The Stovebolt Site Forums The Shop Area The Electrical Bay 57 Electric Temp Gauge Rebuild Questions

57 3100 with the Superman gauge cluster...

It's a pain in the fanny to remove my instrument cluster. I have to disconnect everything in a very short space because none of the attached wires and tubes are of sufficient length to get a hand in. That said...my temp gauge is toast and needs to be redone.

Symptoms: No readings. Sits on zero with and without power. No readings after lengthy runs either. Good 12v from the ignition switch over to the temp gauge "C" side. "H" side has a good ground to the temp sender in the block.

So, if I am to tackle this myself (to save the $59-$75 that everyone seems to want for these), how do I go about it? I have a spare one that looks broke to me too. I can see that there are two studs for the H and C sides, connected to a couple of spools of micro-thin wire.

What test procedures should I use to determine if this wire is shot? What readings would I expect? There are soldered links that look a little fussy on the back of the gauge as well. To be honest, I am surprised it works at all.

Can I test the gauge out of the cluster if I ground the body? Or is that to be avoided?

Thanks in advance for the help,

Jim

those 'spools of wire' are the electro-magnetic coils that cause the pull on the needle, if one or both is internally shorted or open, the gauge is shot - start with a continuity check, no idea what the resistance of each coil would be, but some - you'll prolly find open [infinite resistance]

shouldn't have to ground the gauge to test it, a temp gauge is a meter that reads current flow of the circuit from the hot side to the variable ground of the sender, so you need to be able to put some variable resistance on the sender terminal and attach the ground from the battery to that - I think the only repair you could do is reattach any wire broken free from it's terminal post

DG might try rewinding the coils OTOH, the repro gauges are modern circuits that don't fry so easily, or you might find a used gauge that works

Bill

Thanks, Bill. Just what I needed.

Don't know why I didn't try to continuity check first. Of course, I did check and found 12.something volts across the two terminals. That 'should' indicate a good circuit, right?

Seems there's a fussy little foil layer between the back of the gauge and some insulators on the posts. Will have to make sure that's where it is supposed to be.

Out of curiosity, a bad sender would behave how? I read here that the test for the gauge was to ground the sending unit wire out to 'peg' the gauge. Is there a similar test for the sending unit?

And, finally, when the radiator core is buried, just, with water, can the sending unit be removed without draining down the coolant some?

Thanks again,

Jim

not sure about reading 12V across the terminals, should be like checking 2 points on the same wire maybe a reading from the sender terminal to ground would show 12V on a good gauge because that would be completing a circuit as if the sender were shorted

if grounding the sender wire moves the gauge that'd indicate the gauge is good, sender bad, if it doesn't move the gauge, the gauge is bad - you can check the resistance of the sender [post to threads] cold, then dunk it in boiling water for a minute or 2, pull it out and check again, or have wires attached to the sender that you can meter as the water heats .... also do not use any thread sealer or teflon on the sender when installed to isolate it from ground .... rad level needs to be below the level of the sender, but you'll get a bit of water out the sender hole when you pull it regardless just from water in the block

Bill

Got it. And you're right about reading across the terminals. Sometimes the grey matter has a short too...

Grounding the sender wire (on mine) does not move the gauge (ignition on or otherwise), so I likely have a bad gauge.

Thanks for the help.

Jim

First: Create a "service loop" with all the wires to the cluster so that you can get it out of the dash. This is a must. Next: 90% of all electrical troubles in trucks is the ground. To say you have good ground is not to say you do have good ground, so be advised. You have to have shiny metal touching shiny metal. It's hard to even check for good ground. Now to trouble shoot temp system and to repeat what Bill said. Take wire off sender and be sure it's not touching anything, turn ign on, sender should read at low end of gauge. Then touch this wire to a GOOD ground, gauge should read high. In other words sender is a resistor, high resistance= cold, low resistance= hot. If it passes this test, the gauge is probably good. As stated, to test sender you need hot water or maybe a heat gun (no flame). Connect sender center post to gauge and ground side of gauge to threads of sender with a clip. place in hot water covering only the threads. Place thermometer in there too. Should read same as thermometer or fairly close. These aren't laboratory quality items (gauge and sender) they are old and beat up. I have taken gauge and sender into my kitchen a few times. If you can heat hot water out at the truck, that works too.
Beware the ground!
I have tested items on the bench and touched a shiny metal area and didn't work, touched another area, it worked. Ground is tricky. Sometime you have to dig into the metal with the test probe. Voltage will even arc and make contact, ground usually does not. if you are saying you have good ground because of a visual you have at some bolted joint, that may be wrong. The only fool proof way to check is to set up a wire from the battery neg to the post of the unit you are testing or, on the bench use a power supply like a battery charger with a safety fuse inline. In my opinion. I like to know for sure before I throw away a part or continue to look elsewhere in the circuit. Bypassing ALL existing wiring, with new temporary wiring, pos and neg ,is the only way to troubleshoot electrical on an old truck. Especially gauge and sender items. Also a temp gauge can be tested with a fuel sender or some pots and rheostats. Wish I still had my old slot car controller.
You must buy a compatible sender, I believe that is a T5.