coolant temp high, voltage spec

Markus · Feb 5, 2018

After reconnecting a lot of wiring and installing some new sensors I find that my coolant gauge, an Equus aftermarket unit to give me accurate readings which were fine for ten years, are now wildly over the top. I had a reading of 280 when it should have been under 200 and I know it's the gauge/circuit because the upper hose was hot and I checked the coolant temp with an RF gun by letting some of it out at the sender port and it was only 170 and there was no air pocket there. The thermostat is new as well. There was no steam or pressure when I carefully loosened the radiator cap and the truck was only idling for a few minutes while getting ready to check timing and had barely gotten up to temp.

I checked the voltage to the sender wire and it's around 6 volts, so is it supposed to be 12? I know some sensors don't always get 12 volts but this may explain what's happening. I recall there being some kind of voltage regulator in the IP that may be related to this gauge working properly, is there a way to test it or is the 6 volt reading proof it needs replacing? I'm sure it's the original unit.

Markus · Feb 10, 2018

all good

OK, don't everybody fight to answer this, I get it. I missed the beer shipment so that's fair.

By the way I did check grounding and the signal wire for shorts but the symptoms were not indicative of a short, they just showed a gauge way out of spec but still slowly rising and falling with changing temps.

I found out two things, one is that I needed a new gauge kit but what I apparently really needed was the proprietary sender that came with it. I first tested my old sender and new one and they both worked the same giving a reading that was about 80 degrees too high. I wrote to Equus and they gave me troubleshooting steps but I had neglected to tell them I had used an aftermarket sender for ten years with their gauge since it was fine all that time and I had no memory of getting a special one with their kit.

Their troubleshooting steps started out with a test that was described opposite to what it should have been which I found out based on a contradictory third step so after some testing and thinking it through I realized at least the proper way to test a gauge to prove it's good. They told me to disconnect the wire at the sender and the gauge should peg to max. No, the gauge will just stay pegged at minimum and only pegs to max when grounded which is zero resistance as the gauge responds inversely to the resistance in the sender based on heat. I boiled my sender and at that temp its resistance is 35 ohms, when cooled to 65 degrees it's 350 ohms. When I used to disconnect the lead at the sender with the factory gauge connected it was the same, gauge at zero until I hooked it up again.

What's strange is that I bought an exact replacement gauge and finally installed their sender to get it working but my old one worked with an aftermarket sender for ten years until I reconnected all of my electrical and plumbing recently to start the truck again. I also found out too late I could have bought just their sender but since my old gauge had worked fine with the regular type sender I didn't even think that was the problem. Oh well, it's working but I ended up spending $25 when I probably only needed the $11 sender they sell.

The aftermarket units have a base resistance of about 350 ohms at 70 degrees and theirs is around 2,200 which explains why they're required. It's clumsy but I set up the gauge with a battery connection and a potentiometer and marked where it sweeps from 35 ohms (pegged max) to 350 (pegged minimum) to verify gauge condition and the old one is actually good. So how did my old gauge work all those years with the regular type sender, the replacement is the exact model number? Who knows ... (glug glug)

ericbphoto · Feb 10, 2018

Markus said:
OK, don't everybody fight to answer this, I get it. I missed the beer shipment so that's fair.

By the way I did check grounding and the signal wire for shorts but the symptoms were not indicative of a short, they just showed a gauge way out of spec but still slowly rising and falling with changing temps.

I found out two things, one is that I needed a new gauge kit but what I apparently really needed was the proprietary sender that came with it. I first tested my old sender and new one and they both worked the same giving a reading that was about 80 degrees too high. I wrote to Equus and they gave me troubleshooting steps but I had neglected to tell them I had used an aftermarket sender for ten years with their gauge since it was fine all that time and I had no memory of getting a special one with their kit.

Their troubleshooting steps started out with a test that was described opposite to what it should have been which I found out based on a contradictory third step so after some testing and thinking it through I realized at least the proper way to test a gauge to prove it's good. They told me to disconnect the wire at the sender and the gauge should peg to max. No, the gauge will just stay pegged at minimum and only pegs to max when grounded which is zero resistance as the gauge responds inversely to the resistance in the sender based on heat. I boiled my sender and at that temp its resistance is 35 ohms, when cooled to 65 degrees it's 350 ohms. When I used to disconnect the lead at the sender with the factory gauge connected it was the same, gauge at zero until I hooked it up again.

What's strange is that I bought an exact replacement gauge and finally installed their sender to get it working but my old one worked with an aftermarket sender for ten years until I reconnected all of my electrical and plumbing recently to start the truck again. I also found out too late I could have bought just their sender but since my old gauge had worked fine with the regular type sender I didn't even think that was the problem. Oh well, it's working but I ended up spending $25 when I probably only needed the $11 sender they sell.

The aftermarket units have a base resistance of about 350 ohms at 70 degrees and theirs is around 2,200 which explains why they're required. It's clumsy but I set up the gauge with a battery connection and a potentiometer and marked where it sweeps from 35 ohms (pegged max) to 350 (pegged minimum) to verify gauge condition and the old one is actually good. So how did my old gauge work all those years with the regular type sender, the replacement is the exact model number? Who knows ... (glug glug)

Oh, my, oh my. THAT is confusing. :icon_confused:

So, do you still have a problem? Or did you get it fixed?

I would expect this to be a 12v circuit. I don't have the diagram in front of me and will assume you wired it correctly. But I'm really sure 12v is normally applied to the gauge and then a signal wire goes from there out to the sender. The body of the sender is grounded. So the gauge and sender are in series. As the resistance of the sender changes with temperature, that changes how much current flows through the gauge (technically a meter. But we normally call it a gauge) More current usually causes a higher gauge reading. If your voltage is low, I would expect that to give a lower reading because less voltage at the same resistance means less current through the gauge/meter. (volts=current X resistance or current =volts divided by resistance) So a dirty or loose connection somewhere would cause that.

The best thing you can do is make sure the wiring is correct and that all connections are clean and tight.

coolant temp high, voltage spec

Markus

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ericbphoto

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