79 OEM Tach install

[79Sea30]

Afternoon Electrical Gurus-

Over this winter, I've lurked quite a bit on this site. I'm fairly knowledgeable with mechanics, but electronics are always a struggle for me. Hoping a few people could chime in to help. Here's where I'm at.

The truck is a 79, c30 454, with dual tanks. PO's did some hacks with the fuel system, but eliminated the driver side tank and wiring needed. When I took out the original cluster, the sending wire was literally attached to the back of the gauge vs, through the cluster harness. As soon as I pulled it out, I knew there was something incorrect going on, but figured it would be easier to figure out. To note, I drove this truck a ton last year. No issues with fuel gauge, or sending unit, so I know that circuit was grounded and working with the sending unit wire (pink) directly to the gauge.

In the fall I was lucky enough to find a tach cluster. Being that my cluster was in good shape, I decided to tear in and cleanup/plus add the tach with the lower left small fuel gauge. Tach is working, and was a very easy swap in. I replace with a new circuit board. The fuel gauge worked, but buried itself past full, so I immediately shut the ignition off. Thinking there may be an open circuit, or ground issue. Knowing that I had the sending unit wire that was originally attached to the gauge, I held that wire against the gauge pin on the back, and sure enough, it leveled out to exactly where it needed to be.

However, this is where things go south. I did the same thing as the last owner did to the fuel gauge in it's new position to make sure it all worked. The next time I turned the ignition on, within a minute or so, there was a puff of smoke from behind the cluster. Sure enough, it was the printed circuit right above where the fuel gauge goes. If I follow the trace, it goes back to the #30 pink wire on the cluster harness. For sure, when testing nothing was loose or shorted on a ground, or made contact with anything metal.

After this happened, I immediately checked all the pins to the 78-80 Schematic with a Tachometer. Everything has checked out, and all look to be grounded correctly. I'm thinking that the burnt up circuit could be because the "pink" 30 wire is still in the cluster harness, plus, I had one directly attached. To those out there that are more comfortable than me, should I be able to re-pin the new sender line that was directly attached to the 30 to eliminate that issue?

Thanks again, and sorry for the long one.

Cheers,
Kyle

 

[dvdswan]

I would ensure the cluster connector is wired for your model year, this way there is no confusion. Even if it means buying a new printed circuit. Tachometers are not part of the circuit for the cluster, they have a totally separate pigtail that plugs in.

My guess would be the PO pinched one of the wires for that circuit that pink30 (which is generally power) feeds and shorted it out on something. Or if you're lucky its just dry rotted.

I would chance to say that the problem is in the dash from the tank selector switch to the cluster.

 

[79Sea30]

I would ensure the cluster connector is wired for your model year, this way there is no confusion. Even if it means buying a new printed circuit. Tachometers are not part of the circuit for the cluster, they have a totally separate pigtail that plugs in.

My guess would be the PO pinched one of the wires for that circuit that pink30 (which is generally power) feeds and shorted it out on something. Or if you're lucky its just dry rotted.

I would chance to say that the problem is in the dash from the tank selector switch to the cluster.

Thank you. Yes I already have a new circuit in order and should be here next week. I fixed the burnt one with some solder and will use that to rest until the new arrives.

Tach wise, that was simple. It’s really just the fuel gauge that isn’t working but I’m going to the check everything. I guess I was more curious as if the new lead from sender should be added to the cluster if the original wiring is bypassed.

I did double check the schematic and the pin and color locations matches exactly what it should.

 

[nvrenuf]

I installed a 73/74-ish tach/ammeter cluster in my '80 K10 many moons ago. I pulled the gas gauge and voltmeter wires from the cluster plug and ran them straight to each gauge, everything worked fine. The unmodified temp gauge worked fine too (mechanical oil gauge).

 

[79Sea30]

Thanks all. Think I may have found a few issues that may have caused some issues.

Tank switch was still in the dash. Plug wasn’t grounded and still plugged into ignition fuse panel still and I found a ground that was off of cluster than had been tapped into and removed at one point. But never fully repaired and re grounded.

Re fit my cluster and fired it up. Gauge now reads past full and as soon as I did held the wire to the terminal on the back the level came back down and stopped where it should. I think once I button that’s portion up it should be fine.

Cheers,
Kyle