temp gauge pegged hot

[blue72k10]

Yes I did unplug before testing What would I get if I hadn't unplugged it?

[pjmoreland]

Originally Posted by blue72k10

Yes I did unplug before testing What would I get if I hadn't unplugged it?

You'd get an invalid measurement of the sender resistance because you'd be measuring the combined parallel resistance of the sender and the gauge.

[blue72k10]

Thank you so much for your help in this I'll hit ya back after I get it tested. You've been a real help.

[pjmoreland]

I think you're getting close...

[blue72k10]

Well the good news is the temp sender now does what it's supposed to do ohm wise but the gauge isn't doing the right thing. Now when the truck is running gets to 180 or higher by my thermo gun pointed at thermostat hsg the temp gauge is stuck on cold. After shutting it off and go to try it again in the run position the gauge moves up the scale to the first line. As I start it and run it the gauge drops to cold. Thinking I ruined the gauge I disconnect the connector ground it out and the gauge pegs hot. While the connector is off I check the sender reading and it's around 90-95. Ideas?

[pjmoreland]

The resistance of the sender being in the 90-95 Ohm range at 180° looks correct on the graph.

Just clarifying - does the gauge move up the scale to the first line with the key in the run position but the engine off? And by first line, do you mean the line on the very left side of the scale by the C that I labeled "1" in the image below? At 180°, the gauge should be at around line "2".

[blue72k10]

It just past the c between 1 & 2 with the key in the run position but truck not running. As soon as I start it the gauge drops past the 1 and below the C. It gets pretty close to the 2 but is shy of it. I put a 195 stat in it btw.

[pjmoreland]

Does your truck have the original wiring harness in it?

Did your truck have a gauge cluster with the temp, battery and oil pressure gauges on it originally?

[blue72k10]

Yes original harness Yes gauge cluster original Only thing that's new is the tach equipped gauge cluster I just re did.

[blue72k10]

btw it also has HEI

[pjmoreland]

It could be that the gauge is damaged. There are some additional things you could test though. Here are some things I'd start with. There are four posts on the back of the temperature gauge, as shown in the image below.

1) Use your ohm meter to confirm that you have zero Ohms between the bottom post and the metal of your dashboard. You should have a black ground wire coming from your gauge cluster connector that is clipped to your dashboard, by the way.

2) Use your meter in Volts setting to verify you have 12V on the driver side post with the key in the run position. It should measure around 14.5V with your engine running.

3) Remove the resistor from the back of the gauge, and measure the Ohms of the resistor by itself. It should be approximately 90 Ohms

4) With the resistor still removed, disconnect your sender wire from the sender and measure the resistance between the left and right posts on the gauge. This is measuring the winding inside the gauge that is responsible for pushing the needle toward H. I measured a stock gauge at 48 Ohms

5) With the resistor still removed and sender wire disconnected, measure the resistance between the top and driver side posts of the gauge. This is measuring the winding inside the gauge that is responsible for pushing the needle toward C. I measured a stock gauge at 62 Ohms.

Another option would be to throw your original gauge and resistor in there to see if it works.

By the way, I don't have any direct experience with reproduction gauges, but I have read that some of them don't use an external resistor like the originals did.

[blue72k10]

Would the gauge react they way it's supposed to when grounded out if it was damaged?

[blue72k10]

Let's talk about that black ground wire coming from the gauge cluster connector that's clipped to the dashboard. Are you speaking of the connector that contacts the printed circuit board? Could I connect a ground jumper cable to the silver connector at the bottom of gauge and then to a ground on the dash?

[pjmoreland]

I bought an original used gauge on eBay about a year ago that had perfect paint on it. The seller said he had tested it by connecting it to power and ground and that it had pegged to hot, so he deemed it a good gauge. When I received it, I bench-tested it by using an extra ceramic resistor I have to simulate the sender. It stayed at C just like yours when it should have moved to the second line. It also would peg if I connected the sender wire to ground. The gauge was non-functional. I repeated the test with another known-good gauge I had, and it did move to the second line. So to answer your question, yes the gauge could be damaged and still be able to peg hot when grounded. There are two sets of windings in the gauge that fight against each other. It may be that there is an internal short in the Hot winding that isn't preventing it from working to a certain degree, but its strength is reduced compared to the Cold winding.

I am, however, a bit confused by the behavior where the needle moves up when key is on with engine off, but then it drops when you start. The only difference I can think of in those two scenarios is that the voltage is a little higher when the engine is running because of the alternator output.

[pjmoreland]

Originally Posted by blue72k10

Let's talk about that black ground wire coming from the gauge cluster connector that's clipped to the dashboard. Are you speaking of the connector that contacts the printed circuit board? Could I connect a ground jumper cable to the silver connector at the bottom of gauge and then to a ground on the dash?

Adding another ground connection like that would help if this is a ground issue.

[pjmoreland]

While we are on the topic of ground connections, do you have grounding straps from your valve covers to the firewall?

[blue72k10]

Well you have given me some things to check out in the coming days. We've got new countertops coming tomorrow so garage day is cancelled. I'll check on it probably Thursday after doing the plumbing hook up Wednesday. Thanks again for all the advice. Stay well.

[blue72k10]

Yes there are 2 straps one to each cover

[pjmoreland]

Originally Posted by blue72k10

Yes there are 2 straps one to each cover

Perfect.

I hope your kitchen work goes smoothly. Let us know what you find when you get back in the garage.

[blue72k10]

Should I see a separate black wire from that connector attached directly to the cluster or is this an additional wire for a better ground that isn't factory? I have 3 different clusters and none have a black wire from that terminal to the metal cluster.

[pjmoreland]

Originally Posted by blue72k10

Should I see a separate black wire from that connector attached directly to the cluster or is this an additional wire for a better ground that isn't factory? I have 3 different clusters and none have a black wire from that terminal to the metal cluster.

There should be a lone black wire coming out of the back of the big connector that plugs into the back of the gauge cluster. This lone black wire should have a little clip on it and should be attached to the metal of your dash. By dash, I mean the big metal component that spans the full width of the cab, not the gauge cluster.

The back "tin" or metal of your gauge cluster should be grounded to the cluster flexible circuit by a couple of screws that pinch the flexible circuit to the tin. Then a copper trace inside the flexible circuit should connect from those screws to the lone wire mentioned above.

If you were to add a second wire from the temp gauge lower post to the metal dash, this would be a redundant ground in addition to what I described above. Redundant grounds are a good thing on these old crusty trucks.

[blue72k10]

You're an excellent instructor. I can understand that and when I get back to the shop I'll have a look. In the meantime enjoy the good weather.

[blue72k10]

I'm back to work on the truck. 1) 002 ohms yes to the black wire 2)12v key on driver side post 13.86 running same post 3)84 ohms on resistor by self 4) 51 ohms between lt and rt posts sender wire disconnected 5) 61 ohms between top and left terminal resistor and sender wire disconnected Does it make any difference how that horseshoe thing and resistor are positioned on the gauge and how the nuts are installed? I have them two different ways on two never disturbed clusters.

[pjmoreland]

Hmm. Well those measurements all look good.

The horseshoe piece acts as an insulator for the upper and side posts, so it installs onto those posts first. Here are some photos.

The only thing that has stood out to me so far is the differing behavior of the gauge with the engine off (key in run position) vs. engine running. Maybe we could figure out what the difference is between these two conditions if you were to measure the voltage of the driver, upper, and passenger posts of the gauge. Do this once with the engine off (key in run position), and again with the engine running. Take these measurements with the engine warmed up and with everything hooked up normally. Touch the negative lead of your voltmeter on the metal back of the gauge cluster while taking the six measurements.

[blue72k10]

Just to make sure I understand what you're asking. I need to hook everything back as normal under the dash at the gauge and hook the sender wire up. Take a voltage measurement at the upper, driver, and pass side of gauge with one probe grounded to cluster stone cold and warmed up, correct?