Water temperature gauge malfunctioning.
 

As of the past couple of months the in dash water temp gauge has been steadily reading hotter and hotter even though my mechanical water temp gauge still reads 190. It will climb to hot and read on the hot line and peg beyond that depending if I'm moving or sitting before the engine is even hot enough to think about the thermostat opening. Although the gauge does rise as coolant temp rises, it won't peg with it cold. I've had this issue in the past and a new temperature sender fixed it. I put a new one in and it acts the same if not worse. Does anybody have the resistance values for the temp senders? I tested my old one at about 300 ohms at room temperature and at 212 degrees it is about 30 ohms. I haven't tested the new one yet but will tomorrow. Maybe I got a bad one. The wiring is good, because the gauge works. The fuel, oil, alt and tachometer gauges all work as they should so I doubt the ICVR is causing any issues with this. Either the new sender is bad or my gauge is taking a dump? I don't suspect any ground issues as everything else is working like it always has.

Are you putting any tape or sealer on the sending unit when you install it? That can be a problem. Also realize where all our new parts come from, their reliability seems to be getting worse and worse.

Resistance values on page 100 here:

Fuel Tank Selector & Gauges - Gary's Garagemahal (the Bullnose bible)


The sender operates on the range of 10 ohms (gauge high) to 73 ohms (gauge low). 300 ohms is way out of limits. Was your test lead on the sender body, or the cylinder head or other ground? As Dave has mentioned, sealant or teflon tape on the sender threads could be the source of that extra resistance, depending where you were measuring.

I tested the old one out of the engine in my hand and in hot water on the stove. Yes, there was teflon tape but it had been on there for 7 years without any issues until recently.

The 10 ohm 73 ohm sounds like fuel gauge, not water temp? I did some poking around and found some other people with very similar resistance figures for the water temperature sender. Not sure what is right or wrong here. I did notice that if I have lights on or am using other accessory power all of the gauges will rise a bit, but they've always done that.

The coolant temp, fuel quantity, and oil pressure gauges are all the same inside electrically. The only difference is the label on the face of the gauge. They are all designed for a 10-73 ohm resistance range at the sender.

Well I found the issue. The firewall to engine block ground came loose. I had it bolted to the back of the aluminum head and the bolt loosened up. I relocated it to a better place and the gauge works fine now. My new sender said 145 ohms cold, didn't bother checking it hot but it's probably around 30 or so like my old sender that worked.

See it's all about the grounds!
Thanks for letting us know what the fix was. Did it fix all the gauges?
Dave ----

I bet it fixed more than just the guage. Your main ground from the battery goes to the engine, and also may have a strap around it to the frame. But this does nothing for the cab. The cab is mounted in rubber mounts, the engine/trans/rearend is mounted in rubber mounts. That ground to the cab from the engine block is very important.

Not to rain on the parade, but wouldn’t a loose ground prevent a good sender from showing any temp other than bottom of scale? When I was testing my IP I thought I grounded the temp hot wire to battery ground and the gauge pegged out. Am I recalling what I did and saw incorrectly?

It fixed the temp gauge. Also fixed my aftermarket volt gauge in my triple gauge set. It's kind of cheap and the gauge was swinging a lot. I figured it was just cheap and getting ready to die. Nice and steady now. The other gauges seemed unaffected by the loose ground.

Nice find, thanks for posting an update.

It's very interesting that the other gauges were unaffected, especially for oil pressure. That sender is also grounded via the engine. Maybe it had something to do with the temp sensor being connected to the intake manifold, but the oil pressure sender is connected directly to the block.

Maybe if the conductive path was marginal between the manifold and block, due to sealer and gaskets? The block would probably have been well grounded via the heavy ground cable from the battery, but the manifold may have been an infamous floating ground.

There may have even been some bleedthrough from the ignition system pulses reaching the intake manifold if there was a floating ground. My brain hurts trying to think how the gauge may have responded. Mostly just thinking out loud, trying to make some sense of it all. When the temp gauge (and voltmeter) were acting up, I wonder what would have happened if you turned off the ignition, but then went back to run without starting. This would still power the gauges but remove the ignition system pulses from the equation. My hunch is the gauges would have read fairly accurately.

Moot points, it's what I do...

I suppose the gaskets and sealer probably did cause a bit of a interference. I did notice the gauge read high to pegged both engine running and engine off. I think in the near future I will probably make up another ground from engine block to firewall and connect the battery negative to block optional ground to frame up. I used to have that hooked up but cut it when I pulled the engine out a couple years back.

Oh well, so much for my previous crackpot theory. :o Usually they live a little longer...

That's an excellent idea to add additional grounds, no matter what was actually happening. Glad you've got the problem sorted out.

That is a great idea to add more grounds. Sometimes is easier to add new ones that to try and troubleshoot and find the old corroded original grounds giving problems.

The explanation I remember seeing is yes, the gauges are all the same inside, but they operate SLOWLY as they are just little heaters in there.