temp gauge pegged hot

[pjmoreland]

The 12 gauge pink wire in the TCS schematic that used to power an idle stop solenoid is a good wire to use for powering your HEI. The original coil power wire shouldn't be used because it is a resistive cloth-covered wire that won't provide high enough voltage for the HEI. If that original coil power wire is still there, it would be good to remove it from your truck, or at the very least, make sure it isn't shorting out on anything. Same thing with the yellow wire that runs from the cloth wire to the starter motor.

[blue72k10]

Results from todays work Removed the TCS harness and relay Took voltage readings at ign switch vehicle running red wire 14.3 pink 14.1 The old coil wire is gone so no issue there Haven't got to the yellow wire yet

[pjmoreland]

Hmm. A little bit of voltage drop in the ignition switch. I don't know what to conclude from that.

I know your gauge passed the bench test, but it would be nice to see if it is working correctly still now that it is mounted in the dash. There are a couple of tests that could be done:

1) Use the same resistor you used when doing the bench test to simulate the sender. To do this, first unplug the green wire from the sender. Then connect the passenger side post to your spare resistor, and connect the other end of that resistor to ground. With the key in the run position, the gauge should move to the position it moved to when you bench tested the gauge.

2) Leave the resistor connected from test 1. Try connecting the driver side post to the + terminal of the battery. It would be good to place an inline fuse on that wire. Be very careful not to short it on anything. See if the gauge moves to the bench test position now.

[blue72k10]

I'll do those tests tomorrow. Thanks again for your help and suggestions.

[pjmoreland]

I'm sorry about how this is dragging on so long. You've got a good gauge, and you've got a good sender. That leaves the wiring in the truck. The gauge needs three things as far as wiring goes:

1) Sender wire
2) Power
3) Ground

Those two tests should help determine which of the first two wiring items are responsible for the nonfunctional system.

[blue72k10]

No worries as I'm retired and really enjoy learning new things. I was never interested in electronics early as I had friends that loved it. I was into the mechanical side of things so it worked well doing trade outs. You are very systematic in your testing ruling out one system before going to another.

[blue72k10]

todays results Using post 32 for reference Test 1 gauge moved to 1 when key turned on Test 2 gauge moved to 1 when key turned on Not the results I had hoped for

[pjmoreland]

I wonder if there is something going on with the way the gauge is mounted in the cluster. I think it would be good to do essentially a bench test of the gauge in the cluster, but isolated from the truck wiring. With the above tests, you're almost all the way there. In addition to leaving the above tests 1 & 2 in place, unplug the dash harness connector from the back of your gauge cluster, and then add a ground connection to the bottom post of the gauge.

[blue72k10]

Ground connector already in place I went with the thought of too many grounds is a good thing I'll get back with you after I unplug the truck from cluster Thank you again

[blue72k10]

No change still at 1

[pjmoreland]

We need to isolate the gauge from the truck, one step at a time, until we find out what the difference is between right now and a fully isolated bench test. Maybe there's something going on with the printed circuit, which attaches to two of the posts on the gauge. Can you disconnect one of the printed circuit leads, and then repeat the test? If you get the same result as before (needle at 1), then disconnect the other printed circuit lead as well, and repeat the test.

[blue72k10]

Will do This cluster isn't the original I never saw it work Got it 20 yrs ago in a trade

[blue72k10]

Circuit board disconnected driver first then pass no change still on 1

[pjmoreland]

Time to pull the gauge out of the cluster, but leave it attached to the mounting plate. Then bench test. Also, take some photos that show how the resistor and horseshoe are attached to the gauge.

[blue72k10]

I'll get it done either later today or tomorrow. Have to take one of our fur babies to the vet. Just guessing but you're thinking the gauge maybe toast

[blue72k10]

Was the cluster to be plugged into truck harness I did it with it unplugged

[pjmoreland]

Originally Posted by blue72k10

Was the cluster to be plugged into truck harness I did it with it unplugged

Unplugged

[blue72k10]

good call I'll let ya know can I pm you the pictures again?

[pjmoreland]

Originally Posted by blue72k10

I'll get it done either later today or tomorrow. Have to take one of our fur babies to the vet. Just guessing but you're thinking the gauge maybe toast

I'm thinking something is odd about the way the gauge is attached to the mounting plate. Maybe one of the posts is shorted against the mounting plate or something along those lines. For example, if the top post is shorted, then it would supercharge the winding that pushes the needle in the direction of C.

[pjmoreland]

Measure the resistance between the top and bottom posts. If you get zero Ohms, then there is a short.

[pjmoreland]

Here are your latest photos. A resistance measurement of 1 Ohm between the top and bottom posts is a problem. You've got the components in the right places, but maybe the gauge was pushed to one side so the upper post was touching the metal mounting plate. Try putting the components back on. Leave all four nuts loose initially. Then shift the gauge left and right and up and down in the post holes. Try to find the center of the slop in both directions and then tighten down the nuts. Afterward, confirm you have more like 90 Ohms between the top and bottom posts. If you can get that, then bench test the gauge in the cluster.

[blue72k10]

Back to shop Let ya know how things turn out Would that explain the differences in voltage readings

[pjmoreland]

Originally Posted by blue72k10

Back to shop Let ya know how things turn out Would that explain the differences in voltage readings

Yes, I believe so. If the total resistance of the 12V supply circuit that goes from the alternator, through the ignition switch, and then to your gauge is somewhere around 3 Ohms (just a guess), then having a short on that top post would drop the voltage measured on the driver side post down to the 13.8V you measured. If you didn't have that short on the top post, then the voltage on the driver side post would be 14.2V. This is likely an oversimplification since this isn't the only active circuit in your truck.

[pjmoreland]

For anyone who has been following this thread, it appears that the top post of the gauge was contacting the metal mounting plate, as shown in the photo below where the resistance between the top and bottom posts was basically zero. After repositioning the gauge slightly in the mounting holes, and after performing more bench tests to confirm a still functional gauge, it is mounted back in the truck, and it's working.

To recap, the sender was bad (new from LMC), and the gauge was bad (confirmed with a bench test). The replacement gauge ended up with the top post shorted out on the mounting plate. It started working once the mounting position of the gauge was adjusted.

[blue72k10]

Yes it is fixed finally. After a big struggle we have finally made it to the desired end a working temp gauge. Believe me there is no way I would have been able to do this without pj's expert guidance and patience with me. You all have a wonderful site here filled with very knowledgeable people that are willing to share their expertise and knowledge with others. That's the only way this craft will be able to survive because you can't read something that's not written and what's in peoples minds thru their life experiences hasn't been put to words yet. That's the importance of forums like this and the people who choose to be apart of it. Thank you pj again for all your help and sharing your wisdom and if ever I can pass on what I've learned in my years on this rock I hope that I'm as gracious as pj was and is.