temp gauge pegged hot

[pjmoreland]

Almost. Warm up the engine. Then take one set of measurements with the engine off, but key in run position. Then take another set of measurements with the engine running.

[blue72k10]

Ok I'll get it done in morning after grocery shopping. Thanks again.

[pjmoreland]

I took those voltage measurements of the gauge posts on my truck for comparison to yours:

Post, Stalled, Running
PS, 8.58, 9.66
DS, 12.76, 14.35
Top, 6.90, 7.78

When the engine was stalled, the gauge was pointing at the second line. When the engine was running, the gauge moved about half the width of the needle to the right of the second line.

The ceramic resistor on the back of the gauge plus the C winding inside, and the sender plus the H winding inside the gauge make two voltage dividers that push the gauge in opposite directions. The C voltage divider stays basically the same all the time since it is using a resistor with fixed resistance. The H voltage divider increases the voltage applied to the H winding when the sender resistance drops. Hopefully we will see something odd in the voltage measurements on your gauge that explains the issue or at least points us in the direction of the issue.

[blue72k10]

Thanks for the info and measurements. After some errands I'll get back to it.

[blue72k10]

Here's the results. Pass side stalled 6.6 Run 8.3 Driver stalled 12.3 Run 13.8
Top stalled 6.6 Run 7.4 The major difference is on the pass side of the gauge.

[pjmoreland]

What is the voltage measured at the large terminal of your alternator with the engine running?

[pjmoreland]

Originally Posted by blue72k10

Here's the results. Pass side stalled 6.6 Run 8.3 Driver stalled 12.3 Run 13.8
Top stalled 6.6 Run 7.4 The major difference is on the pass side of the gauge.

Yes, I agree about the lower-than-expected voltage on the passenger side post. That terminal is connected to the sender via several connections and a couple of wires. I'm wondering if you've got some corrosion on one of those connections somewhere between the gauge and the sender. Another possibility would be a less than ideal connection between the sender body and ground. Here are some things you could do to see if either of these things are the case:

1) Run a temporary piece of wire all the way from the passenger side gauge post to the connector on the sender. Make sure it has good connections on both ends. Disconnect the original wire from the sender during this test. See if this makes the gauge start to work normally (warm up the engine).

2) If the above doesn't make any change, leave that wire connected still, and then add a second wire from the negative post on your battery to the sender body. See if this makes the gauge start to work normally.

I used the following website to calculate what the expected passenger side and top post voltages would be, assuming your sender was warmed up to 90 Ohms.
https://ohmslawcalculator.com/voltag...der-calculator
The passenger post voltage is significantly lower than expected at 6.6V instead of 8.0V stalled.

[blue72k10]

14.6 at the screw term on back of alt

[blue72k10]

Just to be sure in step 2 the wire from battery ground would go to lower term on gauge correct? It'll be a bit before I can do those but it will be sometime shortly after noon. Thanks again for your help I'd never been able to figure this out or even where to begin.

[pjmoreland]

Originally Posted by blue72k10

Just to be sure in step 2 the wire from battery ground would go to lower term on gauge correct? It'll be a bit before I can do those but it will be sometime shortly after noon. Thanks again for your help I'd never been able to figure this out or even where to begin.

What you described above would be another good test to do. What I meant was from the battery negative terminal to the brass body of the sender. Just wrap the end of the wire around the sender body as best as you can. Maybe clamp it on there with a pair of vise grips.

It's not good that you're measuring 14.6 at your alternator and 13.8 on the driver side post of the gauge. You're getting voltage drop somewhere in the wiring, or your cab isn't grounded adequately. It would be good to track down this issue first before running the other tests. What is the voltage at the Ign Unfused terminal on your fuse panel with the engine running? Do you get a higher reading if you touch the negative probe of your voltmeter to the negative terminal of the battery? You might have to extend it with some wire.

[blue72k10]

The results of this afternoon's testing. Scenario 1 No change Scenario 2 No change Measuring at unfused connection at fuse panel 13.95 then same connection only neg probe connected to neg side of battery 13.95 Then my idea of connecting gauge direct to neg term no change. I even reran all the measurements from earlier at gauge to make sure I didn't mess up no negligible difference. One thing weird did happen during this whole thing. When I came up from under the dash the gauge's needle did move up past C to first line stayed there for a very short bit then dropped back below the C. Now another thing. Remember that other sender in the right head that had to do with Trans or Spark advance I've had that thing disconnected during all of this. Now in all the years over 40 that I've owned this thing the cab has never been off but the box has been a couple times. Only accident happened to the box sides. I wish I had some hair left to pull out.

[pjmoreland]

Have you thought about swapping in your old temperature gauge just to see if it acts differently than the new one? I don't have any specific reason to suspect there is anything wrong with the new gauge, but you never know. If I had my hands on your new gauge, I would be inclined to bench test it. I'd hook 12V to the driver side post, ground to the bottom post, and I'd use an extra 90-Ohm resistor to simulate the sender by connecting one end of the resistor to 12V and the other end to the top post of the gauge. The needle should move to around the second line.

Back to your recent tests.

Since running the negative probe to the battery didn't increase the voltage measurement at the Ign Unfused terminal, I'd say the cab has a good ground. Sounds like you are getting a voltage drop somewhere between the alternator and the fuse panel. There could be corrosion in the bulkhead connector that plugs into the back of fuse panel on the firewall. To make debugging this easier, see if you are getting a similar voltage drop when the truck is off. Measure the battery voltage at the battery terminals and then measure the Ign Unfused terminal at the fuse panel again to see if you get a significantly lower voltage reading than at the battery.

If you do get a significantly lower reading, then the next task is to figure out where the voltage drop is coming from. Unscrew/unplug the bulkhead connector from the engine side of the firewall. Locate the 12 gauge red wire coming from headlight/battery/alternator harness, and measure the voltage on the blade terminal that's on the end of that red wire. Be careful not to short any of the blade terminals together with your voltmeter probe. Also, do a visual inspection of the terminals. Do they look corroded?

[pjmoreland]

By the way, which sender have you been using for all of these recent tests?

[pjmoreland]

Originally Posted by blue72k10

One thing weird did happen during this whole thing. When I came up from under the dash the gauge's needle did move up past C to first line stayed there for a very short bit then dropped back below the C.

Sounds like maybe you have a loose connection somewhere! I'd go on a wire & connector jiggling expedition to see if you can repeat that behavior.

[blue72k10]

I agree with trying another gauge. I have three clusters to choose from. I will bench test the next one before I tear the installed cluster out. Well that's it for today and another day will come tomorrow. Thank you again.

[blue72k10]

I'm using the 2nd new one that measured 536 ohms.

[pjmoreland]

Originally Posted by blue72k10

I'm using the 2nd new one that measured 536 ohms.

Ok, good.

[blue72k10]

Today's results Not looking real good Took all three clusters apart removing the temp gauges to do bench testing all with the same outcome none of the needles moved. How could I be so lucky to have three worn out/broken gauges. I'll recheck everything tomorrow just to be sure that all my connections where/are good. If they don't move tomorrow I guess I'll be ordering a new gauge. I find it hard to believe that all three are bad. One cluster had a burnt circuit board so that one could be suspect. The orig cluster I don't think the temp gauge ever worked from my owing it and the tach cluster that's going in I have no info if it worked or not. Any other thoughts?

[blue72k10]

I didn't get to do any looking at any of your other thoughts today as I was doing some painting in kitchen. I'll look at those in the am.

[pjmoreland]

Can you provide a detailed description of how you did the bench test? Photos would help.

[blue72k10]

I did it like you suggested on post 62 hooking 12v to driver side term 12v to 85-90 ohm resister hooked to top post and wire to ground on bottom post. I was using alligator clips for all my connections.

[blue72k10]

I'll post some pictures in the am

[pjmoreland]

You will need two 90-Ohm resistors for a bench rest. One is the stock one mounted to the top and bottom posts like normal. Then the second one is attached to the passenger post, with the other end connected to ground. Everything is hooked up like in the image below, except you have the second resistor taking the place of the sender.

[blue72k10]

I have the gauges out of the cluster and off the tin mounting plate. Do I need to put them back on the plate as per picture which is no problem. I misunderstood your directions from that post earlier. So to make sure hook everything up like the picture and adding a resistor to pass term grounding the other end of resistor and grounding the bottom term as well. Correct?

[pjmoreland]

Originally Posted by blue72k10

I have the gauges out of the cluster and off the tin mounting plate. Do I need to put them back on the plate as per picture which is no problem. I misunderstood your directions from that post earlier. So to make sure hook everything up like the picture and adding a resistor to pass term grounding the other end of resistor and grounding the bottom term as well. Correct?

You don't have to mount the gauge to the back plate. You just have to connect the bottom post and the second resistor to negative.