Troubleshooting Temp Sending Unit

[JGolden]

I'm obviously not very well versed in the electrical functions of cars so here I am again with another question.

When I picked up the 74 Dart, the oil pressure and temp gauges didn't work so I traced the wires to find that they had been cut short. I made the appropriate repairs but still don't have voltage to the temp sending unit wire.

So far, I've tested the ground of the unit itself, which is good. I've tested for voltage at the connector along the passenger side of the firewall, and the connection at the bulkhead and have nothing. It's too dark (and the lights are too hot) to test much further tonight so I figured I'd ask you guys for any tips on the next steps.

As far as I can tell, the only connection left is on the back of the gauge cluster. If that's dead where do I go from there?

Are there additional tests I should be doing?

 

[RedFish]

With the ignition switch on, a volt meter should show a pulsing/changing voltage of 2.X to 3.X that will average around 5 volts.
The resistance between the cotact on the sender and ground should be somewhere around 80 ohms when the engine is cold.
zero resistance or infinate resistance is a bad sender. A basic test light wont diagnose the supplied voltage. Hope this helps

 

[JGolden]

Well, I've concluded that the voltage limiter is bad.

I removed the gauge cluster, leaving the leads to the ammeter connected. I probably could have left the connector to the printed circuit board on but to give myself more room disconnected it and instead ran a jumper wire from pin #1 (or A) to the correlating pin on the board.

With a test light (the key turned to on) I found that I had power to one terminal of both the temp and fuel level gauges but not the other. There was power to two terminals of the voltage limiter but not the third. So I am assuming the voltage limiter is bad and will be purchasing one today.

@Redfish

As soon as I am able to find the correct fuse for my multimeter and have installed the new voltage limiter I will test for pulsing voltage and resistance. Thank you for the info, I never would have figured that out on my own.

 

[67Dart273]

If this is all you checked, you have not fully troubleshot the problem

Some things you need to consider:

Did you check for 12V power RIGHT AT the limiter?

(The connector pin/ PC board can give trouble. The pins break loose/ corrode, and the "connector" that the limiter fits into can give trouble)

Since BOTH gauges seem to have failed, it might not be probable, but in my own case, BOTH gauges were not making contact on the PC board. The connection nuts on the studs/ nuts of the gauges were loose/ corroded. Cleaning the board/ retightening the nuts fixed that

BUT ALSO the connector for the limiter is a problem. The springy fingers that the limiter goes into may not be making contact with the board traces. You may need to solder jumpers from the contact fingers to the board traces to make this contact.

I'm not saying an electronic limiter upgrade is not a good idea---but it may or may not fix the problem(s)

 

[JGolden]

All I know is that after installing the new limiter/regulator (whatever it is) I now have voltage at every pin but the temp sensor.:banghead:

I've removed and clean the circuit board from the cluster. I cleaned the posts of each gauge. When testing continuity of the gauges I got 0.00 on the ammeter, 11.6 on the temp gauge, and 11.9 on the fuel gauge.

 

[JGolden]

Does that mean I need a new gauge or should it show resistance??

 

[67Dart273]

I'm not familiar with "every circuit board." Is this yours?

http://premiumdashdecals.com/graphics/PCB Fullsize/67-77 A body Valiant NON rallye front.JPG

(from here

http://premiumdashdecals.com/dash_circuit_boards.htm#abody

Looks to me like the trace is a great big one, so if the gauge sensor does not show voltage, make these checks:

Does the STUD of the power side of the gauge have power? This is important to be sure the stud is making contact with the board.

Does the STUD of the sensor side of the gauge have power? From there, check continuity from the STUD of the gauge to the CONNECTOR PIN on the PC board

 

[JGolden]

Yeah that's the one. Since installing the new voltage limiter the gauge does have power to the power terminal. The sensor terminal has very low voltage. Like I said, I can't find the correct fuse for my multimeter so right now I'm stuck with a test light. On the sensor terminal, the light barely glows. I ran the car for a while and the gauge never moved.

The picture illustrates what I'm talking about:

http://i9.photobucket.com/albums/a84/lbpunk101/67-77AbodyValiantNONrallyefront.jpg

 

[67Dart273]

OK, just go right to the sender stud on the gauge --which will eliminate the rest of the wiring, and clip lead the sender stud to ground, then turn on the ignition and see if the gauge "heads for" the high end.

Double check that the limiter/ power stud of the gauge is for certain making good contact.

After you do that, there has to be no doubt about the gauge itself.

 

[RedFish]

Resistance across the gauge should be 20 ohms. Its common for a mechanical limiter to fail and send too much current through a gauge. If the bi-metal strip inside get cooked the gauge might still seem to work but will be nowhere near correct.

 

[JGolden]

Yeah.... neither the temp gauge or the fuel gauge work when grounded. Looks like I'll be on the hunt.

Thanks for being so patient and helpful!

Redfish - It's obvious now that the gauges or shot one way or another, but for future reference...

If the gauge reads less than 20 ohms, is that the fault of the gauge or the limiter?

 

[RedFish]

Yeah.... neither the temp gauge or the fuel gauge work when grounded. Looks like I'll be on the hunt.

Thanks for being so patient and helpful!

Redfish - It's obvious now that the gauges or shot one way or another, but for future reference...

If the gauge reads less than 20 ohms, is that the fault of the gauge or the limiter?

Lets hold off until tomottow morning. The 20 ohms I stated may or may not be correct for your gauges.

 

[67Dart273]

Yeah.... neither the temp gauge or the fuel gauge work when grounded. Looks like I'll be on the hunt.

Thanks for being so patient and helpful!

Redfish - It's obvious now that the gauges or shot one way or another, but for future reference...

If the gauge reads less than 20 ohms, is that the fault of the gauge or the limiter?

Don't jump to conclusions, first try this: (You may have a bad connection in the chain from ignition voltage, through the limiter/ connections to the gauges

clip one or both of the gauges to ground, and with ignition on AND THE CLUSTER GROUNDED, check the voltage into and out of the limiter

In the photo you posted, the far left terminal of the limiter is ground, the far right is 5v output, and middle is igniton input.

 

[RedFish]

I thought I had the factory spec for those gauges here somewhere but haven't found yet. It may be in the diagnosis proceedure of your factory service manual.
I do have a similar panel here though. i get 12 ohms on one gauge and 15 ohms on another.
The rallye gauges are all built at 20 ohms resistance but their needle swing is controlled by the geometry of the needle connection to the bi-metal
( each gauge is different ).
I'm going to speculate that the bimetal responder and needle goemetery is constant in your gauges and there built in resistance alters the needle responce. That's the only way they could keep the senders resistance the same for all models and change the size of the screen and needle length.