Help me troubleshoot my temp gauge please!

[fishmunger]

I'm getting a pretty consistent 209 ohms across the top and bottom 'resistor' terminals...

[fishmunger]

Thanks for all your help tonite guys, appreciate it. Just want to figure this thing out, it's pretty much the last thing keeping this old girl off the road!

[fishmunger]

Could the extra grounding wire that I added from the bottom of the resistor terminal (at the advice of several threads on this board) be a possible source of my problems? If I'm understanding this correctly, that end terminal is supposed to be grounded anyway right?

[dmjlambert]

Originally Posted by fishmunger

Also, stupid question, but should I be aware of anything before I disconnect my oil pressure sending unit from the back of the gauge? (It still works and I'd like to keep it that way!)

I used a small wrench from my ignition wrench collection that i used to disconnect the oil gauge tube from the back of the gauge. I didn't notice anything to be careful with. I have even moved my truck by starting it up and moving it to another place on the driveway, with the tube disconnected and instrument cluster removed. I let it dribble in a pan down by my feet. When you put it back, don't cross thread it and it's not necessary to over-tighten it, just snug it up.

[pjmoreland]

I get 89 Ohms between the top and bottom terminals on an extra gauge I have. I removed the resistor, and it still measures at 89 Ohms separated from the gauge. The resistor has a red rectangle on it.

[pjmoreland]

Do the left and right posts on your gauge have plastic insulators on them like in the photo to prevent them from touching the mounting plate? I noticed the top post doesn't have an insulator, but it would be bad if it is touching the plate.

[dmjlambert]

Look for something near 38 ohms between top and bottom resistor posts when the resistor is installed on the gauge, and 75 ohms across the resistor when the resistor is removed. If you're getting 209 ohms across the top and bottom resistor terminals when the resistor is installed, that is too much. Your resistor may be bad, or there is corrosion on the nuts or terminals.

[dmjlambert]

It seems like there are color codes for the exact resistance. Mine is white with wire wound around it, and it's 75 ohms. pjmoreland's red one is 89 ohms. That's the readings of the resistors with the resistor removed. There are probably slightly different based on what the gauge requires in order to read properly. Something in the 70's or 80's is probably fine.

[fishmunger]

Thank you both very much! First thing tmrw morning I am going to take it the rest of the way apart and clean the crap out of, check for those plastic insulators, take more readings, and report back here...

[pjmoreland]

I removed the resistor from my guage and applied 12v to the right post and ground to the housing. The needle stays where it is in this state. I can move the needle around by hand, and it just stays where I leave it. If I then ground the left post, the needle pegs hot and stays there even after I remove ground from the left post. It seems the needle's position is a balancing act between the resistor that is always pushing the needle toward cold and the green wire signal from the sender to the left post that pushes the needle toward hot. If the resistor has too high of a resistance, then it won't be effective at pushing the needle to the cold side.

[fishmunger]

OK, so I took the circuit board / resistor all apart and cleaned the crap out of everything. Looked exactly like the picture posted, everything in place, nothing obviously suspect. I still get the exact same ohm readings after cleaning everything.

My resistor has that red rectangle on it. I noticed that the resistor on the gas gauge looked the same. The ohms across the gas gauge resistor terminals is ~49. But across my temp resistor they are ~209.

209 sounds high with what everyone here has said. Does anyone know where I can buy a new resistor?

Should I try swapping the one from the gas gauge over and see if it makes the temp gauge work?

[fishmunger]

Also, with the resistor off, I get an open line if I try to measure resistance across the two terminals. It only makes a connection once the resistor is in place - and then 209 ohms.

I may be confused, but I thought that if I had too much resistance, my gauge would be stuck on cold, not hot? Have I got this backwards?

[pjmoreland]

The resistance of the sender and the resistance of the gauge resistor are supposed to fight against each other to position the gauge. If the gauge resistor isn't low enough, then it might not be able to push the gauge to the cold end. I like your idea of trying the gas gauge resistor on there temporarily.

[pjmoreland]

This place sells the resistors. Look on page 33 (search page 35)

https://www.scribd.com/fullscreen/57...jcbazlklxtunnw

[pjmoreland]

This thread says a resistor isn't used on some reproduction gauges. You have a reproduction gauge, right? Maybe you don't need the resistor at all.

http://67-72chevytrucks.com/vboard/s...gauge+resistor

[fishmunger]

Thanks pj, I will try and order a resistor, and in the meantime, try the gas gauge one.

The gauges are all stock, this whole truck is basically stock and mostly untampered with...

[fishmunger]

shoot, that resistor has been discontinued!

Anyone know anybody selling a used one?

[dmjlambert]

So you're getting 209 ohms across the resistor itself when it's not installed, and also 209 across the resistor terminals on the gauge when the resistor is installed? I was not expecting that, I thought there would be a difference.

[pjmoreland]

Originally Posted by dmjlambert

So you're getting 209 ohms across the resistor itself when it's not installed, and also 209 across the resistor terminals on the gauge when the resistor is installed? I was not expecting that, I thought there would be a difference.

I got the same measurement on mine installed and removed.

[dmjlambert]

I don't know what to think of the situation. I don't know why mine reads differently when it is removed. Perhaps I am mistaken. I did the measurements back in June.

[pjmoreland]

Was your gauge installed in the truck when you took the measurements? Mine was not.

[dmjlambert]

I think mine was installed. There's a lot of variables to think about...

[70StepsideC10]

I just bench tested a couple of gauges I have. One of them was installed in a cluster (not installed in a truck) and the other was just a spare gauge. The gauge installed in the cluster has a blue resistor, the spare one has a red resistor. For the blue resistor I measured 83 ohms when the resistor was removed from the gauge, and the same when it was installed on the gauge but the circuit board was not connected to the gauge. When I reconnected the circuit board to the left and right terminals, the resistor measured 61 ohms. The red resistor on the spare gauge measured 85 ohms when both removed from and installed on the gauge.

Below are resistance values (to ground) for the stock temp sender for these gauges. I found these values online and tested both my gauges and they were fairly close to these numbers. If you want to test your gauge rather than just pegging it hot or cold by grounding or opening the sender wire you can use these values. If you can get your hands on a potentiometer (variable resistor) you can connect that in place of the sender and vary the resistance to ground to test the gauge. I think you said in a previous post you confirmed the wire from the sender to the gauge was good. If this is the case, just disconnect the wire from the sender and connect it to one side of the potentiometer, and connect the centre terminal (adjustable wiper) of the potentiometer to a good ground. Vary the resistance of the potentiometer to the values below and see what your gauge shows, just use your ohm meter to check the resistance of the potentiometer as you adjust it. You will of course have to have the ignition on to power the gauge while doing this. If you can't get a potentiometer you could also just get resistors that are approximately these values and connect those between ground and the sender wire.

Left Line (Cold) - 350 ohms
Middle Line - 76 ohms
Right Line (Hot) - 51 ohms

[fishmunger]

Originally Posted by dmjlambert

So you're getting 209 ohms across the resistor itself when it's not installed, and also 209 across the resistor terminals on the gauge when the resistor is installed? I was not expecting that, I thought there would be a difference.

I couldn't get a reading across the resistor itself with it removed? Always came back as open line? Once on the dash bezel, I could get the 209 ohms...

Thankfully, a board member here anew4961 has dropped 3 NOS resistors in the mail for me to try! Looking forward to receiving those to see how they compare.

[fishmunger]

Originally Posted by pjmoreland

Was your gauge installed in the truck when you took the measurements? Mine was not.

The gauge is still in the bezel, but the bezel has been removed from the dash, wiring harness unplugged, and I too measurements with the circuit board in place and disconnected.

And thank you stepside, if the new (old) resistors don't make a change, I will try what you describe. Good idea