Instrument cluster

[chazdo2]

I had to replace the printed circuit on the instrument panel on my 1971 Chevy. I would like to test the temp, battery and amp guage before re installing. Do you just hit them with 12 volts for a full scale read and 6 for mid scale? Don't want to ruin them if they are not broke. It appears my heating/cooling blower isn't working or the wipers. Fuses are all good and hot. Any easy way to test before pulling them out?

[noxqsz]

Chaz,

To test your gauges you need to duplicate the wiring found in the wiring diagram for your specific model truck. That typically requires a 12 volt power source, a ground, and an input signal from the appropriate sender. For example, to test the gas gauge, you need 12 volts, a ground, and a signal from a variable resistor, which is what is in your gas tank. The variable resistor should have roughly the same range as your gas tank sender. Same for your temp gauge, i.e. 12 volts, ground, and an input from the engine temp sender. If you have a trickle charger, that will work for the 12 volts and the ground, but you'll have to figure out what to use as the sender input, if you don't want to take the signal off the senders in your truck. What I have done is just use some jumper wires with alligator clips on the ends to facilitate the hookup. In any case, be very careful, because you definitely can ruin you gauge if you happen to get the wrong wire to the wrong post on the gauge. Don't even try it without carefully studying a wiring diagram and fully understanding it. Good luck, and remember to be careful!

[chazdo2]

Thanks How about another question. Whats the best way to test the wipers before I pull the wiper motor.

[ray_mcavoy]

Ditto what noxqsz said about testing the gauges.

For the fuel & temperature gauges, you can use resistors or a potentiometer (variable resistor) to simulate the sending units. Connect the test resistor between the gauge's sending unit terminal and ground. And of course hook up the +12V and ground connections to the gauge. Again, as noxqsz said, be extra careful to make sure you have everything connected to the correct terminals on the gauge.

On the fuel gauge, a 90Ω resistor should give a full reading. A 45Ω resistor should give a 1/2 tank reading. And a direct connection to ground (0Ω) should make the gauge read empty.

On the temperature gauge, anything around 250Ω or more should give a cold reading. Around 80 to 85Ω should give a mid-scale reading. And around 56 to 60Ω should show hot.

The "battery" gauge (ammeter) is a bit different. Do not put 12V across it's 2 terminals. Doing so will short out your power supply / battery charger and possibly damage the gauge & printed circuit. Instead, use an ohmmeter to check across the ammeter's 2 terminals. You should get a very low reading like 0.1 or 0.2Ω and might even show zero on some ohmmeters. Alternatively, you could (briefly) connect a small 1.5V battery (like a AA or AAA) across the ammeter's terminals. Doing so should make it peg full scale in one direction. Reversing the polarity of the connections should make it peg full scale in the opposite direction. Again, only keep it connected for a second or two (just long enough to watch the needle movement).

Originally Posted by chazdo2

How about another question. Whats the best way to test the wipers before I pull the wiper motor.

Here is a typical 2-speed wiper motor with the terminals labeled according to the GM service manuals:


Start by making sure you have power on the #2 "power" terminal (and #4 if your wiper has this terminal) with the key on. This is wired directly back to the "wipers" fuse.

Next, use a jumper wire to temporarily ground the #1 "high" terminal. That should make the motor run on high speed if the motor is okay. Similarly, grounding both the #1 "high" and #3 "low" terminals should make the motor run on low speed.

If that all checks out okay, unplug the wiper switch and repeat the test with the jumper wire(s) to ground at the wiper switch connector. I believe the #1 "high" terminal uses a light blue wire, and the #3 "low" uses a black wire to connect it to the switch. If the motor still runs okay with this test, the problem is most likely the switch. Or the switch is not grounded (the switch case is it's ground and needs to have good contact with the metal dash).

[VetteVet]

Here is the wiring diagram Ray points out and another that shows how to wire in an accessory pump in case the factory one fails.

You'll notice that 2 and 4 (C and D), are the same connector where the yellow wire comes from the wiper fuse. some units just run the yellow wire to no. 2 and a jumper to the washer pump.







Remember that the wiper motor has to be grounded to the cab in order for it's motor circuit to be complete.

[VetteVet]

Originally Posted by chazdo2

I had to replace the printed circuit on the instrument panel on my 1971 Chevy. I would like to test the temp, battery and amp guage before re installing. Do you just hit them with 12 volts for a full scale read and 6 for mid scale? Don't want to ruin them if they are not broke. It appears my heating/cooling blower isn't working or the wipers. Fuses are all good and hot. Any easy way to test before pulling them out?

Testing the gauges should be very easy because all you need to see is a reaction of the needle unless you want to check the gauges for accuracy.

1. to test the fuel gauge all you need is a 12 volt power source from the battery to the right hand terminal on the gauge as you are looking at the back of it and a ground from the battery to the gauge body. You should see the needle go past full to the 2 or 3 o*clock position.

2. To test the temperature gauge you only need to apply 12 volts to the left hand terminal as you are looking at the back of the gauge and apply ground from the battery to the right hand terminal and the gauge body. You should see the gauge needle go to the full hot position.

3. Do not apply any voltage to the ammeter since both terminals are positive and definitely do not apply 12 volts to it since you may smoke it. It is designed to measure a very small amount of current. The best thing to do is just measure the ohms across the terminals and you should get very near zero
ohms.
I would use as small a battery as I could find for these test and I will not be responsible for any damage you may incur as a result of using my advice.

[chazdo2]

Originally Posted by VetteVet

Testing the gauges should be very easy because all you need to see is a reaction of the needle unless you want to check the gauges for accuracy.

1. to test the fuel gauge all you need is a 12 volt power source from the battery to the right hand terminal on the gauge as you are looking at the back of it and a ground from the battery to the gauge body. You should see the needle go past full to the 2 or 3 o*clock position.

2. To test the temperature gauge you only need to apply 12 volts to the left hand terminal as you are looking at the back of the gauge and apply ground from the battery to the right hand terminal and the gauge body. You should see the gauge needle go to the full hot position.

3. Do not apply any voltage to the ammeter since both terminals are positive and definitely do not apply 12 volts to it since you may smoke it. It is designed to measure a very small amount of current. The best thing to do is just measure the ohms across the terminals and you should get very near zero
ohms.
I would use as small a battery as I could find for these test and I will not be responsible for any damage you may incur as a result of using my advice.

Attachment 1508864

I must sound like a broken record but THANKS AGAIN TO EVERYBODY!!!! You guys make this stuff easy enough for a 7th grade shop student. Oops if you had a shop class your old like me!!!!

[tdangle]

I belive ray_macvoy has it backwards for the fuel guage. The guage reads E with no ground to the guage. if you short it to ground it will read full. I know this because when my sending unit went out the guage read E with the wire disconnected and full when shorted to ground.

Good luck with your testing.

[VetteVet]

Originally Posted by tdangle

I belive ray_macvoy has it backwards for the fuel guage. The guage reads E with no ground to the guage. if you short it to ground it will read full. I know this because when my sending unit went out the guage read E with the wire disconnected and full when shorted to ground.

Good luck with your testing.

Where did you disconnect the ground side of the gauge? If you disconnected it from the sender it is possible that the sending unit wire was shorted to ground. If you disconnected it at the fuse panel then it might have read full +.


Ray and I have both been helping trouble shoot these gauges for several years and yours is the first that I have ever heard of that reacts this way. When you have no power to the gauge it will not read or move the needle and when there is power to the gauge and no ground it will read towards the 3 o*clock position. If you short the negative side of the gauge it will go to empty. The gauge body also has to be grounded.

This will help greatly.
http://67-72chevytrucks.com/vboard/s...d.php?t=640615

[ray_mcavoy]

Originally Posted by tdangle

I belive ray_macvoy has it backwards for the fuel guage. The guage reads E with no ground to the guage. if you short it to ground it will read full. I know this because when my sending unit went out the guage read E with the wire disconnected and full when shorted to ground.

Originally Posted by VetteVet

Ray and I have both been helping trouble shoot these gauges for several years and yours is the first that I have ever heard of that reacts this way.

Yes, I've never come across a GM fuel gauge that has reacted like that either.

Here is the basic circuit for the 0 - 90Ω GM fuel gauges (the earlier 0 - 30Ω gauges are wired the same, only the resistance values are different).


As you can see, shorting the sending unit connection to ground bypasses the "full" coil and causes all the current to go through the resistor on the back of the gauge and the "empty" coil. That pulls the needle all the way to E.

Now, there are some non-GM fuel gauges that work with sending units that have a higher resistance when empty than when full. Those would read backward compared to a GM gauge when the sending unit lead is disconnected or when it's grounded.