Faulty Temp Gauge Diagnosis - Sanity Check

[Clucas]

I've learned so much in this forum and am hoping to confirm what for some will be immediately obvious. My temp gauge stopped responding at the end of last season and I wanted to establish whether the problem was the sender, the wiring, the gauge, or any (or all) of the above.

I confirmed continuity between sender and gauge, between engine ground and sender and between engine ground and gauge.

Gauge, when cold and without power, reads approx 170 degrees (???) -- does this alone indicate a fault in the gauge? Next, with power on and sender wire disconnected from sender, the gauge stays at approx 170 degrees (does not peg low scale as I thought it would). Shorting the sender wire directly to ground pegs the gauge at full scale, and so does re-attaching the sender wire to the sender.

I'm moving ahead with replacing both the gauge and the sender -- am I missing anything here?

Thanks in advance for the sage advice (or slap upside the head, as warranted).

[JOHN COOKSON]

I'm moving ahead with replacing both the gauge and the sender -- am I missing anything here.
Thanks in advance for the sage advice (or slap upside the head, as warranted).

The gauge is sticking most likely due to corrosion. Time for a new one. It for sure will not get better with time.

TRUE GRIT

[Al Schober]

First, the reading with no power means nothing.
The temp sender is a variable resistor. Mine measures 750 ohm at 70F, and 75 ohm at 190F. So open circuit should peg low, short to block should peg high. You're good on grounding to the block (reads high) but not on the open circuit test (should peg low). This seems to indicate that the wiring is good.
First I'd make sure the gauge is wired properly. Gnd to ground (duh?), B to + 12V and A to the sender. Probably not your problem though, as reversing A and B will peg the gauge low.
Next check would be that you're getting 12V between Gnd and B. If not, fix it. Check the sender - 750 ohms cold, 75 ohms at 190F. Next, replace gauge.
Hope this helps.

[ndutton]

I agree with replacing both the sender and gauge. The sender is clearly shorted internally.

The gauge is likely kaput. I say likely because not all gauges go to zero when shut off. Most do but not all. Take a look at my tachometer in this thread. The needle remains at the last reading until the gauge is energized the next time. I'll say though that I would not trust your temp gauge and accurate temperature monitoring is important.

[Clucas]

Thanks for the feedback and for confirming my diagnosis. I'm going to proceed as planned with replacing the gauge and sender.

[marthur]

The temp senders can be a pain to remove. Ken gave me good advice about using a 6 point socket. I used a 1/2" drive 6 point with a 18" breaker bar. I used pentrating oil and heated the block while cooling the sender with ice (the brass expands and contracts more than the iron). It was still so hard to remove that I feared having to drill it out--but it moved in the end.

[JOHN COOKSON]

If the engine is running get it up to operating temp and with the engine running try breaking the sending unit loose.
The vibration and heat will help.
I had good luck with a closed end wrench and a hammer.

TRUE GRIT

[Clucas]

The temp senders can be a pain to remove. Ken gave me good advice about using a 6 point socket. I used a 1/2" drive 6 point with a 18" breaker bar. I used pentrating oil and heated the block while cooling the sender with ice (the brass expands and contracts more than the iron). It was still so hard to remove that I feared having to drill it out--but it moved in the end.

No problem removing using a closed 6-point wrench. Pleasantly surprised.

[ndutton]

No problem removing using a closed 6-point wrench

Those were once called box wrenches.

[edwardc]

A six-point socket and an 18" breaker bar and mine came right off.

[marthur]

You guys are lucky! Mine was seriously stuck. As in all my tricks and then feet on the side of the hull and strain until something was going to pop! Fortunately it was the sender and not my arms : )

[MikeInMaine]

switched to mechanical..

and this one starts at 100f, which is nice for my raw cooled a4.

http://www.speedwaymotors.com/Speedway-Mechanical-Water-Temperature-Gauge-2-1-16-Inch,65376.html