: 90 Dash install on 78



Wheelman322
07-04-06, 11:12 PM
Anybody know if the dig. speedo from a 90 FWB will wire up to a 78 analog harness. Also wondering what it would take to install the digital climate cluster as well. I might 90 out the whole interior. Any suggestions/advice would be great.

Junkman
07-05-06, 12:43 AM
Can't see how you could get it to work without a lot of work and knowledge. The 1978 uses a driven cable to turn the speedometer, and the 1990 uses electrical impulses going from memory. Where will these pulses come from? I don't believe that it would be worth the amount of work to do this task successfully. I would just start with the year that you like and customize the other areas of the car, but when it comes to the electrical system, they are just too much of a pain to do much with them.... Junk....

N0DIH
07-05-06, 12:48 AM
I think it would be a huge task, but not impossible, but not easy at all.

Don't let it discourage you if you REALLY want to. Get the FSM's for both, that is key. And start to study the wiring.

Broughamadrosis
07-05-06, 09:21 PM
The digital display uses a Vehicle Speed Sensor (VSS) that mounts in the same hole in the trans where the speedo cable drive gear would normally go, driven by a similar gear - so the first question(s) would be if the hole is the same size in the 700R4 and the Turbo400, if the gears will mesh, and if the ratios will jive. A parts book might be a good idea along with the service manuals that NODIH suggested - I think the VSS part number depends on the rear axle ratio.

Another consideration is the Cruise Control on the 78 - it depends on the speedo cable to sense vehicle speed. If you delete the cable, the cruise won't work.
One way around this might be to attach the digital VSS to the end of the cable at the speedo head end.

The VSS is connected to the VSS Buffer, which is a box under the dash that sends the signal to various systems that measure vehicle speed - ECU, speedo, P/S variable assist, cruise control...

One way or another, it could likely be done. The mileage is stored in the cluster memory, so try to find one that's close in mileage to your car.

The fuel gauge is also in the cluster, so you'd need to get a fuel level sender from a digital car - but the assembly also includes an electric fuel pump, and there's no way to know if the sender/pump assembly will fit the mount in the 78 tank, so you might have to swap the tank, and bypass the pump with a regular pickup tube. Another option might be to retrofit the digital sender electronic bits to the 78 sender mechanism...

As for the Climate Control - you're on your own there. I know the CC is tied into the FI ECU, and relies on a signal from the ECU to power the A/C compressor clutch. The ECU will cut out the compressor if the coolant temp is too high, or if it senses WOT. I suppose you could bypass that circuit, and hook the CC head output right to the compressor relay. As for the rest of the system, I think it would take a lot of research, parts, and labor to convert the vacuum system to electronic. You're looking at replacing most of everything behind the dash - and some stuff on the engine-side of the firewall. A parts donor car would definately be in order.

The factory manuals do a very good job at expaining how each system works, so it's going to require hitting the books.

Phantom&RedGoat2004
11-16-06, 03:33 AM
I'm considering an '80 Coupe that I would like to update all the 90's goodies onto. I find this very interesting. I would certainly go for an entire (possibly totalled) parts car as well as a good library of service manuals. I think the key here is to get the 350 computer to communicate with a 425. I know the 425 had TBI, but would it be feasible to get a super-human tuner to tune that out, and re-fit all the sensors to suit the 425? I guess I should do a little homework as well. I hope that you will take on this project and post more on it. PRG04

N0DIH
11-16-06, 09:04 AM
VSS may be the interesting problem, GM changed the VSS rate somewhere along the way, it was like 1001 pulses per mile, not it is like 2002 or 4004. Might be easy to make a leading edge trigger to double it and/or a leading and trailing edge trigger to make it a 4x. Need to dig up the details for sure.

The 80's cars often uses VSS off the dash, a simple optical sensor was picking up from the speedo. So you would have to break in and interpret that to make it work. Could be done. Or, if the PCM is recent enough, just adjust internally to make it think it has 1.54:1 gears or so when it has 3.08's, etc Might be a quick and dirty way to make it work...

Good info Broughamardrosis!


The digital display uses a Vehicle Speed Sensor (VSS) that mounts in the same hole in the trans where the speedo cable drive gear would normally go, driven by a similar gear - so the first question(s) would be if the hole is the same size in the 700R4 and the Turbo400, if the gears will mesh, and if the ratios will jive. A parts book might be a good idea along with the service manuals that NODIH suggested - I think the VSS part number depends on the rear axle ratio.

Another consideration is the Cruise Control on the 78 - it depends on the speedo cable to sense vehicle speed. If you delete the cable, the cruise won't work.
One way around this might be to attach the digital VSS to the end of the cable at the speedo head end.

The VSS is connected to the VSS Buffer, which is a box under the dash that sends the signal to various systems that measure vehicle speed - ECU, speedo, P/S variable assist, cruise control...

One way or another, it could likely be done. The mileage is stored in the cluster memory, so try to find one that's close in mileage to your car.

The fuel gauge is also in the cluster, so you'd need to get a fuel level sender from a digital car - but the assembly also includes an electric fuel pump, and there's no way to know if the sender/pump assembly will fit the mount in the 78 tank, so you might have to swap the tank, and bypass the pump with a regular pickup tube. Another option might be to retrofit the digital sender electronic bits to the 78 sender mechanism...

As for the Climate Control - you're on your own there. I know the CC is tied into the FI ECU, and relies on a signal from the ECU to power the A/C compressor clutch. The ECU will cut out the compressor if the coolant temp is too high, or if it senses WOT. I suppose you could bypass that circuit, and hook the CC head output right to the compressor relay. As for the rest of the system, I think it would take a lot of research, parts, and labor to convert the vacuum system to electronic. You're looking at replacing most of everything behind the dash - and some stuff on the engine-side of the firewall. A parts donor car would definately be in order.

The factory manuals do a very good job at expaining how each system works, so it's going to require hitting the books.

The Ape Man
11-18-06, 04:02 PM
If you have the time and $$$ then find a 4L80E and PCM from a diesel vehicle and change your TH-400 and rear axle. The cluster will be easy then. The HVAC is exactly as described by another poster. DEFI RWD cars used the compuker (ECM) to enable the climate control but they worked on HT-4100 transplants with NO sensors left at all. BTW, the older climate control system is just as good as the later when it is working properly.