Coolant Temp Gauge Help Sought

[Stepside 67]

Hi all. I am trying to get the coolant temp gauge to work on my '67 C10. I bought the full gauge cluster from Classic Industries and am struggling to get the completion of all instrumentation wrapped up. Absolutely no instructions came with the cluster I've called them a couple times and each time they say they'll check with their tech group or the supplier and call me back but have not done so.

So far, I've switched the wires in the cluster connector (going from idiot lights to gauges) as per instructions I found here. I have continuity from one end of the green wire to the other. I have tried two different senders. The needle seems to blip a tiny bit at start up, but won't register any temperature.

Please see the attached photo that I took of the rear side of the gauge. Do I need a separate ground wire to one of the studs? If yes, which one?





Any other advice as to what I need to do to get this monkey off my back will be VERY APPRECIATED.

Thanks!

[kool1972]

I went out and Looked at mine. Not sure why, put I have a jumper from the top terminal to the bottom terminal. See the pic.

[Stepside 67]

Thanks kool1972!

What exactly am I seeing in the two terminals/studs you have there? Looks like some kind of composite. Yes? Is there a wire for sure run between the two? I can't quite make it out.

Thinking more about this problem, does it make sense to this group that I should be able to put my multi tester on the two studs that have printed circuits to them and test resistance as a way to test whether or not the gauge is any good?

[Stepside 67]

Thanks kool1972!

What exactly am I seeing in the two terminals/studs you have there? Looks like some kind of composite. Yes? Is there a wire for sure run between the two? I can't quite make it out.

Thinking more about this problem, does it make sense to this group that I should be able to put my multi tester on the two studs that have printed circuits to them and test resistance as a way to test whether or not the gauge is any good?

[kool1972]

Ceramic resistor. I didn't know mine had it. But it does. Still learning about why its there. I was hoping you had one in the kit for the conversion. Perhaps someone else with experience on those can chime in.

[Steeveedee]

Drawing a blank, but someone here makes stuff for these truck's instrument panels. He'll chime in eventually; maybe he's out enjoying his truck instead of posting here.

[dmjlambert]

I think perhaps you received a cluster that is missing that vertically-mounted resistor like is on kool1972's, unless the gauge in the cluster does not require it.
Also, regarding the sender, I believe you must use a stock type sender for 67-72 or it will not work with the stock type gauge cluster. People like me who have a 3/8 NPT hole in the engine either drill the hole in the head and thread to 1/2 NPT to fit the original sender, put the sender in a 1/2 NPT hole in the intake manifold, or do like me and have the original sender turned down to 3/8 and re-threaded (a couple of board members offer that service). So the question is, are you using the stock type sender for our model year range? And it is a gauge sender, not the idiot light sender? Perhaps you can measure the resistance of it at cold and normal running temperatures and verify it is working.

[dmjlambert]

TBONE1964 is the forum member that usually chimes in on anything gauge related, he's a high octane member and active here.

[dmjlambert]

davepl posted the chart that shows the resistance the sender has at various temperatures.

See my post above, I added more info about checking the sender.

[Steeveedee]

Originally Posted by dmjlambert

TBONE1964 is the forum member that usually chimes in on anything gauge related, he's a high octane member and active here.

That's who I was thinking of! :thumbsup: I'll get this stuff sorted eventually.

[Stepside 67]

Thanks so much, guys. I'll wait in hopes that TBONE jumps in. In the meantime, here are a few pics of my project, now about complete. I need only to resolve the temp gauge issue to let it go.
In the meantime, here's a couple snaps of my project, just about complete:

[tdangle]

You are missing the 80 ohm resistor. Always frustrating to buy something that is not complete and/or no instructions Good Luck

[leddzepp]

If that is an aftermarket gauge I’m pretty sure the resistor is NOT required.

[demian5]

Truck and car shop sells the resistor.

[70 shorty]

I"m curious to see a side shot of the truck. Looks like upsized factory rallys, which I've never seen before. Care to share any info on them?

[Stepside 67]

Hi all!

UPDATE: In an archive search, I found this 2017 post by VetteVet:


There is an easy test to check the gauge.
Take the sending unit wire off and ground it against the block and the gauge should go full hot with the key on. If it doesn't move then the gauge isn't getting a ground from the wire or it isn't getting voltage from the gauge cluster.
It could also be a bad gauge.
__________________
VetteVet


So I did this tonight and sure enough, when I disconnect the lead from the sender and touch it to a good ground, the gauge goes to full HOT when I turn the key to the ON position. As well, when I touch the sender's brass casing to a ground, the needle goes to full HOT. When I reinstall the green lead to the sender and turn the key to ON, nothing happens.
I'm stumped. Can someone help???

BTW, for 70 Shorty - yes. Indeed the wheels are a 20" morph of the 1970's GM rally truck wheel. I've attached a side view for a better look. They're the Delray wheel by Detroit Steel Wheel. The center hubs are real GM service replacement parts. Hope you like 'em. I thought they were perfect for this project.

[dmjlambert]

Disconnect the wire from the sender on the engine. Connect an ohm meter between a clean metal ground point such as the alternator case, and the button terminal on the sender. Measure the ohms. When the engine is warm, mine measures about 135 ohms.

I don't understand the sentence "As well, when I touch the sender's brass casing to a ground, the needle goes to full HOT." The brass case of the sender is ground because it's screwed into the engine.

Post a picture of your sender.

[Stepside 67]

Originally Posted by dmjlambert

Disconnect the wire from the sender on the engine. Connect an ohm meter between a clean metal ground point such as the alternator case, and the button terminal on the sender. Measure the ohms. When the engine is warm, mine measures about 135 ohms.

I don't understand the sentence "As well, when I touch the sender's brass casing to a ground, the needle goes to full HOT." The brass case of the sender is ground because it's screwed into the engine.

Post a picture of your sender.

Will do the ohm check tomorrow. Thank you!

Here's a photo of the sender in place. It's Summit Racing part number HLY-534-2 (3/8" NPT thread) The sender is installed in the cylinder head, between cylinders 6 and 8.

[dmjlambert]

Don't bother with checking the ohms. It is the wrong sender. Here is the correct one.
https://www.amazon.com/gp/product/B000C9SM22/

There is no 3/8" NPT sender that works with the gauge, and you can only use the 1/2" NPT sender with the original part number for the truck.

If you want to put it in a 3/8" NPT hole you need to have it turned down and re-threaded by a machinist. Forum member A1971Blazer turned mine and threaded it, and others have reported forum member brian mac does the work, too. It is not expensive, and you just do it once.

[Stepside 67]

Originally Posted by dmjlambert

Don't bother with checking the ohms. It is the wrong sender. Here is the correct one.
https://www.amazon.com/gp/product/B000C9SM22/

There is no 3/8" NPT sender that works with the gauge, and you can only use the 1/2" NPT sender with the original part number for the truck.

If you want to put it in a 3/8" NPT hole you need to have it turned down and re-threaded by a machinist. Forum member A1971Blazer turned mine and threaded it, and others have reported forum member brian mac does the work, too. It is not expensive, and you just do it once.

Thanks DMJL.

Here's a (perhaps) crazy question: Since the standard/needed sender is 1/2" thread and the cylinder head (for me this is the case for all openings on the engine) is 3/8", can I use a 3/8' nipple and a 1/2" to 3/8" reducer from a plumbing store? I know I can make it fit. The question is whether or not the sender element will interface properly when not extended into the water jacket, but instead recessed a bit in a short nipple. I'd say the sender tip would be recessed approx 1/4".
Ultimately, my rigged assembly look like this:
1/2" sender -> 1/2" to 3/8" reducer -> 3/8' nipple -> cylinder head. Would save me time and expense of modifying a sender to fit the 3/8" opening.

Thoughts?

[dmjlambert]

That has been discussed here in other threads and I believe it didn't turn out to be practical. The forum members I mentioned will get a modified sender to you quickly and then you'll never have to deal with it again for this truck. Depending on what you work out with the machinist, they can buy sender locally and modify it and mail it to you, or in my case I bought the sender and mailed it to A1971Blazer and he mailed it back. I would imagine the turnaround time will be about a week or week and a half to get up and running with the ultimate solution if you PM one of them.

[Stepside 67]

Originally Posted by dmjlambert

That has been discussed here in other threads and I believe it didn't turn out to be practical. The forum members I mentioned will get a modified sender to you quickly and then you'll never have to deal with it again for this truck. Depending on what you work out with the machinist, they can buy sender locally and modify it and mail it to you, or in my case I bought the sender and mailed it to A1971Blazer and he mailed it back. I would imagine the turnaround time will be about a week or week and a half to get up and running with the ultimate solution if you PM one of them.

Thanks again.

I did reach out to both forum members you mentioned as soon as I saw your earlier post. I'm hoping to hear back soon. I need this to get taken care of ASAP because it's the last item to do before I can call the truck complete.

Regards.