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Old 04-07-2018, 06:39 AM   #1
cjensen_68
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Fuel gauge wiring and voltages

The fuel gauge doesn't work in the 1970 truck. It was stuck at the 4 o'clock position. The wiring has been modified. I found the pink wire on the instrument connector (pin3) went to the sending unit via a wire that was installed by the previous owner (since there are dual tanks and the wire went to the switch). The tank sending unit seems good on the side I'm using since the resistance changed over the 70 miles that I drove it and the fuel level is about right for that. It's currently at 20ohms.
Both tan wires had been cut at some time. The tan wire (pin 4) was actually spliced to the tan wire that went to the proportioning valve which from what I can see is wrong (should be pin 2).

Which wire should be going to the sending unit (ping 3 or 4) and where should the other one connect? The diagram I found says the pink is Fuel Gauge Feed which to me is a 12V source.

From what I think, which I can be wrong I don't know; The pink (pin 3) went to the sending unit but should go to a 12V source. The tan (pin 4) went to the brake valve but should go to the sending unit. I isolated the wires and used alligator clips to get the pink to 12V source and tan to the sending unit. The gauge went to empty. I went under the truck to open the wire to the sending unit to test and when I popped back out there was smoke in the cab and melted the alligator clip (I believe the one to the sending unit). That got me thinking I was wrong and wanted to get some opinions from the forum members. I have a new gauge I can put in but don't want to chance ruining that one if the wiring's not right.

I'm wondering if the circuit board could be bad. I get 9.5V on the pink cable when I think that should be 12V. I have 12.5V on the tan cable. I'm confused.
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Old 04-07-2018, 01:17 PM   #2
VetteVet
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Re: Fuel gauge wiring and voltages

The pink wire is the power feed for the gauge cluster. It is on pin 3 as you stated. It originates at the key on side of the fuse panel and runs to pin 3 on the gauge cluster.

Pin 4 is the ground to the sender via the fuse panel. Your gauge reads at 4 o'clock because it is not getting grounded through the sending unit.


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The gauge cluster circuit is shown here. Notice the pink wire feeds the fuel and the temperature gauges, the brake warning light and the temperature warning light for the light clusters. It is shown as a purple wire in the diagram.

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Pin four is the tan or light brown wire that feeds the sending unit side of the fuel gauge and it runs to the fuse panel and thence to the fuel sending unit.

Since you have two tanks, I believe that the tan wire runs to the selector switch and then splits to go to each of the two tanks.

Do you have an electronic switching valve for the tanks or a manual lever switching valve?

If it's electric then you should have a power wire going to the tank selector switch along with the tan sending unit wire. Which ever it is you should never send 12 volts into the tank sending unit as it is a resistor that can get very hot without going through the gauge resistor and the tank resistor. There have been some instances of in tank explosions attributed to this mistake.

You should read 12 volts on the pink wire at pin 3 and you will read 12 volts on the tan wire because it is connected tho the pink wire through the gauge resistor. Once it goes through the load resistance and back to the grounded flange on the sending unit it will read less than 12 volts at that point, as ling as it is grounded back to the battery negative through the frame


This is the only fuel tank switching diagram I have but it may help you sort out the wiring. I think it's for the electric switching valves. It is for an 83 year so that's why I asked you if yours was manual or electric.


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Old 04-07-2018, 01:37 PM   #3
LH Lead-Foot
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Re: Fuel gauge wiring and voltages

The fuel gauge and tank sending unit on my schematic shows 67 - 72. The sender should be 0 ohms empty and 90 ohms full. This is a TAN wire circuit that on a 67, plugs into the front of the fuse box single terminal, that says "FUEL". The PNK wire is B+ keyed with the switch. If you key on/engine off, disconnect the TAN wire and the gauge should move to empty. Most of us would use a old sender or potentiometer or "POT", use it measuring with a DMM and adjust to 90 ohms. To the gauge and ground, key on, the gauge should now read full.
I cannot say what your exact problem is with the information you have provided, based upon what you have done. No one like it when the "Magic Smoke" come out. The fuel gauge has internal resistance and this acts as a voltage drop. Some fuel gauges have "Thermal Limiter" that drop gauge voltage down to 3 volts DC. No one wants full battery available inside the gas tank. I would test your printed circuit for continuity, from terminal to terminal. With the dash or "IP" disconnected, key on and test the pink for voltage. If you have an idea of how much fuel is in the tank, use the DMM to ground, test the resistance on the TAN wire back to the sender, take note of reading and do a little math. One half of 90 ohms is 45 ohms = half tank.

Also. in this same "Electrical" forum, there is a good schematic posted. double check it and do some testing before hanging on new parts.
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Old 04-08-2018, 06:41 AM   #4
cjensen_68
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Re: Fuel gauge wiring and voltages

Vette, the switch is an electrical switch. it's set up correctly. I pulled the dash and opened the wiring. It's a bit of a mess. There is no ignition switch. It's basically a toggle switch with a push button to start the engine but there is a lot coming off of that toggle switch and I don't like it. I have already bought the switch and parts and will be going back to what factory came with. The main hot wire feeding the toggle switch was melted when I opened the harness up.

I was wrong with my initial thoughts on where the tan wire went. I thought it went to the brake switch but it went to the fuse panel. That's why I kept seeing 12V on pin 4 even with the connected off of the cluster. I got that situated now.

I do have another question with the fuel gauge though. Can my fuel gauge be working but have too low of resistance? I rigged it up to how it should be and it worked. I read another post where TBONE1964 did a write up on how to test the fuel system. In there his readings were different than mine on the fuel gauge. I had the dash out and between the feed and sending unit terminals I had 39ohms when tbone had 44 (not too worried with that one) but between the sending unit and ground he had 98 and I have 66. And for the feed to ground he had 140ohms and I had 75. Are those readings acceptable?

I did find a schematic that was color coded from this forum. I had printed it out at work on a 11x17 paper. What a life saver that is.
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Old 04-08-2018, 11:54 AM   #5
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Re: Fuel gauge wiring and voltages

.................................................................................................... ...

Quote:
Originally Posted by cjensen_68 View Post
Vette, the switch is an electrical switch. it's set up correctly. I pulled the dash and opened the wiring. It's a bit of a mess. There is no ignition switch. It's basically a toggle switch with a push button to start the engine but there is a lot coming off of that toggle switch and I don't like it. I have already bought the switch and parts and will be going back to what factory came with. The main hot wire feeding the toggle switch was melted when I opened the harness up.

This is a common problem with our trucks after 50 years of current going through the plastic connector and the red wire which is the main current supply for all the key on items it supplies.
Here is the new switch and the wiring to it. Hopefully your color coded wires will match.


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I was wrong with my initial thoughts on where the tan wire went. I thought it went to the brake switch but it went to the fuse panel. That's why I kept seeing 12V on pin 4 even with the connected off of the cluster. I got that situated now.

I do have another question with the fuel gauge though. Can my fuel gauge be working but have too low of resistance? I rigged it up to how it should be and it worked. I read another post where TBONE1964 did a write up on how to test the fuel system. In there his readings were different than mine on the fuel gauge. I had the dash out and between the feed and sending unit terminals I had 39ohms when tbone had 44 (not too worried with that one) but between the sending unit and ground he had 98 and I have 66. And for the feed to ground he had 140ohms and I had 75. Are those readings acceptable?

The readings do vary a lot, but those are ok. Just remember with the fuel gauge- high resistance mans more fuel reading and low resistance means less fuel reading on the gauge.

Here's a schematic on the circuit. It doesn't show the tank switch you have, just the regular trucks.

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I did find a schematic that was color coded from this forum. I had printed it out at work on a 11x17 paper. What a life saver that is.

The wiring diagrams that I have seen all show a wiring error on the wiper motor wiring which would produce a dead short to ground and blow the wiper fuse the second the wiper switch is selected.

Here it is with the corrected wiring below. Notice the yellow jumper for the washer is connected to the low speed terminal, which is a ground path for the motor.
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Old 04-08-2018, 01:35 PM   #6
cjensen_68
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Re: Fuel gauge wiring and voltages

I had to buy all new parts for the ignition switch since there was none when I got the truck so I have that new pigtail connector and switch. I know all about the brittle wiring/connectors from the chevelle. Had an issue with the solenoid jumping out randomly and thought it was the starter. Changed it and didn't solve the issue. I called it quits for the night and was putting tools away when I turned around and saw flames coming out of the dash and up the windshield. I knew exactly what was wrong right then. I disconnected the battery real quick and hit the dash with a fire extinguisher. It melted all the wires together by the switch and melted the dash which on the 68 was square so now you couldn't see the gauge in that corner. I had to take a torch to it and slowly heat it up and mold it back out the way.
Thank you for the help on this fuel gauge.
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Old 04-08-2018, 01:54 PM   #7
VetteVet
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Re: Fuel gauge wiring and voltages

.................................................................................................... ......
Here's the best diagram I have found on the forum from Boundstaffs thread.

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If you click on it it should enlarge.
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Old 04-08-2018, 02:00 PM   #8
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Re: Fuel gauge wiring and voltages

Well it doesn't seem to enlarge here on the forums so save it to your hard drive and use MS word or whatever office program you have to enlarge it.
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Old 04-08-2018, 02:35 PM   #9
cjensen_68
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Re: Fuel gauge wiring and voltages

That's the same one I was going by. Got it from his thread.
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Old 04-08-2018, 07:58 PM   #10
cjensen_68
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Re: Fuel gauge wiring and voltages

Hey Vette, you happen to know if a 68 chevelle fuse panel will be the same as a 70 pickup? I vaguely remember having a spare one around some where. The previous guy put in a different style fuse panel. Just curious.
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