Hi guys. I currently own an 88 Formula Firebird (stock, 305 TBI Auto) and have had it for about 2 1/2 years now. The speedometer on this car has never worked. I've tried many things, read up everywhere I could find info, and asked lots of people for help. I had pretty much given up on it.

Well, the time is coming soon that I will be selling the car. While I personally may have been able to get by without the speedometer, potential buyers will probably be very turned off that it doesn't work, and I'm sure I'll allready have enough trouble selling it in the winter.



Here's a little more info on the problem:

The Formula model for 1988 was (un)lucky enough to have an electronic speedometer.

I found out shortly after buying the car that the previous owner was a complete idiot when it came to electronics. Among other things, he self installed some lousy amps and somehow managed to blow out the brainbox (computer). The one in there right now I can tell is out of a junkyard, but I figure it must be the correct one for the car to still run well. Why he was messing in that area for an amp is beyond me.....there were all sorts of random sections of heavy gauge wire left back there.

I also found that in addition to blowing out the brainbox, he removed the little yellow box (VSS Buffer I believe it's called) that the electronic speedometers use. I purchased a replacement directly from Pontiac, and snapped in the wiring harnesses that were left hanging loose. This didn't fix the problem.

After that failed, I suspected the computer. I read a few articles on installing electronic speedos in older, cable speedo Firebirds which wouldn't have a computer that supports it. What they did, was run wires directly from the yellow box (VSS Buffer) to coresponding pins on the back of the gauge panel. I tried this, and had no luck.

There IS a sending unit on the trans, and also wires connected to it.


If anyone is able to help me fix this problem I would like to personally call them up and thank them. It would mean that much to me. This has been such a long term aggravation.

 

I did alot of thinking this evening, and then I had this disturbing thought: I mentioned that the previous owner put a junkyard computer in there. I always thought that since the car ran well, the computer must've been an exact match.

Well, after reading around on here and taking a closer look at the data sheet, I realized that if he pulled this computer from an 88 or 89 TBI Camaro, that car wouldn't have had an electronic speedometer, because Camaros didn't get them until 1990. It would still probably run though, because everything else would be the same.

I dunno.....you guys are the experts

 

You have major problems unfortunately. Without a VSS signal the car should not run properly and it should have a constant SES light. VSS controls more than the speedo. It operates TCC lock, and it also controls EGR operation. That being said he may have somehow screwed with the prom. At the very least you will need a good diagram of the circuit and the actual number from the ECM. There is a cross reference on the site here with correct part numbers. If it's as bad as you say you may need the help of a local shop because you will have to start from scratch on this one.

 

Ouch, that doesn't sound good .....I also posted in the PROM section and got a few more responses there.

I was told that on TBI cars, the ECM has nothing to do with the speedometer, but I'm still a bit confused on how the whole thing works. I know the buffer has outputs that go to the ECM, but like I was told, I'm not sure if the ECM is entirely necessary for it to work because this site explains how to install an electronic speedometer on an older car.


Heh....he didn't just screw with it, he threw out the entire ECM and got another from the junkyard.

However, you're right about the SES light....it does come on, but only when I hit about 50mph or so. The fact that it does this tells me that it might be the right computer, because it's looking for a signal and not getting one. Though I could be wrong.

I'm unsure what to do. I don't really know anything about ECM's or PROM's, so I'll need all the help and advise I can get. Later today I'm gonna check and see if I can find a break in the wire between the sending unit and the buffer, but I doubt he messed with that, and if it was anything obvious I probably would've allready seen it.

As for the TCC Lock (torque converter?) and EGR.... I dunno, I'd like to get this whole thing fixed but don't know where to start.