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Location of the engine coolant temperature sensor? Testing fan relays

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Hello, I have been having some over heating issues on my 1996 Seville STS Northstar. I did a complete flush of

the coolant, the water pump seems to be working, the pressure cap is a new one with the correct pressure. I think

I have narrowed the problem down to either the engine coolant temperature sensor (is that what controls the relays to the fans

coming on and off?), or possibly the cooling fan relays.

The fans dont turn on until the temperature is one line under the orange warning area (analog gauge) then the engine

will return to the half way mark for 20 minutes than climb back to that high temp mark than back to the middle. This is

all done under the same driving conditions. Not high revs or racing. Does anyone also know if the relays can be tested

by an autozone or someone similar to see if they are defective?

Also, if someone can show me a photo of the engine coolant temperature sensors location. Is it easy to get to?

Thanks for taking the time to answer my questions.


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Hi Mark, I believe you are looking at #7 in this photo,


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Relays either work or they don't. Your apparently are. Sounds like your ECT sensor is working as it should to me. Get a block test kit from Napa to test for exhaust gases in the coolant so as to rule out a bad head gasket before replacing any parts.

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There is a purge line in the cooling system which can cause overheating when it is clogged. Here is what BBF recommended.

OK, check the air purge line, it begins at the throttle body. I am not sure what it looks like on a 2000, but there is a 'bolt with a hole in it' that is connected to the coolant/waterpump crossover, that if it gets clogged will not allow air out of the cooling system. At the top of your coolant tank, there is a hose that goes in coming from that bolt, while the engine is cool, pull that hose off the tank and see if you have coolant flowing, it should flow quickly...

Are your cooling fans working?

How is your water pump belt and tensioner?

Does your cooling system hold pressure?

Who did your head gasket?, were both replaced? Was the engine timeserted?

I assume that your are using a 50/50 mix of dexcool and water/distilled water?

I assume that the cap on your coolant tank is holding pressure?

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Wow! :o

Thanks for all the quick replies. Let me just say, that I belong to another car forum and the replies

on that board are not nearly as fast. You guys are amazing!

That diagram is great. I never would have found it. That coolant sensor is

in definitely in an award place. Are there any tips on getting it out and the new one in? Is is easier from the top or

from under the car. I wont be using a lift, just some ramps.

Ok, so it sounds like the relays are working.

Tomorrow I will get out to NAPA and get one of those test kits,

and check to see if that other line is possibly clogged.


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look straight down using the egr valve as your reference. the temp sensor is screwed into the coolant crossover manifold. i am not sure what your gauge will say if you unplug it as a test? what does your temp gauge do when you drive at 20+mph? does it stay in the normal range? turn on your a/c. that will force your cooling fans on. just a fyi.

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