Temp Gage Not Working

[Richard R.]

Re: Temp Gage Not Working

I just installed a new temp gage,turned the key on and the needle started moving backwards to the 250* mark so I turned the key off. What do you think is causing this new problem? I am lost for solutions to a simple gage causing all this trouble.

[Patrick H.]

Re: Temp Gage Not Working

I just installed a new temp gage,turned the key on and the needle started moving backwards to the 250* mark so I turned the key off. What do you think is causing this new problem? I am lost for solutions to a simple gage causing all this trouble.

I think Tim may have had a clue for you.
I do not think it was the gauge since you proved it would go to zero and to 215 (which is better than your 110).

To follow him, I also suspect it is a ground (possible the gauge's ground in the instrument cluster) or the firewall connector.

[Jack H.]

Re: Temp Gage Not Working

I tested the resistance in the green wire and it read .000 so that should eliminate that problem.

Huh? If you're saying you checked the resistance of the green wire from the engine to the temp gauge with the temp gauge DISCONNECTED, then a reading of zero ohms is expected. You're essentially verifying the continuity of the wiring run from the temp gauge to the temp sender.

It you left the temp gauge connected and simply checked the resistance from the temp sender to ground to get zero ohms, something's rotten in Denmark! You'd expect to see the internal resistance of the gauge's pointer needle inductor and that's NOT zero ohms...

the new sending unit in the car reads .979 ohms and the old sending unit reads .879 ohms at 60* in the garage, higher then the 700. Is that a problem?

Once you're below 120F on the guage, the resistance of the temp sender is UNDEFINED. Search the archives and you'll find a prior post with the temp vs. resistance characteristics of the temp sender.

There's really not a lot of correlation between various 'room temperature' resistance readings of temp senders. Expect them to vary in the 400-1000 ohm range (depending on unit). This is what you're seeing/reporting.

Bottom line, a 60 ohm resistor substituted for your temp sender should produce a temp gauge reading of 230F (engine started, and charging system active). If it doesn't, then you're fighting problem(s) elsewhere.

A bad connection in the firewall sandwich plug to the fuse block, bad temp gauge, bad instrument cluster ground are among the normal 'suspects' and it's beyond me to trouble shoot further. Mr. Goodwrench is still in business and there are competent auto electric shops out there if this is over your head.

[Stephen L.]

Re: Temp Gage Not Working

Richard, how many poles are on the rear of the gauge?

I had a similiar problem on a 72 Monte Carlo (erratic readings)
I set up a bench test and found the problem to be a ground.
That is what I'd do with your old gauge to test out the setup.

Viewing the rear of the gauge:
If your gauge has 4 poles you must ground the bottom pole.
Left pole is from sender
Right pole is 12vdc
Top pole has no connection.

I found that the gauge mounting plate ground was inadequate causing the gauge to do funny things.

By doing this test you can eliminate the gauge and sender (if it works properly) and concentrate on the wiring and mounting. My system used a printed circuit across the cluster on the Monte Carlo. I don't know what yours has.....

I used a Decade Resistance box but you can use any resistors for the test.
160 ohms indicated 100 deg
72 ohms indicated 210 deg
48 ohms indicated 250 deg

These degree readings are identified on my gauge with 3 "tic" marks between the numbers.

Call me if you want to discuss... 864 963 2991

Steve

[Richard R.]

Re: Temp Gage Not Working

I found the problem ! I backed off the nuts that hold the insulators and connectors on the back of the temp gage and low and behold by grounding the sender wire gage went to full hot, removed the green wire from ground and returned to zero. then statred the car and gage go's to abount 190*! I didn't have the nuts over tight just a good snug but I guess that was too much. Unbelivable! Thanks for all youor help guys!

[Richard G.]

Re: Temp Gage Not Working

Getting a gauge from a parts house is a huge issue. One would think that it is listed for you Automobile it would read correctly, right? Just not the case. The parts house sending units vary all over the map. Sometimes owners have to bury a resister inside the wiring to get it correct. This is the reason I use GM sensors whenever available.

[Jack H.]

Re: Temp Gage Not Working

Several issues here...

First, you started talking about the problem of getting a good gauge. Factory orignals were calibrated a certain way and I've yet to see someone who does instrument restoration claim they have the factory correct/original tooling for calibration... Once the pointer needle is removed, you can kiss your factory original gauge calibratino bye bye!

Second, now you're talking about various temp senders. That's a different issue and it DOES take two to 'tango'. Both the gauge and the sender have to be in-cal/in-spec.

The sender issue is clouded by the fact that even AC made running changes to the temp sender's temperature vs. resistance profile over time. So, not all AC XYZ parts are alike!

Plus, temp senders offered by aftermarket suppliers are spec'd for MORE THAN ONE application. Go to the mfgr's web site and pull the spec's on his specific sender. I can almost guarantee you, you'll find they claim compliance to one or more GM specifications as well as certain Ford and Chrysler spec's.

How can you get there...have your cake and eat it too??? That's because there's a game of specsmanship afoot. I've yet to see one of these 3rd party suppliers qualify their claims by saying 'FULLY' conforms to this/that spec. So, what they're saying is that SOME part of the sender's overall temperature vs. resistance profile agrees with the cited specification(s)...

Bottom line, what was important then was for the temp reporting system to be accurate at the HIGH END of the dial (engine about to overheat and enter thermal runaway). Designers didn't really care about low scale or mid-scale temperature accuracy.

BUT, we're used to today's modern cars that ARE accurate across the full range of the temp guage. PLUS, we've generally just restored 'baby' and we're looking for the first sign of an engine 'opps' issue under normal, mid-scale, driving conditions which makes us prone to be hair triggered on the issue of temperature reporting accuracy.

Anyway you slice it, you can NOT 'modify' a temp sender by adding series or parallel resistance to force it to be accurate across the entire dynamic range of the guage IF both the gauge and the temp sender were in 'spec' (their relative specs) to start with. The more resistance 'tweaking' you do from nominal, the more distortion you're generating to the overall system...

With a small degree of t******* (maybe 10-20 ohms), you're probably OK. Go beyond that and you're rolling dice...

[Richard G.]

Re: Temp Gage Not Working

I am glad Richard found the issue with his gauge. Always happy to see a resolution to an issue posted so we all learn something.

Some of the new Fords have factory temperature gauges than are on a switch. One for up to temp and another for hot. They fool the driver by slow reaction times on the gauges. When I found this information I was flabbergasted. Then I remind myself its all about the money. However it can be done and done cheapest that is how it will be designed. Now the other sensor that feeds the computer is another story.

I was off base as I was thinking the gauge was reading incorrectly not no-functional.

All great point
Good luck
Rick