Wonky temp gauge

[eaglescout526]

So let’s get right to it. Been working on an 86 2.5L. Got everything squared away and what not. But this “overheating” issue may not actually exist. That being said the temp gauge is acting weird. So it’ll slowly climb up to temp like normal but once it hits 220 it just climbs after that and doesn’t stop. Now I’ve taken an IR temp gun to it to see temps and well I didn’t see anything that looked to be overheating. But I had the son of the owners truck do some things and he removed the t-stat only to find the oddest thing I’ve seen this gauge do. It got to 220 and stopped moving. But each time he revved the engine, the temp would climb with it but come back down.

 

I’ve cleaned every damn ground I can find and even added a new dash ground but no dice. It helped a little but not a lot. Now I ohm tested the temp sensor plug and got about 70ohms and did not like seeing that one but. But I’ve hit a brick wall.

Any thoughts?

[jdog]

How old is the coolant?

I've heard strange stories that the coolant gains electrical properties and could be screwing with you. 

Only other thing would be swap the temp sensor, see if it remains.  How's the gauge ohm out?

[eaglescout526]

26 minutes ago, jdog said:

How old is the coolant?

I've heard strange stories that the coolant gains electrical properties and could be screwing with you. 

Only other thing would be swap the temp sensor, see if it remains.  How's the gauge ohm out?

actually running water right now to make sure it cools properly. So no coolant yet. Had to flush it a couple times due to brown crap in the engine. That could be messing with it. But I’ve swapped temp sensors around. One that I have and a new one. New one I swear could be a switch as it wasn’t doing anything. Even after 5 minutes of running the gauge remained at 100*. Here’s the weird part. The gauge ohms out like it should. Unplug it and it goes to 100*. Ground it and it pegs to 260*. And I’ve tried this with two different gauges already. Swapped clusters too thinking the printed circuit was going bad. 
 

The only thing I can think of is someone messed with the wiring. It does have an aftermarket alarm system, original battery wires have been cut and replaced for some reason and an aftermarket stereo not that that should affect anything. 

[eaglescout526]

And I did check all ground ohms between the battery and engine firewall ground and all good. But once you get into the interior that’s when there’s resistance and I really don’t want to tear into this truck to make things right. 

[Ωhm]

Not sure on this. Disconnect the connector at the CTS and connect ohmmeter between CTS and battery ground. Make sure meter in NOT in AUTO range (select 10kΩ scale). Now if ohmmeter act like your gauge, suspect sending unit, otherwise suspect gauge or wiring.

[eaglescout526]

Simple enough Ωhm. I should be able to do that. Just need to get my alligator clips from home and try it tomorrow. I don't know why I didn’t think about that. I did resistance test on the sensor before start up and during and watched and compared the Ohms to what it should be which I was getting between 300-110 ohms and that translates to between 180* and 220* so it was reading like it should but not accurately because it is a Napa sensor. 
 

But this was tested at the engine ground strap not battery ground. 

[eaglescout526]

I should add that I hooked up the REM to this MJ and watched the CTS in the manifold get above the average range for temps. But that could’ve been before the T-stat could’ve opened up and it’s supposed to get above average to warm up the manifold. 

[zomeizter]

If the IR temp sensor shows normal operating temperature, more than likely there is no overheating issue. I went through what you describe except on a GM truck, turned out to be the resistor on the gauge causing the wonkyness. I made a resistor "cluster" in the signal wire with the correct resistance value to match my IR sensor reading, now the gauge is more accurate than ever. I would put my money on your sensor-to-gauge circuit being the culprit.

[eaglescout526]

Thats what I am thinking. Somewhere between that wire and the cluster is unnecessary resistance. Ill get it figured. What bothers me is how without the t-stat, the gauge reacts to the throttle. Almost makes me wonder if I have the plug on the cluster upside down but I doubt its that possible.

[89 MJ]

IIRC, it might be possible to put that connector in upside down.

 

I think having the thermostat removed might be part of the issue. 2.5s only have a mechanical fan, yes? If it does and if it has a clutch, I’d bet that is worn out and then the coolant is circulating too quickly for that worn out clutch on the fan to cool the water.  To me, that explains why it increases when revved, but will cool down later. 

[eaglescout526]

2 minutes ago, 89 MJ said:

IIRC, it might be possible to put that connector in upside down.

 

I think having the thermostat removed might be part of the issue. 2.5s only have a mechanical fan, yes? If it does and if it has a clutch, I’d bet that is worn out and then the coolant is circulating too quickly for that worn out clutch on the fan to cool the water.  To me, that explains why it increases when revved, but will cool down later. 

It might be, I do plan to double check.

 

As far as the t-stat goes, we did have one in. One was in while diagnosing and cleaning ground. Took it out on the suspicion that it might be bad. Swapped another in and still got the same results. The mech fan actually moves air like it should through the radiator which is good. But what I mean the gauge moves with the throttle I mean it moves like its a tachometer. hits 220 and stays like its should if it was actually operating at nominal temp. I sent you a PM on FBmessanger

[89 MJ]

2 minutes ago, eaglescout526 said:

It might be, I do plan to double check.

 

As far as the t-stat goes, we did have one in. One was in while diagnosing and cleaning ground. Took it out on the suspicion that it might be bad. Swapped another in and still got the same results. The mech fan actually moves air like it should through the radiator which is good. But what I mean the gauge moves with the throttle I mean it moves like its a tachometer. hits 220 and stays like its should if it was actually operating at nominal temp. I sent you a PM on FBmessanger

I just saw it. That is weird. 

[cruiser54]

There is a tool for testing the gauge. 

 

And there's this:

 

 

[cruiser54]

and here's some more stuff for you to save.

 

 

[eaglescout526]

That’s good stuff right there cruiser. Was it in the FSM? 

[cruiser54]

Yep. 

 

Easy to test what you've got now. 

 

Sender reading correctly ohms vs temp?

 

Correct sender? 

 

Cluster reading correctly ohms vs temp?

 

C100 connector have the resistance? 

 

You guys need to freely save and share the stuff I post up. It's not MY stuff. It's OUR stuff. 

[eaglescout526]

Should make it very easy. Owner hasn’t reported any issues since removing the T-stat. I don’t like it but oh well for the moment. I do have plans to make sure that gauge is correct. His engine is still spitting up brown crap that will need to be flushed out so I think that contributes to the problem along with the C100 probably being gummed up with that tar.

 

I verified that it is the correct sender and ohms match somewhat due to it being aftermarket. Only thing I can think of is the cluster plug is either in wrong or I tore off some wire insulation and it’s grounding out at the speedometer cause it is an 86.