Temp gauge help
 

Hello,
The temp gauge in my truck wasn't working, it was pegged hot all the time. and I think I found the problem. The 90 ohm resistor listed in the picture is reading infinite resistance, or open loop. So basically, the gauge was not able to ground itself. I jumped the two terminals with a 1000 ohm resistor I had lying around. And it began working somewhat normally.

How important is it to find a 90 ohm resistor? Would a 100 or 200 ohm resistor do the job? I noticed in the picture that some models don't use a resistor in that spot. Thanks!

Re: Temp gauge help
 

Interesting question. They say a 270 to 330 ohm resistor will make the gauge read cooler so I would guess it's not terribly critical. However, the closer to 90 ohms the better. Seems like 100 ohms is so close you'd never know the difference.

Re: Temp gauge help
 

I believe you would need to match the resistance of the original part in order for the gauge to read accurately from one end of the scale to the other. To test, heat some coolant on a stove and measure with a candy thermometer and dip the sensor in it and see how the gauge reads. If the coolant is very hot, such as 240, you will want the gauge to be showing a hot reading. It may be easier to get a new gauge or pull the stock resistor from a used cluster. It could be a member here has a stack of junk clusters and can offer you one. I see the LMC catalog has the gauges for $30 but don't know if they come with the resistor.

Re: Temp gauge help
 

I need to add that the statement in the photo that states "the silver post is grounded to the cluster" is not always true. If the temp gauge had a retainer clip on it from the factory and it was removed, the temp gauge has now lost its ground to the cluster. Without the retainer, the silver post has nothing to connect it to the cluster tin. This is true with porcelain resistors only as far as I know. You can run a short ground wire from the silver post to the tin mounting screw and should have a good ground, if everything else works as it should.

Anytime someone ask about a temp gauge problem, I will ask if they have a fuel gauge problem as well or the other way around?

If you have the correct temp sending unit, meaning for the truck as built, the 90 ohm resistor on the back is the correct resistor for the gauge.

Also note that there are some manufactures of aftermarket gauges that require no resistor as it is built in to the gauge. Having a resistor on one of these temp gauges will make them read incorrect.

Hope this helps.

Re: Temp gauge help
 

Hi there, my fuel gauge was working properly. It was just that the temp gauge would peg all the time when power was applied, on or off the vehicle. I tested it on the work bench with a 9 volt battery. But anyway, I think I'm in buisness, I took apart an old delco radio and I found a 100 ohm resistor. I installed it in the gauge, and it appears to be working correctly. I'm gonna instal it in the truck and test it out. Thank you!

Re: Temp gauge help
 

Oh, and when I said I tested it..... when I apply power, from a 9v battery, the gauge reads cold. When I ground the sending unit wire to the gauge, the gauge pegs hot. Now I'm gonna install it in the truck to see how accurate it actually is. I'll report back.

Re: Temp gauge help
 

Here is my 2 cents worth. For the cost of a new repo gauge I would go this route. I build on a budget like everyone else but your time and frustration level is worth something too. LMC sells replacement gauges for a reasonable price. I would go this route unless you are retired and have nothing but time on your hands....

Re: Temp gauge help
 

Just wanted to update this thread for anyone that stumbles upon it. I put the gauge back into the truck with the 100 ohm resistor as shown above, and it works like a charm. When the engine is up to operating temp, the gauge reads just to the right of center.

Re: Temp gauge help
 

Truck and car shop sells new ones that fit like OG, but what you did is awesome.

Re: Temp gauge help
 

I'm afraid that I couldn't recommend this workaround. The resistor is a fixed value and the sending unit resistor is a graduated curve. What that means is that the resistor is linear and the sending unit is variable. This means that the gauge will read cooler across the band and might give a cooler reading than what is actually the value.
Having the correct sender and gauge resistor is the best way to go IMO.

Comparing the reading with a known good gauge and sender and the resistor on the gauge itself would give a better indication of the correct value for the gauge resistor.