Temperture gauge is pegged on high

[Seventy Two]

I had to check outfor a few days to prep and attend my sons wedding.
Today i tested as per recent suggestions and got thr following readings in ohms:

Negative post on gauge: infinate
Positive post on gauge: infinate
Negative on circuir board: 185
Positive on circuit board: 78.6

All thoughts/suggestions are welcome.

[Seventy Two]

Originally Posted by Seventy Two

I had to check outfor a few days to prep and attend my sons wedding.
Today i tested as per recent suggestions and got thr following readings in ohms:

Negative post on gauge: infinate
Positive post on gauge: infinate
Negative on circuir board: 185
Positive on circuit board: 78.6

All thoughts/suggestions are welcome.

All readings were to ground

[dmjlambert]

Originally Posted by Seventy Two

Well the results are in! Trying measure the ohms on just the resistor, I got nothing at all.

Did you fix this? I think that is the next thing to try.

[Seventy Two]

Originally Posted by dmjlambert

Did you fix this? I think that is the next thing to try.

I will see if i can pick up a resistor and give it a try.

[Seventy Two]

Re Set: This is not a cold case just yet.
I went back through this thread and decided to divide and conquer the 20DG lead from the sensor back to the dash.
I separated the wiring harness at the bulk head and tested the 20DG from the sensor end to the engine side of the bulk head. I got 0.7 ohms and neither end grounded out. Perfect!
Then I checked the 20DG connector on the inner side (cab) of the bulk head connector to ground and got 134.8 ohms! The 20DG wire inside the cab is virtually grounded. But to where? According to the standard wiring diagrams the 20DG wire from the bulk head should go directly to the ignition switch and then onto the gauge cluster.
I will track this down next but until then, can anyone clarify how the wiring should look at the ignition switch? I find the wiring diagrams a little vague in this regard. A picture is worth a thousand words right.

[Seventy Two]

Oops! to clarify, I do not have a 20DG wire going to my ignition switch. I will figure out where it is going (incorrectly) though.

[dmjlambert]

The dark green wire goes to the ignition switch only on trucks with warning lights instead of gauges. That is to ground the sender during engine start and give a bulb check like you get for the brake light. You don't have that connection.

I don't think you need to worry about this additional wire testing. You already took care of it well enough on post 64 on the previous page of this thread.

[Seventy Two]

Originally Posted by dmjlambert

The dark green wire goes to the ignition switch only on trucks with warning lights instead of gauges. That is to ground the sender during engine start and give a bulb check like you get for the brake light. You don't have that connection.

I don't think you need to worry about this additional wire testing. You already took care of it well enough on post 64 on the previous page of this thread.

Thanks for clarifying the wiring diagram. This makes sense as the 20DG wire appears to go directly to the gauge cluster plug #6 connector.
Now I'm stumped again.

[tdangle]

so to simplify, the wire going from the sending unit to the connector at the gauge cluster has continuity and is not shorted to ground, with the wire disconnected at the sending unit and unplugged at the gauges, Correct? If that is the case the problem is in the gauge cluster.

[dmjlambert]

The problem may be that vertically mounted resistor on the back of the temp gauge. Seventy Two ohm'd it and found it has no reading. I don't know what the reading those blue ones are supposed to have. My factory gauge has a wire-wound-on-white-cardboard resistor that reads 75 ohms. I'm pretty confident that if the resistor is required for the gauge, it should have some resistance.

[Seventy Two]

Originally Posted by tdangle

so to simplify, the wire going from the sending unit to the connector at the gauge cluster has continuity and is not shorted to ground, with the wire disconnected at the sending unit and unplugged at the gauges, Correct? If that is the case the problem is in the gauge cluster.

Hi Terry, yes your recap of the continuity and not shorted to ground is exactly correct.

[Seventy Two]

Originally Posted by dmjlambert

The problem may be that vertically mounted resistor on the back of the temp gauge. Seventy Two ohm'd it and found it has no reading. I don't know what the reading those blue ones are supposed to have. My factory gauge has a wire-wound-on-white-cardboard resistor that reads 75 ohms. I'm pretty confident that if the resistor is required for the gauge, it should have some resistance.

I agree, and Terry also suggests it may be in the gauge cluster, so why not the resistor as it seems way off from other reading that have been indicated in the forum.

Since I have the entire gauge cluster out of the truck i will pull the resistor and head down to radio shack and see if I can pick up a 60 and 90 ohm resistor to try out. Any other suggestions while I am there?

[dmjlambert]

I don't think they'll have anything similar enough. I suggest just PM TBONE1964 and see if he has one laying around that he can stick in an envelope and mail you.

[Seventy Two]

Originally Posted by dmjlambert

I don't think they'll have anything similar enough. I suggest just PM TBONE1964 and see if he has one laying around that he can stick in an envelope and mail you.

Thanks for the tip. I will follow up with TBONE. In the mean time I did pick up a 62 and an 82 ohm resister. On my tester they read as 61.3 and 81.7 ohms respectively. Until I can source an original, I thought I would give them a shot. If nothing else, it may be a hint that I'm going in the right direction, or not.

[Seventy Two]

Originally Posted by Seventy Two

I will follow up with TBONE.

I hope I got the right TBONE. All I found listed was T-BONE and T BONE.

[dmjlambert]

TBONE1964
Here is his profile
http://67-72chevytrucks.com/vboard/member.php?u=119465

[Seventy Two]

Originally Posted by dmjlambert

TBONE1964
Here is his profile
http://67-72chevytrucks.com/vboard/member.php?u=119465

Posted via Mobile Device

[Seventy Two]

Thanks DMJ, BTW, check this out. I just took it for a spin and snapped this photo. I'm stoked.

[dmjlambert]

Good deal, so that was it.

[tdangle]

Yes inquiring minds want to know. My guess is the wire from the sending unit shorted on a sharp edge somewhere and in messing with it cleared it.

[Seventy Two]

DMJ and Terry, I'm convinced it was the resistor based on the wiring harness being in generally unmolested form. Also, before taking it for a spin, I was able to peg the temp needle hot AND Cold with the key on and the sender wire grounded and disconnected.

Of the two resistors I picked up for $1.00 each, I chose the 62 ohm resistor simply based on the popular info in this thread. Probably one of the cheapest but challenging fixes I've had to deal with in a while.

BTW, for what its worth, after installing the resistor into the cluster, the resistance across the (vertical) posts dropped to 45.5 ohms WITHOUT being connected to the cluster plug. After hooking up to the cluster plug the resistance dropped to 42.5 ohms.

I like were the gauge sits after the trucks been warmed up and I can actually tell when the thermostat opens up as it tends to drop the needle by about a needle width. Now it is a matter of time to see how it holds.

I hope to get a chance to help someone else on this site. It is great! I certainly appreciate everyone's input. Thanks to all who waded in on this one.