Tach install in Factory Clock car

Kolbey Baltz

Not-quite-so-new-guy
Apr 26, 2016
32
8
Eastern, Pennsylvania
Okay so I have two *wiring* questions:

1.) I FINALLY got the elusive silver faced tachometer cluster for my ‘78 LS swapped Pontiac. I swapped the tach with my clock setup and changed the “clips” on the circuit tape to match the tach cluster I removed the tach from. Soldered a jumper for V8. Went to splice the tach wire from my LS PCM into the stock harness and I don’t have a pin on the dash plug with the factory “white” tach wire. Figured, okay. I traced the three clips on the cluster for the tach back to the plug leads. I found the ground, found the pink power, and what I believe to be the “signal” pin because it leads to nothing on the dash plug. So I soldered the LS PCM tach lead to that pin, fastened it down, turned the car over and got nothing. Any thoughts? I’ve read on here that if your car came with the dummy light cluster (mines original clock and dummy cluster) you will NOT have the white tach wire pin in your dash plug. That checks out because the upper left two pins are non existent and the one clip leads to on of the empty pin slots. I’m curious as to what I can do? I figured running my tach signal directly to the clip would bypass even needing to add a pin to the dash plug but that obviously didn’t work either. Everything else works though, all my rallye gauges work and the speedo works with no issues.

2.) Also, I purchased a third brake light. My car being a ‘78, which wire should I tap into just for the brake lights? When I go look at the brake switch, I only have two wires, an orange and a blue. I read a bunch of mumbo jumbo about having to swap a bunch of stuff, to get JUST the brake lights to turn on and not the turn signals. Is it true? I’ve read about the logic box but can’t seem to find one (probably searching for it with the wrong name). And yes, the third brake light does work, I bench tested it. Would I be able to buy a later year brake switch from a vehicle with the third brake light? Or will that mess up things since I’ve also read that the brake switch wiring is then picked up by the turn signal/hazard wiring?

Thank you all! I’m stumped and I’m usually quite good at figuring this kind of stuff out. The Facebook groups have left me with no responses as of now. After a few hours of troubleshooting and diagram researching, I found nothing conclusive.
 

Anubis

Royal Smart Person
Mar 30, 2012
2,155
113
West Michigan
Did you check to see if you’re actually getting a signal from the ECM? That would be the first step. For the center stoplight or CHMSL the easiest and best way in my opinion would be to get a tail light harness from a newer G body. They aren’t that difficult to remove. The signal obviously comes from the brake pedal switch which If memory serves me has an extra contact.
 

Kolbey Baltz

Not-quite-so-new-guy
Thread starter
Apr 26, 2016
32
8
Eastern, Pennsylvania
Did you check to see if you’re actually getting a signal from the ECM? That would be the first step. For the center stoplight or CHMSL the easiest and best way in my opinion would be to get a tail light harness from a newer G body. They aren’t that difficult to remove. The signal obviously comes from the brake pedal switch which If memory serves me has an extra contact.
Yeah admittedly, I just assumed I was getting a tach signal. I’ll have to get a meter on the wire with the car running and see what happens. The way I have it done, I really don’t see how I wouldn’t be getting a reading. It’s the same thing as having it plugged in and tapped into the factory wire. So it must be the PCM wire being the issue. If the PCM isn’t the issue, I’ll just swap my circuit tape over from the original tach cluster. As for the brake lights, that’s an option. I was hoping for just a splicing and soldering option. I wanted to avoid taking a bunch of stuff apart
 

Gonzo1970

G-Body Guru
Supporting Member
Sep 30, 2018
842
93
Third Brake Lights became mandatory in 1986.

-Gonz
 
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Anubis

Royal Smart Person
Mar 30, 2012
2,155
113
West Michigan
As for the brake lights, that’s an option. I was hoping for just a splicing and soldering option. I wanted to avoid taking a bunch of stuff apart
I think I have a clean, uncut tail light harness from an 1986 Salon. It includes the CHMSL wire you need. PM me if you're interested. I'm making a trip out to the shop this weekend.
 

5spdCab

G-Body Guru
Dec 29, 2019
640
93
Tukwila, Wa.
Okay so I have two *wiring* questions:

1.) I FINALLY got the elusive silver faced tachometer cluster for my ‘78 LS swapped Pontiac. I swapped the tach with my clock setup and changed the “clips” on the circuit tape to match the tach cluster I removed the tach from. Soldered a jumper for V8. Went to splice the tach wire from my LS PCM into the stock harness and I don’t have a pin on the dash plug with the factory “white” tach wire. Figured, okay. I traced the three clips on the cluster for the tach back to the plug leads. I found the ground, found the pink power, and what I believe to be the “signal” pin because it leads to nothing on the dash plug. So I soldered the LS PCM tach lead to that pin, fastened it down, turned the car over and got nothing. Any thoughts? I’ve read on here that if your car came with the dummy light cluster (mines original clock and dummy cluster) you will NOT have the white tach wire pin in your dash plug. That checks out because the upper left two pins are non existent and the one clip leads to on of the empty pin slots. I’m curious as to what I can do? I figured running my tach signal directly to the clip would bypass even needing to add a pin to the dash plug but that obviously didn’t work either. Everything else works though, all my rallye gauges work and the speedo works with no issues.

2.) Also, I purchased a third brake light. My car being a ‘78, which wire should I tap into just for the brake lights? When I go look at the brake switch, I only have two wires, an orange and a blue. I read a bunch of mumbo jumbo about having to swap a bunch of stuff, to get JUST the brake lights to turn on and not the turn signals. Is it true? I’ve read about the logic box but can’t seem to find one (probably searching for it with the wrong name). And yes, the third brake light does work, I bench tested it. Would I be able to buy a later year brake switch from a vehicle with the third brake light? Or will that mess up things since I’ve also read that the brake switch wiring is then picked up by the turn signal/hazard wiring?

Thank you all! I’m stumped and I’m usually quite good at figuring this kind of stuff out. The Facebook groups have left me with no responses as of now. After a few hours of troubleshooting and diagram researching, I found nothing conclusive.
I did my own 3rd brake light on my '78 Caballero. Directly below the GMC letters on the tailgate, I drilled 3 small holes and filled them with red LED bulbs.
20200706_200943.jpg

I had also realized that the turn signals would be a problem, therefore my solution was to go directly to the brake light switch with a dedicated wire to the 3 bulbs, bypassing the turn signal switch. I don't recall specifically which wire I tapped into at the switch, but it would be the one energized by activating the switch.
 

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