temp gauge pegged hot

[blue72k10]

Ok I get it. Are you going to be around tomorrow

[pjmoreland]

Originally Posted by blue72k10

Ok I get it. Are you going to be around tomorrow

I should be around most of the day. I'm at a flat track race right now, waiting for it to start, so that's why my replies are lacking detail. I'll hook up a gauge on the bench later so I can send some photos.

[blue72k10]

Enjoy the night and I'll look at pictures later. Sorry to be such a problem child but thank you for your help in this.

[pjmoreland]

It's a challenging problem you've got here.

[pjmoreland]

Here are some more specific details on bench-testing the temperature gauge. It can either be attached to the metal mounting plate or not.

1) Make sure the horseshoe insulator is installed first onto the upper and side posts.

2) Attach the stock 85-Ohm resistor to the top and bottom posts. Put a washer under the resistor on the bottom post so the resistor doesn't get damaged when securing it with nuts

3) Connect the bottom post to the negative post of a 12V battery (black wire)

4) Connect the driver side post to the positive post of a 12V battery (red wire)

5) Connect a wire to the passenger side post (yellow wire). Connect the other end of that wire to a resistor in the range of 80-130 Ohms. The resistor from the back of a fuel gauge can be used. Connect another wire to the other end of the resistor (green wire). Connect the other end of that wire to the negative battery terminal.

The needle should move away from C. A 130-Ohm resistor should move it to around the second line. An 85-Ohm resistor should move it about twice as far as the second line (last photo below).

[pjmoreland]

I created a video that shows how to bench test the temperature gauge.

https://youtu.be/pK8lQ0DFHOo

[blue72k10]

Just watched the video. Very easy to follow I'll let you know how it goes later. Thank you again.

[blue72k10]

I think we're heading in the right direction. I tested all three gauges and found that two work as they should per test and one didn't pass. I bet you know what one failed, yes the one that was in the tach cluster failed. So I'm now going to install the original gauge from the original cluster into the tach cluster and try it all hooked up. I'll do the other tests you suggested looking at the bulkhead connector closely. I looked for a bench testing video prior to installing and couldn't find one to the detail of the one you just did. Thank you again for doing the video for all of us that struggle with these issues. I'll let you know the results as I move ahead.

[pjmoreland]

That sounds promising!

[54blackhornet]

PJ you are a Saint for the electricaly challenged ! Thanks for your support !

[pjmoreland]

Originally Posted by 54blackhornet

PJ you are a Saint for the electricaly challenged ! Thanks for your support !

It's a head scratcher foe sure. I'm not the one sweating away doing the actual work, thankfully

[blue72k10]

Well todays wrap up. Got the tach cluster all put back together and installed. Thankfully no leaks at oil pressure and the temp gauge now works. It just nudges the second line until the therm opens then drops down to first line. I've got a 195 stat in it so I thought it would go to the middle of range but right now any victory is a win in my book. Now to the other readings you asked for from battery to un fused at fuse panel running and stalled it changes about .8v on both. Didn't get to the bulkhead today so that's a Monday thing. I got wrapped up with finishing on a winning note today I did some things that I knew wouldn't cause me any issues. As 54blackhornet said you Mr. Pj are a real blessing to us that are electronically challenged. Have a great rest of your weekend.

[pjmoreland]

When there are multiple issues with a system, it makes it quite challenging to figure out what is going on. So far, you've had a bad sender and a bad gauge. I suspect there is something going on with your wiring as well, which would explain the difference in voltage between the alternator output and the Ign Unfused terminal on the fuel panel. That would be crazy if there were three things keeping your temperature gauge from working correctly.

I'm a little nervous about your second sender from LMC. I have no specific evidence to feel that way, but I just don't trust it since the first one was so messed up.

I've got a 195 thermostat in my truck, and the gauge usually hovers slightly to the right of the second line when warmed up. It will climb maybe the space between lines 1 & 2 again on a hot day. It never gets to the middle of the gauge. I've owned a few of these trucks over the years, and that's how all of them have been.

[blue72k10]

Finally got the truck back together after painting and what a pain that was. The temp gauge is as we left it but in your post you mentioned that you thought the sender maybe suspect and if that's correct what sender do you recommend that will have the correct ohm ratings? I haven't gotten to the bulkhead connector yet because I've been headlandscaper for our property. I did though come across some loose wires that I can't find where they would go. I can't seem to get the pics to load as the forum says they're to big in PDF form. Any ideas on how to make them smaller file size?

[pjmoreland]

I don't have a good reason for suspecting the second sender other than the fact that it is the same kind as the one that was bad. You measured the resistance, and it seemed reasonable although it is lower than two of my senders.

The sender I've got in my truck is ACDelco G1852. It's $15 on Amazon.

Sounds like your photo files are too big. You will have to reduce the size using something like Microsoft Paint, or maybe your phone has the ability to decrease the size.

[pjmoreland]

Where is your sender mounted in the engine? Is it installed in one of the heads, or is it in the intake manifold?

[blue72k10]

I'll order the GM part today. It's in the left head.

[blue72k10]

I'll work on resizing the pictures. Not very tech savoy though.

[pjmoreland]

If you're able to access the sender without burning yourself, it would be useful to know what the sender resistance is when your engine is warmed up. The wire needs to be unplugged from the sender when taking the measurement. Measure the resistance between the sender connector post and the sender body. Also, measure the resistance between the sender connector post and the negative terminal on your battery to see if you get the same result. This second measurement will confirm that you have a good ground connection.

[blue72k10]

I'll get that test done after I figure out how to get those pics uploaded.

[pjmoreland]

Here are your photos. I'm headed out for a bike ride right now. Maybe someone else will chime in on what the wires are. The wires in photo #4 appear to be for your starter (purple), temp sender (green) and brake safety switch (tan). I can't quite tell what that fourth wire is. It might be ignition power to your coil.

[blue72k10]

Results from recent test After warm up connections at sender peg and body 93 ohms Sender peg and neg side of battery 90 ohms

[blue72k10]

From the graph you sent earlier it appears that the sender is within specs .

[blue72k10]

The other thing is originally this truck was a stick and I'm not sure if those extra wires were for that set up.

[pjmoreland]

I think that bundle of wires you found might be for TCS. Personally, I think it would be a good idea to remove that harness from your truck. Here's a thread that has some photos and a schematic of the TCS system.
https://67-72chevytrucks.com/vboard/...d.php?t=808748

I reread all of our posts to refresh my memory. At this point, this is what you've got:

-A gauge that passed bench testing
-A sender that is the appropriate resistance when the engine is warmed up

An issue that needs to be tracked down is that you measured 13.8V on the driver side post of the gauge with the engine running, but you measured 14.6V at the alternator. You also measured 13.95V on the Ign Unfused terminal at the fuse panel. Here's the path the voltage takes to get from your alternator to the temperature gauge:

1) Red 12 gauge wire from alternator post to the firewall bulkhead external connector
2) Red 12 gauge wire from fire wall bulkhead internal connector to ignition switch
3) Pink 12 gauge wire from ignition switch to fuse panel Cluster Feed fuse and Ign Unfused terminal, when ignition switch is on
4) Pink 20 gauge wire from other side of Cluster Feed fuse to the connector that plugs into the back of the gauge cluster
5) A copper trace on the gauge cluster printed circuit to the temperature gauge

Somewhere along this path the voltage is dropping from 14.6V down to 13.8V. You have already found that it has dropped down to 13.95V at the Ign Unfused terminal on the fuse panel, so you could work your way back from that point toward the alternator. My suggestion would be to check the voltages on the red and pink wires on the back of your ignition switch while the engine is running. You should be able to insert your multimeter probe into the connector and get contact with the terminals on the ends of the wires.