Gauge issues

[HiredGun]

Recently I was given my 72 CST. It was a barn find and had been under a tarp for over 15 yrs. That said I knew it wasn't going to be a simple project. Got the truck up and on the road in about 3 months, but without a working tac or fuel gauge. I've been reading around on here and from what I've gathered, the "tan" wire is from the sending unit and that pretty much acts as the ground for the gauge and all the gauge is reading is how much of a ground it has as the float moves.

1st off, the TAC:
I connected the wire to the distributor. when I put my lights on the gauge lights up (sometimes). I'm thinking thats a simple ground problem. I found a wire that came from the tac and is a male spade clip (all probably rigged work from my grandfather as it was his old track truck). I found a weird clip from the wire harness that clips on underneath the dash. I stuck the spade clip between that clip and the dash and the gauge reads under 1k rpms but nothing else

2nd, the fuel gauge:
NOTE that I have a plastic tank from either a Blazer or Saburban complete with a brand new sending unit. The unit has a port that sends the signal to the gauge and another that you ground to the frame I'm assuming? I've ran wire along the frame to the fuse panel. The gauge has always read "over full". And I dont have a meter to check this.

I see me very shortly removing all the wire harness and starting fresh

[VetteVet]

Do you have points ignition or the HEI distributor?



1 wire from the back of the tach (marked coil) to the tach post on the HEI on the engine (spade connectors on each end) - light brown wire from factory if you are going for that look. If you have points then the brown wire goes to the negative (distributor side of the coil).

1 wire from the back of the tach (marked 12v) to an ignition unfused connection on the fuse block under the dash - could use the cigar lighter opening if you don't have a lighter. Usually a red wire.
Make a ground wire from the tach to the dash mount.

That is it...

Over full on the gauge means that the ground side of the gauge from the fuse panel isn't going to the sender and through the sender resistor to the frame ground back to the battery. If you have run a wire to the fuse panel from the sender then you need to ground the other terminal to the frame. They are normally grounded by the sender mount and then through the tank mount. A plastic tank will not ground so you have the extra terminal for the ground.

[HiredGun]

I should have posted pics with a thread like this. I'm headed out now to take pics.
I have a distributor, the only two spade clips on it are "BAT" and "TAC". From the distributor i know I'm good. It's just this cluster that I don't know about. My grandfather said it all worked for him when he had it (over 15 yrs ago) and that it should all just work because he never messed with it. It's an aftermarket tac, pretty much the only factory gauges I have is speedometer and fuel.

The sender is grounded to the frame and not the tank. I probably wasn't too clear on that. I was saying I grounded to the frame because the tank is plastic and I know it wont ground to plastic. Right now the sender is disconnected from the fuse box and it still reads at 3'oclock (fast full)

[HiredGun]

Here's the pics I promised:

Red wire is going to my tach. The brown wire is from the battery

My fuse box. I cant figure where that clip goes with the yellow and white wires

How the tach was grounded

Not to insult your intelligence, but for those having the same issue, here's what the clip looks like when disconnected from the dash:

Off the back of the cluster the yellow wire in my hand is the same ground wire that's in my hand in this pic


From the distributor the wire is coupled to this black wire

how the tach reads

How the fuel gauge reads

Wire from the sending unit

[VetteVet]

Man I'm getting some bad vibes off those pictures. First off it appears that you have the tach and distributor wires reversed on the cap unless they are crossed somehow. The brown wire goes to the Tach post and the red wire goes to the BAT terminal post.

All those wires changing colors just confuses me. You should have red positive wires and black negative or ground wires. The gauge light wires can hook to any key on power unless you want to dim them then you have to hook to the gray wire on the cluster plug.

The fuel gauge will never work as long as the sender wire is unplugged from the fuse panel. If you plug it in and the gauge doesn't go back to the normal range then the sender wire has an open in it or the sending unit is bad or not grounded properly.
Get a multimeter and read the ohms between the sender wire and the ground wire and you should get some where between 0 and 90 ohms.

[HiredGun]

Originally Posted by VetteVet

Man I'm getting some bad vibes off those pictures. First off it appears that you have the tach and distributor wires reversed on the cap unless they are crossed somehow. The brown wire goes to the Tach post and the red wire goes to the BAT terminal post.

All those wires changing colors just confuses me. You should have red positive wires and black negative or ground wires. The gauge light wires can hook to any key on power unless you want to dim them then you have to hook to the gray wire on the cluster plug.

The fuel gauge will never work as long as the sender wire is unplugged from the fuse panel. If you plug it in and the gauge doesn't go back to the normal range then the sender wire has an open in it or the sending unit is bad or not grounded properly.
Get a multimeter and read the ohms between the sender wire and the ground wire and you should get some where between 0 and 90 ohms.

yea, I really need to just rewire everything. Alot of the harness was cut because my grandfather went to manual gauges. I'm running a Ford solenoid before my starter, and the relay by the horn is deleted (new alternator)

[HiredGun]

oh, the brown IS connected to the battery, and the red goes to my tach. I've traced the wires. So I'm just assuming the ground is bad on the tach.

And for the fuel sending unit, I disconnected it to show you it reads the same connected or not. But it looks like I'm getting a new tool lol

[HiredGun]

oh, the brown IS connected to the battery, and the red goes to my tach. I've traced the wires. So I'm just assuming the ground is bad on the tach.

And for the fuel sending unit, I disconnected it to show you it reads the same connected or not. But it looks like I'm getting a new tool lol

[HiredGun]

I think I found the reason, with your help. I took the cluster apart and accessed the back of the tach. Looked like i was grounding the "IGN SW" port. Tach is grounded with the rest of the cluster already. The "IGN SW" I'm assuming is ignition switch?

Also, on my fuse box theres two terminals under "Fuel GA". Which one do I plug into from the sending unit? I had it plugged into the top one (see pic) and I'm hoping that isn't sending any type of charge.

[VetteVet]

Originally Posted by HiredGun

I think I found the reason, with your help. I took the cluster apart and accessed the back of the tach. Looked like i was grounding the "IGN SW" port. Tach is grounded with the rest of the cluster already. The "IGN SW" I'm assuming is ignition switch?

That would be correct/ now here's a little tip. Run a small gauge red wire from there over to the fuel gauge power side terminal. You can see this in the picture below.



Then run a brown wire from the tack terminal out to the distributor TACH terminal. 16 gauge is big enough for these two wires.

Then run a 12 gauge red wire (Not brown) from the ign unfused terminal on the fuse panel to the BAT on the distributor



Also, on my fuse box there's two terminals under "Fuel GA". Which one do I plug into from the sending unit? I had it plugged into the top one (see pic) and I'm hoping that isn't sending any type of charge.

Your picture didn't come through but here's one. The fuel ga terminal is on the end of the panel and the ign'unfused is shown on the bottom. There's two ports so you can use either one. These are keyed on by the ignition switch. If you don't like my tach power wire then you can run the red wire from the tach to this terminal as well.

[HiredGun]

I'm going to use your way. Seems cleaner and I have enough random wires running around lol.

What's the terminal to the left of the "Fuel GA"?

My fuse box has the "Fuel GA" terminals up top and the "IGN Unfused" on the right.

[HiredGun]

????

Tach moves in the opposite direction now from just under 1k to zero. I must be missing something simple here. Here's the back of my Tach:

Black wire runs to the distributor and the yellow to the back of the fuel gauge.


It just hit me tho: If my fuel gauge would work properly then the tach might work properly? So I'm thinking about getting the fuel gauge working first, then focus on the tach.

[VetteVet]

Originally Posted by HiredGun

I'm going to use your way. Seems cleaner and I have enough random wires running around lol.

What's the terminal to the left of the "Fuel GA"?

My fuse box has the "Fuel GA" terminals up top and the "IGN Unfused" on the right.

Your yellow wire is the power wire for the tach and the black wire is the signal wire that goes to the distributor Tach terminal. The other wire is just a ground for the tach and the light wire can go to any dash light or the head light switch.

Here is a picture of the fuse panel with the fuel ga terminal on the left and you can see the tan wire plugged into the end terminal. The one next to it is unfamiliar to me and not used on most trucks. Just on the bottom is a red wire that is plugged into one of the ign-unfused terminals. You could run your yellow wire to this for key on power. It would be great if you knew someone that would let you wire the tach into their truck to test it and see how it worked. if it did the same thing then it might be fried.

[HiredGun]

Originally Posted by VetteVet

Your yellow wire is the power wire for the tach and the black wire is the signal wire that goes to the distributor Tach terminal. The other wire is just a ground for the tach and the light wire can go to any dash light or the head light switch.

Here is a picture of the fuse panel with the fuel ga terminal on the left and you can see the tan wire plugged into the end terminal. The one next to it is unfamiliar to me and not used on most trucks. Just on the bottom is a red wire that is plugged into one of the ign-unfused terminals. You could run your yellow wire to this for key on power. It would be great if you knew someone that would let you wire the tach into their truck to test it and see how it worked. if it did the same thing then it might be fried.

Attachment 1052258

I think I'll do that tomorrow to see if theres a problem with the fuel gauge. If my tach works from the fuse box and not my fuel gauge, what does that mean about the fuel gauge?

Also, I started it and forgot to plug the harness into the cluster and it did the same thing as its doing now. It pulsed backward to zero. So I'm thinking the fuel gauge must not be getting power somehow. Just thinking out loud lol

[VetteVet]

Originally Posted by HiredGun

I think I'll do that tomorrow to see if theres a problem with the fuel gauge. If my tach works from the fuse box and not my fuel gauge, what does that mean about the fuel gauge?

Also, I started it and forgot to plug the harness into the cluster and it did the same thing as its doing now. It pulsed backward to zero. So I'm thinking the fuel gauge must not be getting power somehow. Just thinking out loud lol

Well if you didn't plug the harness plug into the cluster then you definitely won't get power to the fuel gauge or anything else in the cluster. If you plugged it in and the tach still didn't work then check to make sure you have the tach wire on the power terminal of the fuel gauge and not the sender terminal. If the plug is pinned correctly then the pink wire on pin three is the one that powers the fuel gauge. The sender is the tan wire on pin 4 that goes to the other side of the fuel gauge. Try the fuse panel connection and see if that makes a difference. If your tachometer ground is connected to the cluster then make sure the cluster is grounded to the dash of to the cab to a good ground.

[Putter]

Fuel tank....
The sending unit in the tank is just a ground (if I recall correctly). One side of the sending unit (since your tank is plastic) should go to a frame ground. The other should be the tan wire heading to the cluster. This is just a ground.
I believe on a normal METAL tank (in-cab or rear mounted) there is ONLY the tan wire...which would be the signal to the gauge.

On your harness, position 4 should be a 20-gauge tan wire to the tank
Position 3 should be a 20-gauge pink...which is the fuel gauge FEED from the fuse box (+ voltage).

Hope that helps sort it out!
Look here for the wiring positions...
http://gmcpauls.com/Tech%2520Tips1.htm

[HiredGun]

well... I wired the tach to the unfused port on the fuse box and the tach is pegged out at 10k rpms. this is enough to piss off the pope.

for the fuel gauge, I'm waiting till I get a mutlimeter

[HiredGun]

Originally Posted by VetteVet

Man I'm getting some bad vibes off those pictures.

I'm seriously thinking of ripping everything out and starting fresh. I wanted to do a frame off restore. I want to stay as close to original as possible with the electrical and such, only keep the big block and lower it a bit. My plans were to just get the gauges working enough to safely drive and enjoy it until that time comes.

[HiredGun]

I bit the bullet and bought all new gauges except my speedometer. I installed the new tach and it works great along with my Vacuum, oil pressure, temp, and voltage.

I have a new fuel gauge in my old cluster and before I plug anything in from my sending unit I want to make sure I plug it in the correct location. (note: I'm using my existing cluster, just all new gauges)

Do I plug the wire from the sending unit into my fuse box? or on the back of the fuel gauge? (its a stock fuel gauge I got at cruising the coast)

[VetteVet]

You can go to either place but the fuel gauge terminal on the fuse panel is easier. If you have power on the fuel gauge. (pink wire) then turn the key on and run a ground wire to the fuel terminal on the fuse panel and the gauge should go to empty. If it does then plug the sending unit wire there and the reading should match the fuel in the tank.

[HiredGun]

Originally Posted by VetteVet

You can go to either place but the fuel gauge terminal on the fuse panel is easier. If you have power on the fuel gauge. (pink wire) then turn the key on and run a ground wire to the fuel terminal on the fuse panel and the gauge should go to empty. If it does then plug the sending unit wire there and the reading should match the fuel in the tank.

Awesome! You've been a big help! I'll go out and try right now. Sadly I haven't had time to fix all my rat-nest of wiring that I have. I've been rebuilding my daily but I'm about to finish that project so my truck can get the attention it needs/deserves!

[HiredGun]

Oh, and I'm assuming normally the gauge gets power when the harness is plugged in right? No other wires are needed?

[HiredGun]

This is what it reads when I unplug the wire-harness off the back. It reads the same when the harness is plugged in and when I connect a grounded wire to the fuel gauge connection on the fuse box it still reads this. Pretty much no change at all and this is a brand new gauge.