brealytrent

Am I overheating, or is it all just a ruse?

For about as long as I've had the car, the little red light indicating the coolant is getting too hot would come on after driving through lots of city traffic. I did a little research and found that the viscous clutch of the fan had lost the majority of it's fluid. I repaired and filled the clutch (it is holding the fluid now) and that has helped somewhat - It no longer flicks the light on in traffic unless I turn on the A/C, which then it kind of flickers on and off until it gets hot enough to stay on consistently unless I get up to highway speeds.

Now another issue about this:

The gauge itself doesn't seem to be reading correctly. The temperatures the gauge reads when the light actually turns on is either halfway between the two white lines (rare) or when it is sitting on the first white line (common). The car normally operates with the gauge between the bottom white section and the first white line.
Are there two separate temp sending units that actuate the gauge and the warning light? I figure this is an issue with a ground, but I checked both the sending unit and the ground under the dash and they are both clean. I'm starting to question whether the car is even actually overheating and maybe the gauge is actually reading right and it's running cool most of the time!

A couple additional notes, my car is an '84 euro 'twin dizzy' S and has an aftermarket custom aluminum radiator.

Jetdriver69

My 82 did the same thing. At the time, I still had the viscous fan and old Behr radiator.

The coolant temp gauge would go past the 2nd white line toward the red, especially in traffic and the AC on.

When I measured the temps with an IR gun at the top rad hose, the temps were a little high, but not approaching melt down.

Since then, I have replaced the old Behr with a 928MS aluminum rad and the viscous and condenser fan with a FF Dynamics single electric fan.

I also wired in a variable resistor in line with the temp sensor (1st picture). I bought mine from Radio Shack for $5. That way you can adjust where coolant gauge needle reads in relationship with the actual coolant temp.

I extended the right lead (power lead) off the temp sensor and connected it to the variable resistor. The resistor will have 3 leads, use the center and either one of the other leads for your hook ups. Do not use both of the outside leads or it will not work.

Then from the outside lead of the resistor run that line back to the temp sensor. (2nd picture) All you are doing is introducing a resistance that will allow you to adjust the gauge.

My electric fan kicks on at 185-190F, so when my engine was warm I measured the coolant temp with an IR gun at the rad hose and then adjusted the resistor until the gauge needle was in the middle of the two white lines. Simple.

This is essentially the same concept as pulling the pod and adjusting the potentiometer on the back of the voltmeter to it more accurate, which i did this weekend and it is not as bad as I thought it would be. Only the coolant gauge has no potentiometer and you will have install one.

I know this akin to "putting lipstick on a pig" and it doesn't solve the basic issue of an inaccurate coolant temp gauge, but short of installing a second temp gauge somewhere in the car, is it a quick and simple solution that seems to work well.

GlenL

You don't mention the aux fan coming on. That's another thing to check.

The light and gauge are separate signals. Cleaning grounds is often (always?) a good idea.

brealytrent

Here's of video of the issue at night. Sorry for the potato quality, and ignore the Schlusslicht light, that's a separate issue!

brealytrent

Bump

bureau13

As GlenL stated above, the gauge and the overheat light are separate circuits, although they might be both fed from the same physical sensor location. I know when my gauge wasn't working, the overheat light would still come on. In my case, I replaced the sensor (driver's side on the water bridge) and that fixed the gauge, which revealed that the overheat lamp had been right all along...i was overheating because the sealing ring behind the thermostat had decayed into nothingness. In any case, if your gauge is acting normally but your lamp comes on, it could be that the sensor feeding the lamp warning is bad. IF both gauge and lamp come from the same sensor (there are two wires connected to the sensor, and I believe it's grounded through the body, so that makes sense) then just replace it and see...it's a cheap part, and they do go bad.

bureau13

Another test can be done with a ohmeter, I don't recall what the values should be cold and hot, but that info is on here somewhere...

Mrmerlin

I would suggest to replace the temp sensor,
NOTE the erratic readings indicate an internally shorted sensor,
and or one of the wires is shorting
Also cut off the old female connectors.

Replace both of them and use fresh heatshrink

NOTE one is bigger than the other connector,

use some deoxit on the sealing ring of the sensor,
and the new connectors

Get an IR gun so you can test the temps in real time

Wisconsin Joe

Infinite resistance (open circuit) is cold.

No resistance is hot.

A "quick and dirty" check is to unplug the larger wire and ground it out. That should peg the needle at full hot.

bureau13

To be clear, you'd measure each pin vs. ground, correct? Not across the pins.

brealytrent

Which sensor where? Where's the one for the hot coolant lamp?

While the actual gauge appears to read smoothly and doesn't jump, it does seem to read low... However you need to take the car on a long drive on a hot day with the A/C on to get the auxiliary electric fan to kick on.

Mrmerlin

please reread what I posted,
you will find only one sensor on the water bridge that has two different sized female connectors.
This is the sensor for the gauge and lamp

brealytrent

Ah, okay. I was confusing the other temp sensor with the one you were referring to, yes. I'll look at that connection tomorrow.

brealytrent

looked at the temp sending unit and you were correct: the connectors were a little rough and the insulation had rubbed off of them and allowed them to possibly touch. After replacing the connectors and taking for a short drive I don't see any hot coolant light flicking on, but a longer drive may be needed to see if this did indeed solve the problem. The temp gauge seems to read pretty much the same as before, maybe marginally higher now.

Mrmerlin

I would replace the temp sensor and the connectors.
NOTE cut the wires back to fresh wire,
solder them on,
use heatshrink