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Old 06-28-2022, 06:28 PM   #1
72c20customcamper
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85 dual tank gage question

I believe the printed circuit board went on my 85 as the fuel works sometimes and the others not at all . I would like to install a fuel gage in place of the volt gage in this gage pack . My question is what wire in the harness running to the plug at the printed circuit board is for the gage . My thinking is if I get a new gage and tap into the wire I should be able to run one aftermarket gage for both tanks . I plan on doing the circuit board this winter and fix it properly then
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Old 06-28-2022, 11:15 PM   #2
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Re: 85 dual tank gage question

Mark
Here is a link from a Member (Vendor) on the board of his wiring diagrams.

P.S. I just purchased a vinyl floor from him (It was A great purchase)

Anyway, here is the Link

Link: http://forum.73-87chevytrucks.com/sm...?topic=11766.0

Hope this helps.
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Old 06-29-2022, 08:22 AM   #3
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Re: 85 dual tank gage question

Quote:
Originally Posted by Getter-Done View Post
Mark
Here is a link from a Member (Vendor) on the board of his wiring diagrams.

P.S. I just purchased a vinyl floor from him (It was A great purchase)

Anyway, here is the Link

Link: http://forum.73-87chevytrucks.com/sm...?topic=11766.0

Hope this helps.
Those are the Haynes manual wiring diagrams that Chris posted on his board.

GM wiring diagrams are available right here. Look in my signature.

The other thing to look at is the dual tank theory of operation thread.
https://67-72chevytrucks.com/vboard/...d.php?t=754061
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Old 06-29-2022, 09:44 AM   #4
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Re: 85 dual tank gage question

I'm not good with diagrams specially wiring . But from what I can tell the #30 on the plug is to the gage. I'll just trace up the harness to find the plug i assume it's on the firewall .
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Old 06-29-2022, 10:56 AM   #5
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Re: 85 dual tank gage question

Ordered the gage auto meter 2316 for 65-97 gm . Be nice not to have to use mileage for fuel stops . Iíve had to switch over on the road not fun at 65 mph
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Old 06-29-2022, 11:23 AM   #6
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Re: 85 dual tank gage question

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I'm not good with diagrams specially wiring . But from what I can tell the #30 on the plug is to the gage. I'll just trace up the harness to find the plug i assume it's on the firewall .
The pic you borrowed is for the 73-80 solenoid valve systems.

You have a 1985 with the motorized valve. Read post #2 in that thread. Post #2 covers 1981-1991.
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1986 M1008 CUCV SOLD
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2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
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Old 06-29-2022, 12:43 PM   #7
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Re: 85 dual tank gage question

I see that now thanks . I guess I need to tap the B pin ?
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Old 06-29-2022, 03:28 PM   #8
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Re: 85 dual tank gage question

A & C come off the tank senders to the switched contacts of the SPDT switch inside the valve.

B is the common terminal on the gauge switch inside the valve and the B wire terminates to a molded rubber disconnect on the RH frame rail. The wire runs from there up to the bulkhead plug under the brake booster.

Honestly it's usually not difficult to diagnose and fix the fuel gauge in the dash.

The tank ground wire is one of the more common failure points in the 1973-1986 fuel gauge circuit. It either gets left off by someone replacing the tank or the wire rots off the ring terminal at the frame or the .250" disconnect on the sender locking ring rots off the wire.

If the stock fuel gauge is nailed at 3:00 you likely have a bad tank ground wire.
Easy to test and not awful to correct. Take a set of jumper cables and attach one end to the frame and the other to the pinch weld on the selected fuel tank. If the fuel gauge needle suddenly comes to life with the KOEO (key on engine off) you'll need to fix the tank ground wire between the frame and the sender locking ring. Many moons ago I attached an alligator clip jumper wire from the frame to the pinch weld on the tank on my 78 GMC to get me by for a couple weeks til I had time to fix the problem properly.

The other thing that'll nail the fuel gauge at the 3:00 position is an unplugged sender wire or damage to the wires in the sender circuit. There's only three of them and they're fairly accessible.
If the valve is unplugged you've likely found your problem.
I've commonly seen the sender wire get chowdered up where it runs along the frame rail. It's either unplugged from the harness wire that runs up to the bulkhead plug under the brake booster or something happened to it between the valve terminal B and the molded rubber round pin disconnect on the frame rail or right after the disconnect. None of that is difficult to repair.

No amount of brandy new fuel gauges will read anything without a good tank ground and good sender wires from the tanks all the way to the gauge.
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2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8


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Old 06-29-2022, 03:36 PM   #9
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Re: 85 dual tank gage question

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A & C come off the tank senders to the switched contacts of the SPDT switch inside the valve.

B is the common terminal on the gauge switch inside the valve and the B wire terminates to a molded rubber disconnect on the RH frame rail. The wire runs from there up to the bulkhead plug under the brake booster.

Honestly it's usually not difficult to diagnose and fix the fuel gauge in the dash.

The tank ground wire is one of the more common failure points in the 1973-1986 fuel gauge circuit. It either gets left off by someone replacing the tank or the wire rots off the ring terminal at the frame or the .250" disconnect on the sender locking ring rots off the wire.

If the stock fuel gauge is nailed at 3:00 you likely have a bad tank ground wire.
Easy to test and not awful to correct. Take a set of jumper cables and attach one end to the frame and the other to the pinch weld on the selected fuel tank. If the fuel gauge needle suddenly comes to life with the KOEO (key on engine off) you'll need to fix the tank ground wire between the frame and the sender locking ring. Many moons ago I attached an alligator clip jumper wire from the frame to the pinch weld on the tank on my 78 GMC to get me by for a couple weeks til I had time to fix the problem properly.

The other thing that'll nail the fuel gauge at the 3:00 position is an unplugged sender wire or damage to the wires in the sender circuit. There's only three of them and they're fairly accessible.
If the valve is unplugged you've likely found your problem.
I've commonly seen the sender wire get chowdered up where it runs along the frame rail. It's either unplugged from the harness wire that runs up to the bulkhead plug under the brake booster or something happened to it between the valve terminal B and the molded rubber round pin disconnect on the frame rail or right after the disconnect. None of that is difficult to repair.

No amount of brandy new fuel gauges will read anything without a good tank ground and good sender wires from the tanks all the way to the gauge.
I’m doing this because none of the gages work . My fuel gage will at times will read . But 99.9 % of the time it’s at some point no specific spot on the gage it stays where it last read
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Old 06-29-2022, 03:41 PM   #10
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Re: 85 dual tank gage question

If none of your gauges and lamps are working the first place to look is the ignition power and the ground wires to the cluster.

Ignition power is pinkish red with a black stripe. Look in the wiring diagrams for the fuse that feeds the cluster. It's marked INSTR LPS and it should be 5A. The other fuse to look at is the 20A IGN/GAU but that one will likely cause other issues.
Ground is black.

I've found the main instrument cluster ground unplugged from the buss bar above the parking brake pedal more than once. Easy quick fix.
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Old 06-29-2022, 03:52 PM   #11
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Re: 85 dual tank gage question

The senders are fairly new as is the switch . I tried the ground to frame no response . I doubt I would have a ground issue on both tanks at the same time.

This the bulkhead you referred to? No other connections there .
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Old 06-29-2022, 03:56 PM   #12
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Re: 85 dual tank gage question

Quote:
Originally Posted by hatzie View Post
If none of your gauges and lamps are working the first place to look is the ignition power and the ground wires to the cluster.

Ignition power is pinkish red with a black stripe. Look in the wiring diagrams for the fuse that feeds the cluster. It's marked INSTR LPS and it should be 5A. The other fuse to look at is the 20A IGN/GAU but that one will likely cause other issues.
Ground is black.

I've found the main instrument cluster ground unplugged from the buss bar above the parking brake pedal more than once. Easy quick fix.
I miss spoke the only gage working was the oil pressure gage but it was pegging out then reading normal . I traced it to the sensor behind the distributor. I had fixed that but took it out to install the mechanical gage

Dash Lamps are working fine
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Old 06-29-2022, 04:11 PM   #13
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Re: 85 dual tank gage question

Only one fuel gauge is connected through at a time. If the valve is working then that's unlikely to be the problem on both tanks.

The bulkhead plug is the big square Packard plug that most of the wires pass through into the cab. It's near the booster.
Name:  Bulkhead plug.jpg
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The 16ga pink wire next to the light blue wire at the bottom left on the pictured bulkhead plug is Circuit 30.
Circuit 30 is the GM name for the fuel gauge wire from the frame disconnect that plugs into the valve sub harness wire to valve terminal B.
The bulkhead plug is shown in the first full page of wiring in the 86 wiring diagrams.

You can backprobe the bulkhead plug with a T-pin to hook up your meter. OTOH disconnecting the bulkhead plug isn't rocket science. Unscrew the bolt and the plug comes apart like Lego blocks. Unplug the battery before you disconnect the bulkhead plug.
I'd take a meter and check for continuity from the bulkhead plug to terminal B on the valve plug. If you don't have continuity you've found your problem area.
It's usually chewed up down under the cab near the valve subharness disconnect.
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1986 M1008 CUCV SOLD
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Last edited by hatzie; 06-29-2022 at 04:21 PM.
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Old 06-29-2022, 04:26 PM   #14
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Re: 85 dual tank gage question

Quote:
Originally Posted by hatzie View Post
Only one fuel gauge is connected through at a time. If the valve is working then that's unlikely to be the problem on both tanks.

The bulkhead plug is the big square Packard plug that most of the wires pass through into the cab. It's near the booster.
Attachment 2201685

The 16ga pink wire next to the light blue wire at the bottom left on the pictured bulkhead plug is Circuit 30.
Circuit 30 is the GM name for the fuel gauge wire from the frame disconnect that plugs into the valve sub harness wire to valve terminal B.
The bulkhead plug is shown in the first full page of wiring in the 86 wiring diagrams.

You can backprobe the bulkhead plug with a T-pin to hook up your meter. OTOH disconnecting the bulkhead plug isn't rocket science. Unscrew the bolt and the plug comes apart like Lego blocks. Unplug the battery before you disconnect the bulkhead plug.
I'd take a meter and check for continuity from the bulkhead plug to terminal B on the valve plug. If you don't have continuity you've found your problem area.
It's usually chewed up down under the cab near the valve subharness disconnect.
Iíve had them apart before . Iíll check continuity but my gut tells me the circuit board is messed up
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Old 06-29-2022, 04:57 PM   #15
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Re: 85 dual tank gage question

I just checked it has continuity from the block to the b pin on the fuel plug so one item off the list . What Iíll do is disconnect the pink wire from the harness plug at the firewall and run it to the new gage . Iíll have it Friday so Iíll know by Sunday if itíll work.

Thanks for the help hatzie
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Old 06-29-2022, 05:09 PM   #16
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Re: 85 dual tank gage question

I'd clip the circuit 30 wire 6" from the instrument cluster plug and hook your gauge up there.
No wires to drag through the firewall just across the dash.

Download the 86 wiring diagram. It'll show the position of the circuit 30 wire on the instrument cluster plug. It's in one of the corners of the plug.

When you're ready to use the dash again solder the wire back together.
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Old 06-29-2022, 05:16 PM   #17
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Re: 85 dual tank gage question

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Originally Posted by hatzie View Post
I'd clip the circuit 30 wire 6" from the instrument cluster plug and hook your gauge up there.
No wires to drag through the firewall just across the dash.

Download the 86 wiring diagram. It'll show the position of the circuit 30 wire on the instrument cluster plug. It's in one of the corners of the plug.

When you're ready to use the dash again solder the wire back together.
Thanks . I canít believe it but monkeying around with the wires and itís working again first pic left tank second is right . I know the right is full as I just filled it . Left has a couple of days would need to look at miles but half would be about right
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Old 06-29-2022, 07:39 PM   #18
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Re: 85 dual tank gage question

Well that didnt last long . Turned it off and restarted about 20 minutes later and its sitting at 1/2 tank LH or RH

But the ac is working well
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Old 06-29-2022, 10:47 PM   #19
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Re: 85 dual tank gage question

Quote:
Originally Posted by hatzie View Post
Those are the Haynes manual wiring diagrams that Chris posted on his board.

GM wiring diagrams are available right here. Look in my signature.

The other thing to look at is the dual tank theory of operation thread.
https://67-72chevytrucks.com/vboard/...d.php?t=754061
Hatzie
I am so sorry that I posted that info.

Hope you get this issue resolved Mark.

Just keep the tank full
And I hope to see you at the Show
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Old 06-30-2022, 07:28 AM   #20
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Re: 85 dual tank gage question

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Hatzie
I am so sorry that I posted that info.

Hope you get this issue resolved Mark.

Just keep the tank full
And I hope to see you at the Show
I'll get it squared away I've been writing down the miles i fill at and pretty much know when to get gas . Would be a real problem if i had only the one tank. I have had to switch over at highway speed it's not fun .
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Old 06-30-2022, 08:38 AM   #21
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Re: 85 dual tank gage question

Quote:
Originally Posted by Getter-Done View Post
Hatzie
I am so sorry that I posted that info.
Why? You're being helpful. That's what we're here to do.

Quote:
Originally Posted by 72c20customcamper View Post
I'll get it squared away I've been writing down the miles i fill at and pretty much know when to get gas . Would be a real problem if i had only the one tank. I have had to switch over at highway speed it's not fun .
Sounds like you have corrosion in the connectors or damage near the bulkhead plug or the valve.

The tin plating on Weatherpak tower terminals to the valve corrode if they're left unplugged.
The brass Packard terminals corrode when the black dielectric goo that GM sprayed the plugs with degrades.

Caig DeoxIT D5 and a set of carburetor cleaning brushes might be a worthwhile investment.
I've seen small diamond files for cleaning Canon plug terminals but I figure if they're that corroded it's time to replace them.

Carb Cleaning Brushes on Amazon

I can get Deoxit D5 at a guitar store. Musicians use it to clean the plug contacts on their guitars, keyboards, mics, and amps. I use it on other electronics. I used to pick it up at a local Radio Shack but they're disappearing. :-(
Be sure to get the DeoxIT D5. They make a "fader" cleaner and lube for potentiometers that won't work as well on connector terminals.

Once the bulkhead terminals are treated with Deoxit, scrubbed some, and sprayed again it wouldn't hurt to add some fresh dielectric grease.
CRC 02083 Spray Silicone Dielectric Grease would be a reasonable choice. You can probably use a lube straw on this to reach into the female terminals and treat the bare wire at the back of the connectors.
I wish I knew what that black dielectric grease that GM sprayed into them on the assembly line was. It protects things for half a century and more. I've taken apart the bulkhead plugs on late 1960's GM, AMC, and Chrysler cars and found the terminals look like new.
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1986 M1008 CUCV SOLD
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8


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Old 07-01-2022, 08:36 PM   #22
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Re: 85 dual tank gage question

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Why? You're being helpful. That's what we're here to do.



Sounds like you have corrosion in the connectors or damage near the bulkhead plug or the valve.

The tin plating on Weatherpak tower terminals to the valve corrode if they're left unplugged.
The brass Packard terminals corrode when the black dielectric goo that GM sprayed the plugs with degrades.

Caig DeoxIT D5 and a set of carburetor cleaning brushes might be a worthwhile investment.
I've seen small diamond files for cleaning Canon plug terminals but I figure if they're that corroded it's time to replace them.

Carb Cleaning Brushes on Amazon

I can get Deoxit D5 at a guitar store. Musicians use it to clean the plug contacts on their guitars, keyboards, mics, and amps. I use it on other electronics. I used to pick it up at a local Radio Shack but they're disappearing. :-(
Be sure to get the DeoxIT D5. They make a "fader" cleaner and lube for potentiometers that won't work as well on connector terminals.

Once the bulkhead terminals are treated with Deoxit, scrubbed some, and sprayed again it wouldn't hurt to add some fresh dielectric grease.
CRC 02083 Spray Silicone Dielectric Grease would be a reasonable choice. You can probably use a lube straw on this to reach into the female terminals and treat the bare wire at the back of the connectors.
I wish I knew what that black dielectric grease that GM sprayed into them on the assembly line was. It protects things for half a century and more. I've taken apart the bulkhead plugs on late 1960's GM, AMC, and Chrysler cars and found the terminals look like new.
Iíll get some of the DeoxIT . I used CRC electrical contact cleaner as I had it .

Now Iím a little stumped . The directions on the new gage says to run a ground wire from the back of the gage to the ground on the sender . But I have two tanks guess I could pigtail it . But Iím not sure as to why it would need to go directly from the gage to sender wouldnít a ground to chassis work ? Since itís just really measures the current going through the sender .
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Old 07-03-2022, 03:03 PM   #23
72c20customcamper
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Re: 85 dual tank gage question

I wired it to chassis ground not the tanks . It’s working

Right full left 3/4
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72 c20 custom camper Husky edition,
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1977 Suburban sold
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