And now my temperature gauge is not working

[Rich69shortfleet]

I've been chasing various electrical issues and thought I had them all resolved today. Everything is working again in the dash except now the temperature gauge has quit. This has become a whack-a-mole game.

I grounded the wire at the sender and the gauge pegs hot. So the gauge and wiring are presumed good. I have an aftermarket temp gauge also so I know the engine is getting warm. I actually want to remove the aftermarket gauge once and for all so I need to get the factory gauge working again.

With the engine warm I do get continuity through the sending unit, though with some resistance as you would expect since that's the way the units work.

If the sending unit is okay, the wiring is okay and the gauge is okay, what is there left to check? Not sure what part of the puzzle is likely to be the source of the problem since they all seem to be working when tested independently. Everything else in the dash works; lights, other gauges, etc. so it isn't a ground problem. Is there any way to test the gauge independent of the sending unit to make sure it can do more than full cold or full hot?

In the past when the truck was cold the gauge sat at the lowest point on the scale. Now the gauge is sitting at a lower point (off the scale) to the left of the scale unless it is grounded when it goes to the peg to the right past the end of the scale. This is an original factory gauge, the underdash harness is a good original gauge harness and the gauge has worked fine for years (other than reading low probably due to a modern sending unit, plan to get the supposedly good one from Lectric Limited soon).

[Mike (Asheville)]

I went through most everything after I bought my truck. The temp gauge moved up only very slightly if at all after warming the engine up from cold. It was clearly not reading correctly. Upon inspection, I found that one of the POs had installed an idiot light sender. (I know you have a sender that works with a gauge but the resistance can be materially different and needs to be matched to the gauge.)

In my case I needed a 3/8 NPT sender because one of the POs had installed newer cylinder heads (a common upgrade), but I could only find the matching sender in 1/2 NPT (for the original cylinder head).

Fortunately, there are a couple of members here who turn down the larger senders to fit.

I got mine from A1971Blazer and he did a great job and my gauge works perfectly.

If this sounds like your problem do a search and you'll get more detail.

[stsalvage]

this is how its suppose to look if you use a edalbrock performer Spread bore> like using the 1967 thermostat housing that goes straight and the hose i use that fits perfect number on hose is 3898556 TH/GM/AU

here is the pictures

[Rich69shortfleet]

I have to restate that the temp gauge used to work just fine prior to my work under the dash. The sending unit is in the head just like it has been all along (I have a 76 sb 400 in the truck now though). The engine has multiple grounding straps attached to the body and chassis. Yesterday when I was mucking about with it and was messing with the wire connection at the sending unit it worked for about 30 seconds and then quit again, went to full cold. The wiring harness on the engine was new about two years ago and my test grounding the wire made the gauge react so I'm at a loss as to what to suspect or replace. The sending unit was new when I pulled the engine and regasketed it about 5-6 years ago.

My best guess at this moment is that the gauge got fried or whacked out of calibration when I was working on the previous cluster issues and I had the temp sending wire grounded for a minute or two. BUT, if the gauge will still react to grounding, pegs full hot, it is still alive. I don't want to randomly replace parts if they aren't needed.

I wish I had a spare temp gauge to test with.

[VetteVet]

Originally Posted by Rich69shortfleet

I have to restate that the temp gauge used to work just fine prior to my work under the dash. The sending unit is in the head just like it has been all along (I have a 76 sb 400 in the truck now though). Yesterday when I was mucking about with it and was messing with the wire connection at the sending unit it worked for about 30 seconds and then quit again, went to full cold. The wiring harness on the engine was new about two years ago and my test grounding the wire made the gauge react so I'm at a loss as to what to suspect or replace. The sending unit was new when I pulled the engine and regasketed it about 5-6 years ago.

My best guess at this moment is that the gauge got fried or whacked out of calibration when I was working on the previous cluster issues and I had the temp sending wire grounded for a minute or two. BUT, if the gauge will still react to grounding, pegs full hot, it is still alive. I don't want to randomly replace parts if they aren't needed.

I wish I had a spare temp gauge to test with.

It sounds like you have a problem with the wire, because the gauge isn't getting a signal from the sender. Does the gauge read the same if the wire is off the sender and not touching the ground? Another cause of a no reading is when we use Teflon tape on the sender threads preventing it from grounding to the cylinder head. Did you get an ohms reading on the sender to the head?

Here is a picture of the gauge with some tips.

[Rich69shortfleet]

Using my multimeter I got continuity, with some resistance of course, between the connector on the sending unit and the ground wire at the radiator support, so yes, the sending unit is grounded to the engine, chassis and body.

The reading at the gauge is the same with the wire connected or not connected. But, as I said, grounding the wire pegs the gauge so the wire doesn't appear to be the issue. The wire has two connectors for different styles of sending unit connectors and with the wire connected to sending unit, the multimeter connected to the second lead and the other end of the multimeter grounded I get continuity. I guess I could check continuity between the wire connector at the gauge and ground to make sure a signal from the engine end of the wire is making it to the gauge. Maybe I knocked something loose at the fuse panel when I was working on other issues which creating more resistance thus making the gauge read nothing (as if it were full cold) but enough gets through to ped it to full hot with a stronger signal. I don't know, grasping at straws here. When the gauge pegs the needle flies to the right without any hesitation. A spare temp gauge would be able tell me a lot.

Does anyone know how accurate and reliable the reproduction temp gauges are?

[VetteVet]

Did you do a continuity check on the grounding terminal on the gauge and the cluster body?

[Rich69shortfleet]

Originally Posted by VetteVet

Did you do a continuity check on the grounding terminal on the gauge and the cluster body?

Not yet. Will do soon.

[Rich69shortfleet]

The ground is good at the gauge.

Since the gauge does react when shorted and the wiring gets the signal there I today concentrated on the sending unit. I found that with the truck completely cold there is absolutely no continuity across the sending unit. Not knowing if this is perhaps correct, I turned to a spare sending unit I have lying around and it reads some resistance but has continuity. I then checked the sending units in my two Chevelles and they too both have continuity but with some resistance across the sending unit.

Also, knowing that the sending unit that the truck needs and that the Chevelles use may be different and the two different sets of readings may be correct for each vehicle, I looked up the correct sending unit on Lectric Limited's site and found that both my truck and my Chevelles use the same sending unit, so, if my Chevelle sending units have some continuity at cold, the one in the truck should too. Assuming the sending unit in the truck may not have failed altogether (it had continuity the other day when hot) it apparently is beginning to fail and now simply is not providing the proper amount of resistance through the heat range to make the gauge function. At least that's my current working theory.

Perhaps someone out there with a currently properly functioning factory gauge can check the continuity on their sending unit when cold and post up if they see any continuity at all?

[racerop]

I seem to have the same problem although I
Haven’t dug into the issue as deep as you

I had a gauge that read fairly low when the
engine was warmed up and when I took off the
Wire and grounded it the gauge pegged hot.

I suspect the sending unit.

As of the last few times starting the engine to
warm up the gauge doesn’t come off of cold

It may have gone completely bad . I haven’t
checked continuity or resistance of sending
Unit

[Rich69shortfleet]

It just occurred to me I can test it by taking my spare sending unit, connecting it to the wire and grounding it and then blasting it with my heat gun to warm up the sending unit. I'll have to do that soon.

[dmjlambert]

davepl posted this chart on another thread
http://67-72chevytrucks.com/vboard/s...d.php?t=725468

I suggest heating a pot of coolant to about 240 and dunk the end of the sensor in, to check that the gauge reads near the high end. It will give you more accurate reading.
When my gauge was never coming off of cold it was because I had the wrong sender. I would imagine senders can fail.

[Rich69shortfleet]

Originally Posted by dmjlambert

davepl posted this chart on another thread
http://67-72chevytrucks.com/vboard/s...d.php?t=725468

I suggest heating a pot of coolant to about 240 and dunk the end of the sensor in, to check that the gauge reads near the high end. It will give you more accurate reading.
When my gauge was never coming off of cold it was because I had the wrong sender. I would imagine senders can fail.

That's a useful chart.

[Rich69shortfleet]

Looks like it is the sending unit. Just very odd that it went south just after I did other electrical work. Anyway, I took my spare sending unit, connected it to the sending unit wire, grounded it and turned on the key. Gauge read cold. Expected. I then warmed up the sending unit with my heat gun and heated it to about 180 degrees. They gauge then pegged. Hmm. I guess this is why this sending unit was sitting on the shelf as it was reading almost no resistance when at anything above 130 degrees. So I had to run an errand so I drove the truck and got it all warmed up. According to the aftermarket temp gauge it was warmed to 170-180 degrees. The factory gauge didn't budge. When checked with my multimeter it show almost no continuity at all, the resistance was almost off the chart. So, apparently that sending unit is bad as well.

Time to buy a new sending unit I guess.

[Rich69shortfleet]

Aaaaand it miraculously started working again today. It still reads low. Funny thing, the day it starts working again is the same day my new sending unit from Lectric Limited arrived.

[Rich69shortfleet]

So I got the new sending unit from Lectric Limited installed and I don't notice a lot of difference in performance from the one I had before (at least when it worked). The new one does seem to provide more needle movement once things start getting pretty warm but I expected more improvement. Which leads me to ask: what temperature does the middle of the stock gauge generally represent?

I still have my aftermarket gauge installed. When the temp was 180 the needle on the stock gauge was just about at the first tick mark to the right of the C. At about 200 (I had to idle in place for a while with the AC running to get this warm) the stock gauge needle is a bit to the right of the tick mark. Once I shut the truck down the temp rose to about 225 and the stock gauge got about 2/3rds of the way from the C and the middle. That's as hot as I could get things to go. Are these needle positions about equal to what you would expect for a stock gauge? I'd think it would have to go to 230 just to get halfway, then engine would be ruined before the gauge would ever reach the red H area which kind of defeats the purpose of the gauge having a "danger" zone. At least I know what various positions on the stock gauge really represent, but still.

Any feedback would be appreciated.