Grand Prix tach swap how to

[G-Body_Vet]

I figured I'd post this for anyone wanting to swap a factory tach into their existing Pontiac cluster. I originally planned on swapping the entire cluster but the printed circuit was damaged.

To see if it would work I removed my original cluster from my '78 and compared it to the damaged one with the tach. My original only had a speedometer and a blank silver plate where there's normally a clock. Other than one additional circuit to the speedometer, the printed circuits were identical.

I won't go into details about removing the clear cover, tach or speedo. It's only a few bolts and almost self-explanatory.

Pic of damaged circuit (broken copper strip where the connector goes)

The cluster on the left is the one from my car. The one on the right is the one with the tach. Notice the silver clips on one with the tach. These clips make the connection between the studs on the tach's circuit board to the printed circuit on the cluster. I removed the "center/top" one and placed it on the table. My original already had one clip installed at the "center/top" position.

After pulling out all the clips you can see the contact marks on the copper strips. Use a decent set of needle nose pliers and take your time to remove them from the inside. Back one end out at a time and they should pop out. If you try forcing them you'll most likely bend them pretty bad or damage the plastic on the cluster.

After taking the two retaining bolts out of the tach (12 and 6 o'clock), the tach can simply be pulled out. Here's the inside of the donor cluster before removing the remaining clips that connect to the tach's circuit board pins.

Here's my original cluster with the clips installed. Notice that this printed board is much nicer than the one from the cluster that orinially had the tach. All there is left to do is install the speedo. It's also a good time to check the bulbs for broken filaments and change them while it's out.

 

[79loserbluebu]

Good thread. Shame the Chevy clusters don't use the same logic and have 1 part number for the circuit piece.

 

[G-Body_Vet]

Good thread. Shame the Chevy clusters don't use the same logic and have 1 part number for the circuit piece.

Thanks. Although moving a few clips is no big deal, hopefully it will help someone else down the road. I wish these forums would've been around in the early 90's.

 

[Bonnewagon]

Yup, the factory did a smart thing for a change. Why make different circuits? Now, all we need is a source for those clips. There are 2 sizes, and you can enlarge the opening to fit a big one, but not vice-versa. The Chevy gauge cluster uses really small ones you can't use. When I swapped my tach/gauges in, I didn't have another housing to pirate the clips off of. Good tutorial, I always forget to mention those clips, otherwise it's a straightforward swap.

 

[G-Body_Vet]

Yup, the factory did a smart thing for a change. Why make different circuits? Now, all we need is a source for those clips. There are 2 sizes, and you can enlarge the opening to fit a big one, but not vice-versa. The Chevy gauge cluster uses really small ones you can't use. When I swapped my tach/gauges in, I didn't have another housing to pirate the clips off of. Good tutorial, I always forget to mention those clips, otherwise it's a straightforward swap.

While we're at it, maybe we should add something about using the intended tach wire from the connector. From a previous thread you mentioned it was a white wire.

By eyeballing the connector and following the printed circuit on the cluster, the white wire appears to coincide with the tach. I'm not sure if they coded any of the wires a different color for different years so I double checked.

In this pic I'm pointing at the white wire on the far left of the connector (pin 7). I tried tracing it behind the dash and it goes into a rats nest of wires. It appears to run through a loom of wires that were taped together from the factory. Although I like doing things the correct way, I'm impatient and would rather know if this is indeed the right wire before I have to remove the dash to find where it ends. I removed the headlight switch, wiper switch, under-dash valence panel, and floor duct trying to find it but I can't. It would be much easier to do a little cut and splicing rather than hunt for a wire that runs to nothing.

Also note I would've been S.O.L. had I tried to plug and play the donor cluster. The pin with the white wire corresponds to the damaged copper strip on the cluster.

 

[Bonnewagon]

Let me answer in this post, as no one will see the PM I sent G-Body_Vet. GM is very consistant with it's color codes, even across makes and models.The white wire is the tach wire and disappears into the harness. But I don't know how far it goes in the harness and I haven't had a harness apart to verify if it goes anywhere. So I just pull it out as far as I can, clip it, and solder a new wire to the white and feed it through the firewall and on to the distributor. I checked in my manual for the firewall bulkhead connector and there is no cavity for the tach. So I assume that since most cars did not have a tach, they did not continue the white wire to the bulkhead. They must have spliced in a wire (brown, I think, right?) and fed it through the firewall some other way. There are several "not used" cavities in the bulkhead connector to use for the tach wire routing, but you would need a half-stripped car to get at it and insert the wire and pin. Anyone with a factory equipped tach car and who has investigated the wiring, please chime in as to where the tach wire routes and if it uses the bulkhead connector to pass through the firewall or what. Also as long as it is out, check that the tach is in the "V-8" position. On the rear of the tach, on the bottom, there are 3 possible positions for the "shorting bar". It's a plastic prong with a steel spring clip under it. Position #1 is for V-8's.

 

[pontiacgp]

I havn't had the need to work on the wiring in the cluster but I can say that it is a brown wire that comes though the firewall for the tach and that is the factory set up for the tach in my 86 and previous 85 gp.

 

[Bonnewagon]

Steve, where does it come through? My 86' Pontiac manual shows the white wire coming out of the big main harness just before the ignition switch connector, as it crosses over the steering column. It goes into "Connector 480" which is a single terminal connector that is taped to the harness right before the steering column. My guess is that the wire to the distributor connects there and then heads to the firewall. Perhaps since the dash is apart you can see the connector I'm talking about, or just splice in a wire as I did.

 

[pontiacgp]

It comes out of a small grommet on the firewall ...the grommet is left of center and about 6- 8" from the top of the firewall....I'm doing this by memory but I can take a run over to my car tomorrow to confirm it wit measurements if you need it...

found this...you'll see the brown wire from the tach filter going down and it goes through the firewall close to that location...

 

[G-Body_Vet]

Steve, where does it come through? My 86' Pontiac manual shows the white wire coming out of the big main harness just before the ignition switch connector, as it crosses over the steering column. It goes into "Connector 480" which is a single terminal connector that is taped to the harness right before the steering column. My guess is that the wire to the distributor connects there and then heads to the firewall. Perhaps since the dash is apart you can see the connector I'm talking about, or just splice in a wire as I did.

I went back out there and took a 2nd look and I think I found it! Does this look like a winner to you? I took a few pics so you could get an idea exactly where it is. It's pretty much in front of the brake switch.