Hi, I'm posting here in addition to electronics, because I think the question has alot to do with the ECM/PROM.


Car:
Stock 88 Formula Firebird
305 TBI, Automatic


Problem:
I've owned this car for about 2 1/2 years and the speedometer has never worked.

I found out shortly after buying the car that the previous owner was an idiot with electronics. The radio wiring was a joke, he had the license light shorted out, and he also somehow blew out the original computer while trying to hook up a stereo amp (don't ask me how or why....there was all sorts of random sections of wire by the ECM).

In addition to that, he also removed the VSS Buffer (little yellow box) that the cars with electronic speedometers use (mine). I purchased a replacement directly from Pontiac. This did not solve the problem (but it was a step).


Question:
I always thought that since the car ran well, the junkyard computer he replaced it with must've been an exact match.
However, 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.

So, does this sound like I'm on the right track, and if so, what should I do?? You guys seem to be experts on F-Bodies, and nobody else up to this point has been able to help me out. If I'm forced to buy a whole new ECM from Pontiac, I'll have to pay through the nose, and I'm not even sure if that would fix the problem.

Please help me out.

 

Do you have a buddy with a comparable computer to what you may need....? If so, swap it out take it for a spin, if your speed'o suddenly blazes to life... You know you wont be wasting your money on a shiny new one...

 

Unfortunately, no. My good friend had an 87 for a year or so while I had mine, but his was carbed with a cable-driven speedo.

Third Gen F-Bodies are getting a bit less common around here.

 

On the TBI cars the ECM has nothing to do with the speedometer. The VSS feeds the buffer box (yellow) . The buffer box has four outputs: speedo, ECM, cruise and a spare.

RBob.

 

So even if I had the ECM out of a Camaro without an electronic speedometer it should still work??

I can't think of any other explanation. I bought a new buffer directly from Pontiac and it didn't work. Then, I ran wires directly from the buffer to the back of the gauge panel according to how this site says, and still nothing.

I'm not disagreeing with you (you're more knowledgable than me), but if the ECM has nothing to do with the speedometer than why does the buffer have outputs for it??

HOWEVER, now that I think about it, the SES light comes on at approximately 50mph and gives me a code 24 (VSS Circuit), which means that the ECM is looking for a signal and not receiving one.

What else could the problem be....the sending unit on the tranny?? A broken wire between the sending unit and the buffer??

 

With the ECM reporting a VSS error then it too is not getting a signal from the 4-out buffer box.

Yes, the sending unit or wiring from it to the 4-out buffer seems to be bad. It is difficult for me to explain to someone how to troubleshoot. I can troubleshoot when I am there, difficult to explain.

I would start at the VSS sensor in the transmission, double check the wiring to the 4-out buffer. Just a visual check first. Then check the continuity of the VSS sensor.

If not then remove sensor and check that the gears are OK. It is possible the sensor is bad. Spin the sensor by hand and see if a voltage appears.

If have that then should have the same signal at the input of the 4-out buffer.

If have the AC signal to the 4-out buffer then check for power to the 4-out buffer.

RBob.

 

I'll have to get the car jacked up to be able to get a good look at the sending unit, since these F-Bodies are so damned low.

I removed the passenger side hush panels and tried following the yellow and purple wires from the buffer back to the sending unit, but they take a strange route back towards the drivers side and probably behind the gauges. I lose track from there and haven't got the time today to dismantle my entire dash.

Does anybody know where these wires exit to go to the transmission?? If I could find that point, I'd strip a thin section, then test for continuity between there and the connection to the buffer. Would save alot of work.

 

In the tail shaft on the drivers side. you cant miss it since its about a 1-1/8 inch wide.

Kat

 

Well, that would've been my next question as I'll be checking that out later, but I meant, where do the purple and yellow wires from the sending unit enter/exit the interior??

Thanks for all the help so far everyone.

 

I have a similar problem my 89 iroc has 24 code and speedo just sits at 0 and the only thing I can think of is vss buffer but I can't find it can anyone help I've had the whole dash off and I can't find it Please help

 

In this vehicle I think that the speedometer is cable driven. This means the VSS buffer is attached to the back of the speedometer.

Look for a small lime-green box someplace near the speedometer, it will have a molded three wire set going to the back of it.

May want to first check that this vehicle has a cable driven speedometer. Look at the driver/ rear of the transmission and check for either a heavy cable exiting the trans (cable driven) or if it has a VSS with two wires plugged into it (will then have a buffer box under dash on passenger side).

RBob.

 

They come in through the C100 connector in the firewall in the front drivers-side corner... Big honkin bunch of wires going into a large plug packed with some evil tar-like substance. Turns out that was the issue with my '91 speedo not working after swapping out the tired TBI for an '89 TPI motor and harness... 89 top 91 the VSS wires change locations in the plug.

Hope this helps!!

-Brad

 

I am doing this exact same swap, switching from a 91 tbi to a MAF tpi. I made the mistake of buying a camaro harness and found out it doesn't have the vss hookup. I am getting a gta harness with the vss on it. Can you tell me the exact location on the C100 connector where I should relocate the vss wiring? I've been asking this question everywhere and no one has answered. Thanks so much.

 

I don't remember the actual pin locations... I found out where mine whent though by setting my DMM to ohms and then just probe between your purple at the VSS buffer and the firewall side of the plug (not the plug itself), same for the yellow wire. Those pins are an absolute PITA to change though... I found it was easier to use the poriginal 'donor' harness from my '91 to extract the pins from. Hope this helps, and feel free to holler if you have any other questions and I'll do my best to answer them for you =)

-Brad

 

So technically all I need to do is take the vss from my tbi harness and put it where it's supposed to go on my tpi harness and that's it? I don't need to do anything else to my harness or buffer??