Hello I own a 1984 4spd stock Crorvette.

I was checking the voltage on the Coolant Temperature Sensor that talks to the ECM, front of the engine. It has two wires.
Voltmeter on 20 volt scale with the positive lead on the black wire and negative lead grounded to the motor read 0.01-0.02 volts. Engine temperaute was 186 F. I thought that it should read 0.8 volts.
Did I do this correctly. Are the numbers saying that the sensor is no good?

thank you

 

You have to measure the resistance of the sensor, which changes w/ temp. Google "GM coolant temp sensor resistance" for a chart.

 

Why are you checking your coolant temperature sensor? Do you think it's not showing the correct temp? 186 deg is a good thing!

 

Yes I know that the temperature is good. The fan comes on at 205 and the car stays cool. But my idle is going up as temperatre increases. So after an hour of running the car went from 800 rpm to 1600 rpm. Someone suggested that the CTS maybe giving a bad signal to the ECM.
I have a temperature chart. What I need is the configuration of the volt/ohm meter leads. The CTS has two wires a whits ans a black. Which one gets the positive lead and then do I connect to ground or to the other wire?
Thank you

 

For the price of a CTS - just replace it and see what results you get with the replacement! You mention you know what it does and it seems you mention it does it with regularity. Just replace it!

 

The cts should get a 5 volts from the ecm, the other side is what the ecm reads, so you need to see if you have that 5 volt reference. Then check the cts at different temps to see if your off the chart.

 

Mid america has a cts the signals the ecm at 195, its rather inexpensive .

 

84 fan works directly off a standalone switch ; the ECM plays no part in fan operation

DVM on ohm scale ; one probe on each terminal of the CTS ( not the wires going to it )
Measure resistance of the CTS ( cold )
Is resistance correct with chart?
Warm engine up , measure resistance ,
is it correct with the chart?

If not replace CTS ; $14
http://tpiparts.net/inc/sdetail/10956/497

In fact given the age of your car , I would just replace it anyway

 

Not accurate. There is an interesting combination of pullup resistors on the CTS signal to the 5 volt reference inside the ECM. The CTS signal to the ECM is the yellow wire. The other connection on the CTS goes to ground (black wire). The pullup resistors and sensor make up a voltage divider.

I said, "interesting combination of pullup resistors" above because the ECM actually switches back and forth between two different pullup values to get a more accurate reading when the coolant is hot and cold.

The CTS has a negative temperature coefficient. It has very high resistance when cold and it goes down to a very low resistance when hot. You'll see close to 5 volts cold and a low voltage when hot. There is a chart in the FSM that looks like this:

 

I know I should just replace it. but I really want to learn how to determine whats wrong not just keep changing parts until it runs right.
I did that with my 91 GMC. turns out all I had to do was adjust the IAC. After ding MAP, TPS and EGR.

My quetion is teting the reitance of the CTS.
I have to remove the sensor. Coolant will come out, corrrect.
Put the CTS back in heat the engine.
Then remove the CTS and test the resitance again?

 

Test it in a pan of hot water, use a cooking thermometer to get an accurate temp.
Does the cts on your vette look like a knock sensor? If it does, that's an early design. Your best bet (if its bad) is to up grade to the newer style sensor & pigtail. Any 85 up GM efi engine would have that style.

 

Mine has two wires Black and white. So I need to test the actual sensor?

 

If the car is a couple days cold assume the temp something close to ambient air BUT you could pack it in ICE in the car and cool it substantially more than that, do a check with it cooled with the ice, remove the ice wait until it's likely reached something close ambient again, check then and maybe once more after warm up. I can understand the desire to learn.

 

Why
Just test it in place on the intake; hot and cold like I stated above
And yes , if you remove the CTS , coolant will come out

 

Look at the CTS chart I posted earlier. The temperature vs. resistance curve is VERY non-linear. You only need to check it cold and hot.

It has the same curve as the temperature sensor for the dash display. Because it is so non-linear it is VERY inaccurate at low temperatures. That's why the display shows "LO" until the temperature reaches 150°.

 

Thank you

 

I can appreciate that you tried to measure voltage because that's all the ECM can work with. The 5 volt reference drops as more of it's potential reaches ground and that drop is used to calculate temp. And you will not see the full 5 volts at the ECM Pin as long as the Sensor is connected so if you want to verify it, you will need to isolate the Sensor; ie, disconnect it.

I like testing with a Scanner as figuring out Temp vs Resistance isn't always that easy/accurate. Wait until it's sat so that you can see whether or not it's at ambient - or close to it since 25 plus year old wiring is going to have resistance and skew the reading. You can also compare it to the Intake Air Sensor which should be about the same. Then, with the Scanner hooked up, start it and it should rise to thermostat, cut back a tad when it opens, and then rise to whatever temp is burned into the Fan Switch (for this Year). Otherwise, just getting another one for 9 Bucks at any Discounter makes a lot of sense too - though that isn't going to help you if it's the ECM. When mine went out, it was toggling between 230 and 250 and cold starting was difficult. When it did start, the Fan was on (ECM controlled) and there was no fast idle. Went through all the diagnostics before determining it was the ECM. Yours is the opposite as when it's hot, it speeds up. You might just see if someone spliced in a new connector (replacements usually come with one) to make sure there isn't a weak crimp or crappy solder joint, either of which could muck it up hot.