The Stovebolt Site Forums The Shop Area The Electrical Bay Temp gauge pegs out

I have a New elect. temp gauge, new wiring, worked great at the first, now it pegs out all the time. ??? Any Ideals? Thanks, Larry

Maybe finding a ground before the sender?

hot rod,

The electric gauges rely on a variable ground that is supplied through the sending unit. I don't know what kind of setup you have, but you might be able to isolate a gauge/wire problem from a sender problem by disconnecting the wire at the sender.

If you disconnect the wire at the sender with the key switched ON the gauge should peg either MTY or FULL. When you do this, don't let the wire touch ground!

The next part of the test requires you to touch the same wire to a good ground, like a engine component. This should make the gauge read the opposite of what it did on the first test.

If you got the gauge to swing from one extreme to another...HOT to COLD or vice versa, it probably means the wire and gauge are fine and it's the sending unit that has gone south.

This troubleshoot is similar to one for a fuel gauge. The operating voltage for the electric gauge has to be the same as the trucks voltage, and the sending unit resistance value must be matched to the gauge.

If you bought the gauge and sender as a package, this should be the case. If the wire going to the sender is chaffed or burned through, it might be going to ground. This would cause a false reading.

The location of the temperature sending unit must proper. The threaded portion of the gauge must not be insulated from the block with tape or pipe dope.

If the gauge worked at first, and doesn't now, one would think some component failed. Do some more detective work and see if a clue pops up.

This assumes the engine isn't actually heating up!

Stuart

Sometimes people put thread tape on the sender unit threads to seal it like other coolant related fittings. The tape does not always allow for a good ground. I have removed the tape and cleaned up the threads on both surfaces and fixed a similar problem.
Fred

Hello gang... Grant here. My 59 w/stock 235 and factory gauges does the same thing. Rarely will it work correctly, but often pegs hot... nope, it doesn't even burp an ounce of coolant, so it is definitely not hot. A professional repair shop in MN tested gauge unit, it's good. I tested wires for continuity, all good. I replaced the sender, and the replacement came with sealant on threads... could this be causing my irratic problem? Sounds like the same story as Hot Rod. let me know what you think??? Grant.

Grant,
From my experience not all of the replacement temp senders have the correct ohm rating that is compatable with the gauge. We went through about 5 of them before finding one that would work correctly. One of them acted like a tachometer, when you reved up the engine the gauge would peg, let off and it would go back down. Sometimes you can figure it out through trial & error, but it's frustrating. Remove the sealant and see if it helps.
Fred

rfs56trk, hello and thanks for the reply. Electrical is my weakest area, what ohm rating would be for the original unit/gauge? I have some great parts people at my call and they have found many needles in haystacks for me. Thanks. Grant.

Grant,
I wish I could answer that question. I'm not able to find it in any of my books. I did a search on this site under "temperature gauge" and found lots of postings, and it seems to be a universal problem. I'll do some more research. I have had better luck at Pick Your Part for this type item than through most parts suppliers.
Fred

I think [sometimes!] that the TF gauge is 30 ohms .... but that may be the fuel gauge

the correct sender should be available at NAPA etc - works for all the early trucks/engines, the one I got there for my 283 works fine with the original gauge

Bill

I've been working this same problem in a 49 GMC with a 54 Chevy 235 for the better part of a year. The electrical system has been converted to 12v and has the standard resistor/ballast in the dash/gauge circuit to keep the instruments operating at the original 6v. When the engine is started the temp gauge stays pegged to the cold side for about five minutes. When the 180 degree thermostat opens the gauge needle slowly climbs without hesitation to the max 220 degree position. I've done all the gauge/sender component tests and all indicate a good sender and good gauge component. Both 12v and 6v senders act exactly the same. With the guage indicating 220 degrees I've dropped a thermometer in the radiator and consistently gotten temps in the 180-185 range. Other than the temp gauge, I've never had any indication of overheated coolent. Lately I've been thinking about putting a rheostat between the sender and the gauge to vary the resistance and see if I can get the gauge temp indication to match the temp I'm reading on the therometer dipped in the radiator. Anyone tried this? If you've not tried it, what do you think?

Mike

Itcmikem, Your on top of it but still might need a little help. The variable resistance of the sensor much be matched to the gauge, but we already know this. If you got a variable resistor (0 to 500 or 0 to 1000 ohms) plus an ohmmeter and a themometer you've got it licked. Put your sensor in a pan of water but suspend it so the end with the wire lead doesn't get wet. Hook your ohmmeter to the sensor wire lug and the metal (brass?) base. Put it on the stove and build a fire under it. When the water temp. reaches say 180 degrees read your ohmmeter. What the heck, take an ohm reading every 10 degrees above 100, you'll see why below. Your done with this part. Clean up the kitchen before your wife catches you. At your truck, hook one end of your variable resistor to the wire from the gauge to the sensor. Ground the other side of the resistor. Turn your switch on. Vary the resistor until your gauge reads say 180. Turn key off and remove resistor from system. Read it's value with the ohmmeter. This value is what the gauge wants to see at 180 degrees. If the reading from the hot water test doesn't agree, then you need to find a sensor that has the right resistance for a this temperature. You can carry this test even further by using the variable resistor to give you such values as when the needle comes off the "cold" peg. Also all the values of temperature, 170, 180, 200, etc. Hope this helps.

Itcmikem, I don't think you can get away with using a ballast resistor to drop a 12v system to 6 volts for your gauges. It's tougher than that. The ones I think it would bother most are fuel, temperature, and maybe oil pressure. They make 12v to 6v inverters, I have one in mine. Or you might use a 6v Zener diode, not sure. Maybe someone else needs to jump in here and help.