Temp Gauge No-Go

[N233TX]

Well, somehow the car (1500, '75 model) being down for 6 weeks for brake work kayoed the temp gauge. No movement off the cold position. First order of business will be to disconnect the sender and drop the assembly in some boiling water, hoping to shock it out of slumber. Beyond that, I don't have any ideas.

Thought I would check with more knowledgeable people and see if failure on the instrument panel end is likely and if replacement is inevitable.

Gauge was super-accurate until now and inspection of the connection line shows no damage from owner/operator rants during brake work.

TIA Jim

 

[JPSmit]

It is pretty basic, so, yes, check temp. Another thought is that it may have got bumped and the needle is binding on the glass, keep us posted.

 

[dklawson]

Your first check does not need to involve removing the sender from the engine and can be done in place.

Disconnect the green/blue sending unit wire at the sending unit. With the ignition switch in the run position, hold the terminal on the end of the wire to a bare metal grounding spot on the engine and have a friend watch the gauge. If it goes to "H" (over as much as 30 seconds) then the gauge and voltage stabilizer work. The sender is bad.

What is the gas gauge doing? If it is also refusing to come of "E", then the problem is likely to be the voltage stabilizer or power to it.

 

[N233TX]

OK, above test showed no needle movement and gas gauge performs perfectly. Dropped the sender in near-boiling water and needle moved off peg but only maybe 1/8" closer to the 6 o'clock position than when driving yesterday. Normally the needle is at about 6:30 on the dial. The thermostat is new. Even one stuck open would move the needle more, don'tcha think? Seems like it's registering but reading low... Sender looked pretty nasty; think cleaning it would help? Doesn't make sense it would go bad allofasudden-like...

 

[dklawson]

I have to ask a stupid question after having posted my last message. This IS an electrical gauge isn't it? You are not talking about a capillary tube mechanical temperature gauge are you?

Assuming this is an electric gauge, if you saw no needle movement when you shorted the green/blue wire to ground, the problem is not the sending unit. Shorting that wire to ground allows the maximum current to flow through the gauge and simulates what a really, really, hot sending unit would see/do. No movement on the gauge when you bypass the sending unit means the gauge, voltage stabilizer, or wiring is bad.

After failing the sending unit bypass test, the gauge should not have moved at all in your boiling water test. To test the sender in boiling water it is necessary to leave the green/blue wire connected to the sender AND connect a ground wire between the sending unit body and the car's chassis ground. If you do not provide a ground wire between the sending unit body and the car's chassis, there is path to ground for the current (nothing to make the gauge work).

Again assuming this is an electrical gauge there are a couple of other tests you can do but they require more work. Remove the temperature gauge from the dash but leave the wires attached to it. There should be a solid green wire on one gauge terminal and the other terminal should have the green/blue sending unit wire on it. Make a short jumper wire and connect it between ground and the gauge terminal where the green/blue wire is. If you turn the ignition on and the gauge goes to hot, that says there is a break somewhere along the green/blue wire going to the sender.

If the jumper wire does not make the gauge read hot, that says there is either a problem with the green wire supplying power to the to the gauge or a problem with the gauge itself. The solid green wire comes from the " I " terminal on the voltage stabilizer. Since the gas gauge works, that says the problem is not in the voltage stabilizer. You can test the green wire on the temperature gauge by setting a multimeter to measure Ohms and connecting its leads between the green wire on the fuel gauge and the green wire on the temperature gauge. You should measure something close to zero Ohms. If the meter shows an open circuit, replace the green wire to the fuel gauge. If there is low resistance, clean all the connections and try again. If there is still no movement in the gauge when you short the jumper wire to ground, replace the gauge.

 

[N233TX]

Problem solved...apparently. I went through the drill of grounding the sending unit in the hot water and the needle moved more toward normal than before. Just curious, I pulled the ground wire off with the unit still submerged and the needle remained in place, receding as the water cooled. Took the car out for a good, hard run and after 20 minutes the needle crept back to a position just shy of the aforementioned normal position.

At this point I am going to leave well enough alone and postulate that maybe there was some slight interference on the mechanism behind the glass which was scraped off on the most recent run-up. Don'tcha kind of hate it when things "fix themselves?" Thanks to all who provided all the good trouble-shooting ideas

Jim