Temp Gauge Acting up 65

[Mike M.]

It started acting up.If I disconnect the sender it goes full left which is correct. Connected it will go full right on a cold engine. It I turn ignition switch on and off a few times it will usually start working proper before starting the car. The last thing if its full left and I start the car and take it for a drive once the car heats up it will start working on its own. I can't see where turning the key on and off should have any effect on this. Except that a voltage spike and circuit gets completed. Once it starts working it never fails untill the next time engine is cold. It's not completing the circuit for some reason. Could the engine getting hot have any effect on the sender completing the circuit? Also I think this sender has be replated could that effect it. I was thinking of taking it out and a wire wheel to some of the threads and see if that helps. Has anyone ever heard of anything like this. Connections under the dash seem good. Its 40 gegrees in the garage and I'm not sure what what the ohms reading should be. At around 70 it should be 600 to 700.
Any ideas. Thanks Mike

[Jack H.]

Re: Temp Gauge Acting up 65

The temperature vs. resistance profile of the sender is UNDEFINED 100F. At 180F, the T-R curve calls out allowable resistance to be between 100 and 118 ohms...

On the erratic nature of things, think this way: When you start the engine, you're doing a COUPLE of things...

(1) You're changing the level of the power rail from the battery's normal, at rest, state of charge (could be down into the +10V range for a discharged battery) up to the level of the voltage regulator + alternator charging circuit working to restore/maintain battery voltage (typically 13.8 VDC or higher, depending on battery charge state).

(2) You're creating mechanical vibration(s) which can affect the integrity of electrical interconnect junctions.

(3) You're creating underhood heat which can have an affect similar to #2.

Then, you throw in actual motion of the car when you go on the road and you get even more shock/vibration variables.

If the components of the electrical system are in good repair, #'s 2 and 3 shouldn't have any affect. If they do, it's time to start looking for the source(s) of the problem(s) and fixing them.

The temp gauge is actually a current meter reporting the flow of branch circuit current from the dash, through a variable resistor (the temp sender) to ground. To do its job properly it has to have a known supply voltage (that changes in these cars based on the operation of the charging circuit), solid ground reference(s) (there's one at the engine where the temp sender is installed and another at the instrument cluster where the gauge is physically installed) and the 'nest' (wiring interconnect) has to have integrity.

[Joe C.]

Re: Temp Gauge Acting up 65

Mike,

Agree with Jack's evaluation. Good place to start, is to eliminate wiring/ground problems.
Make up three jumper leads with alligator clips both ends. First short lead goes from BODY of sender to a good ground (neg batt terminal). Second jumper goes from cluster housing to a good ground (dash brace). Third, long jumper goes from sender lug to temp gauge sender input terminal.

Joe

[Mike M.]

Re: Temp Gauge Acting up 65

If the cluster ground was the problem I should have other gauges not working proper I would think. I'll try running a ground dirrect from the cluster to the sender when its pegged full right. See if that helps. Could the sender be failing and cause this. Mike

[Joe C.]

Re: Temp Gauge Acting up 65

Mike,

You are right about the other gauges, and some of the cluster lights, as well. I suggested the cluster ground jumper, to eliminate ANY possible wiring problem. You are astute enough to realize that, though. Your fuel gauge, and any light bulb with a single lead (12v) would not work with an open cluster ground.

Joe

[Mike M.]

Re: Temp Gauge Acting up 65

Tried the first 2 Joe. Doing the same full right. I need to get a smaller alligator clip to get one on the guage. Turning the key on and off (not starting the car) sometimes it works fine. When it starts to work I have to leave it off for a few minutes to get it to fail again. Now I can't get it to fail. It can't be fixed because I haven't done anything. Did this 2 weeks ago and was fine after I drove it and it started to work again. I wish it would just fail and not work this would be easier.

[Joe C.]

Re: Temp Gauge Acting up 65

Mike,

Never mind what the gauge is doing, let's first determine whether the wiring is at fault. Once you make up the jumper from the sender to the gauge "sender input" terminal, be sure to disconnect the dash harness wire (I cannot tell you the color code, from memory, but if you wish, I will look it up for you). Please report any differences in gauge behavior, if any. If there is no difference, then we'll take it to the next level.

Joe

[Mike M.]

Dash Harness Wire

I'm not sure what you mean by the dash harness wire. If you mean the power to the gauge it should be the purple the green goes to the sender. Mike

[Joe C.]

Re: Dash Harness Wire

Mike,

Okay. Assuming that you have BAT v. to the gauge (purple), then disconnect the green wire to the sender and jump it.

Joe

[Mike M.]

Thats what i thought you meant *NM*

[Mike M.]

Wiring Seems Good

All the wires check out unless I moved something and everything is making contact now. I'll be trying it over the next few days and see if it will fail to the right again. I can't get it to fail now. If it does I will change the sender. I don't want to pull the cluster to get to the temp gauge. Thanks for the help we will see what happens here. Mike