to see if this will get everything straight.

 

second big line on stock guage is 200 degrees.for firebird/trans am anyway

 

1991 305 TBI - we were concerned that the fan wasn't turning on soon enough too. We replaced our stock fan switch with a cooler one. Had to do some searching for it and ordered it from a small local supplier.

Like mentioned above, it's on the passenger side and a real PITA to get to. We had ours on a hoist and it was easier, but you will get wet (unless you drain the coolant). Do NOT use any pipe tape or dope on the threads (it needs to get it's ground through the block).

Once changing ours, the fan was kicking on and off great. Before changing it, we had it on a scanner and did find out that our gauge was WAY off. If you can find someone with a good scanner to verify your gauge reading, that would probably be helpful (or at least comforting).

There's a good post on replacement switches here:
https://www.thirdgen.org/forums/cool...ernatives.html