Hi all,

89 Camaro RS 305TBI VIN E.

Got a really weird one I can't explain.

My PCM Coolant Temp sensor, and my Gauge Coolant Temp sender (which are different things if you arent familiar) were in disagreement by about ~35 degrees ( PCM showed ~185 while gauge showed ~220)

I verified the correct coolant temperature using a laser temp gun, the PCM has it correct. From now on CTS will be referring to the SENDER not the SENSOR

I ordered a new one wire CTS, of the correct part number for this car. I DID NOT buy ACDelco.....But please read on as it seems deeper than just that:

With the new CTS, the gauge immediately pinned to max and stayed there on the cold start. I thought that was odd, as it would require sensor being basically fully open to ground......


This is what I've tested:

• Disconnected the single wire from the CTS, gauge reads 0
• Connected a jumper between the wire that normally connects to CTS, and connected it directly to ground. Gauge maxxed out
• Connected a 130ohm resistor between the above jumper and the ground (to simulate CTS showing ~220 according to the below chart), Gauge reads 220

This should show that not only does the wiring to the gauge work, but the actual gauge itself works, and displays the current temp if given the correct impedance from the sender. I expected that the CTS I purchased was in the wrong impedance range for this vehicle or something. after the other above tests, the car was showing about 178 degrees on the PCM. At this time, I tested the impedance between the spade terminal on the new sender, and the block itself. I got 330 ohms. This is exactly right, yet with the gauge plugged in to the sender, it still shoots to max.

I am lost to say the least. What gives? I ordered a GM Genuine CTS, but something tells me that can't be it.

 

I'm subscribing to this one and hope you find out the cause.
I once replaced the sender on my T/A for a similar reason and though it didn't max out straight away, the gauge did go up with engine temps and then smoothly went to max.
All that even though the part seemed to be the right one.

 

It's a 35yo indicator, far from a precise gauge for the factory and it runs through multiple connections and a printed plastic circuit. You need a signal control, variable rheostat to check the exact sweep of the gauge at each increment.

 

Hello did you read my post? I used resistors to test the wiring, gage and, cluster PCB. If there were issues with that, or if it was just off because it's old, then me putting a resistor in the chain of the correct value would not show a correct temp. And it does.

I also hooked up a 10K pot (what I think you mean when you say variable rheostat) and was able to smoothly vary the gauge between 0 and Max.


The issue is with the sender, and the sender only.


Thanks

 

It is possible that the 'precision engineered" replacement part is incapable of passing the current necessary to operate the gauge. If you were testing with ½W resistors and/or a 1W potentiometer, and the sensor element from the other side of the international date line was capable of passing mere mW for a "modern digital" PCM gauge input, you may have found the root cause - BAD engineering assumptions. Even if the sensing element (NTC thermistor) was made by one company in Malaysia and shipped to Mexico for assembly into a threaded package to fit your coolant port, the original unit was likely a lot more robust than the replacement.

For example, look at the TE 44000 RC series, rated at 30mW. Any dash gauge requiring 3/4 of a watt to indicate is going to swamp that thing, making it completely stupid. A high-impedance PCM input wouldn't do that, and when inexperienced and over-generalizing junior amateur engineers design a replacement for your application they only know the last 30 years of PCM-linked devices on a CAN bus, and have no cognizance of how an actual, stand-alone measuring device might function. As a result, you get the best that "modern science" can offer as applied by modern education.

I strongly suspect this is precisely the kind of oversight that is involved with us no longer being able to get a functioning brand-new MAF sensor as well. The people who originally designed the systems are no longer around to instruct the new hires just how ignorant they are about the actual application(s).

 

Oh - and get off my lawn...