temp gauge issue

[ryknot]

So I drtive a 75 620 as  my dd and have for the last year. I cant seem to get my temp gauge to work properly. The truck does not run hot yet my gauge insists that it is. The temp sending unit is good ( well i have tried 3 different ones and the gauge  continues to read hot)  The wire that goes the the temp sending unit is in good shape. Is it commen for a temp gauge to go bad? Any thoughts?

[laotsu]

search "gauge voltage regulator".

[MicroMachinery]

The gauge CAN go bad.. it's an electrical part. They fail. Common? I wouldn't say so, but these are 40 year old vehicles we're talking about.

[ryknot]

search "gauge voltage regulator".

Great! Thanks man. Gives me a good starting point.

[datzenmike]

The 620 voltage regulator feeds the fuel and the temp gauges. It provides a regulated 8 (or so volts) so the gauges read accurately when the charge system is at 14.5 volts driving or 12.8 idling with lights, wiper and heater on. If it fails, both gauges will read high or not at all.

Both senders provide a path to ground so grounding the sender wire will cause a full hot or full gas tank reading. If the gas gauge reads normally and you have replaced the temp sender then either the temp gauge is faulty or the wire to the sender is somehow close to a ground reading. If you pull the temp sender wire off does it still read something? If yes then the gauge is getting a resistance to ground added to the normal sensor reading. If it does not read with the wire off then the gauge is at fault.

[ryknot]

The 620 voltage regulator feeds the fuel and the temp gauges. It provides a regulated 8 (or so volts) so the gauges read accurately when the charge system is at 14.5 volts driving or 12.8 idling with lights, wiper and heater on. If it fails, both gauges will read high or not at all.

 

Both senders provide a path to ground so grounding the sender wire will cause a full hot or full gas tank reading. If the gas gauge reads normally and you have replaced the temp sender then either the temp gauge is faulty or the wire to the sender is somehow close to a ground reading. If you pull the temp sender wire off does it still read something? If yes then the gauge is getting a resistance to ground added to the normal sensor reading. If it does not read with the wire off then the gauge is at fault.

Thanks Mike, makes sense.  The gas gauge has been giving me some issues as well, i thought the sending unit was giving me some hassles, reading beyond full for way to long. The temp gauge reads nothing when the wire is removed and pegs hot when grounded so Im figuring the the gauge or the regulator is tits . . your help is greatly appreciated.

[ggzilla]

> The temp gauge reads nothing when the wire is removed and pegs hot when grounded

 

That is exactly how it should work.

 

Now test the sender. It should read 100 ohms at 80 degrees, 20 ohms at 180 thermo.

 

The regulator may be OK and the gauge bad. Also if the optional 190 thermo has been fitted it should read above halfway on the gauge.

[datzenmike]

So I drtive a 75 620 as  my dd and have for the last year. I cant seem to get my temp gauge to work properly. The truck does not run hot yet my gauge insists that it is. The temp sending unit is good ( well i have tried 3 different ones and the gauge  continues to read hot)  The wire that goes the the temp sending unit is in good shape. Is it commen for a temp gauge to go bad? Any thoughts?

 

 

To be clear, if temp and gas are reading incorrectly it's likely the volt regulator that is common to both. On the 620 the regulator is located in the temp gauge. The points can be cleaned with very fine fine emery cloth.

 

[ryknot]

To be clear, if temp and gas are reading incorrectly it's likely the volt regulator that is common to both. On the 620 the regulator is located in the temp gauge. The points can be cleaned with very fine fine emery cloth.

 

 

Understood, ill give it a shot.

[LenRobertson]

When I started using my 510 this spring,  the gas gauge and heat gauge would both pin on high (temp took a bit until the engine got warm). I assumed the gauge voltage regulator was bad, but before I could tear the dash apart I saw the water pump was leaking. In the process of changing the water pump I noticed the wire to the temp sender was laying against the exhaust manifold. It and a second wire both were melted through the insulation to bare copper. I taped them up and rerouted that harness. My gauges work fine now. I doubt if my experience has anything to do with the original posters issue, but I thought I would throw it here in the thread for anyone in the future having gauge problems.

 

Len

[MikeRL411]

When I started using my 510 this spring,  the gas gauge and heat gauge would both pin on high (temp took a bit until the engine got warm). I assumed the gauge voltage regulator was bad, but before I could tear the dash apart I saw the water pump was leaking. In the process of changing the water pump I noticed the wire to the temp sender was laying against the exhaust manifold. It and a second wire both were melted through the insulation to bare copper. I taped them up and rerouted that harness. My gauges work fine now. I doubt if my experience has anything to do with the original posters issue, but I thought I would throw it here in the thread for anyone in the future having gauge problems.

 

Len

 

You have quoted the most common and obvious cause of most of the posts regarding this problem.  Please, may the gods, not the fools, direct the ignorant to this posting!  Now to listen to those fools dump on this reply.  With age comes wisdom.  But not tolerance for ignorance!

[ggzilla]

Are you saying that the most common cause of a temp gauge reading high is wires melted on the manifold?

 

The truck does not run hot yet my gauge insists that it is.

[datzenmike]

While that's not impossible, it's unlikely, as the temp sender and wires (at least the L series) is located away from the exhaust. If it was rerouted or modified by the owner it's more likely. Any grounding of the wire, if long enough produces a full scale reading. A bare wire rubbing a ground could produce a higher reading than normal.

 

Anyway the OP says the gas gauge also reads higher than expected so there is a commonality.

[Z-train]

I had a "double fault".Both gas & temp weren't reading.I replaced the temp guage and it rectified the temp gauge issue.But the fuel guage was still bad.Turns out it was a pinched wire were the loom exited the cab under the passenger seat.

[LenRobertson]

While that's not impossible, it's unlikely, as the temp sender and wires (at least the L series) is located away from the exhaust. If it was rerouted or modified by the owner it's more likely.

 

 

Absolutely guilty as charged! When I threw the L20b in the 510 I wasn't paying attention to the temp sender wire relative to the exhaust manifold. I ran it that way for several months last year with no problems. When I did the Spring Start-up this year I had no spark which turned out to be tarnished connections where the wires from the coil plug onto the dizzy matchbox. As I was fixing that problem I must have pushed the temp send wire over onto the manifold. I felt stupid when I realized what had happened, but I was so relieved not have to tear into the dash that I wasn't too upset. It isn't unknown for me to create a new problem while fixing a prior problem. :D

 

Len

[datzenmike]

Tell me about it. I used a popsicle stick to remove some Zeebart under spray from the rad fins on my '68. Worked fine before and leaked after. Spent a whole Sat morning finding another.