I am swapping in a 87 L98 into a 91 firebird that had a L03. The 87 used a cable. The 91 is electronic. How do I get the speedo to work?

 

are you swapping the trans also? If not you wont have to do anything. If you are swapping the trans just swap the tail section and the correct parts off the output shaft

 

no trans is from the 91.

 

bump

 

Which trans are you using?

 

using the 700 that is in the 91 now

 

Then, nothing

 

got some good info here from a fellow third gen owner thought id pass it along:

Locate your yellow buffer box, there should be a yellow wire on the bottom and a purple wire. These two wires go to your vss sensor on the speedo. Ok, unplug that altogether. Next , you have a brown wire going from the speedo to the buffer box.Ok, Locate the other brown wire near it and notice that IT goes from the buffer box toward the ecm. It actually goes to a clear plastic box called the c207 connector. Ok, cut and connect both brown wires at the buffer box. This will give you a straight shot from the c207 to the speedo. Ok, now look at the c207 connector. It is located near the ecm under the dash. You can't miss it. Notice where the brown wire comes out of it and goes toward the speedo. This is the same brown wire you cut and connected at the buffer box. Now locate a grey wire on the other side of the c207 connector. Grey wire should be on pin D labeled on the c207 (ecm side of the connector) Now again notice the brown wire on the other side of the connector , it should be on pin C. You will need to open the connector so the wires can be taken out and moved etc. Again notice the brown wire, the other side of the connector should have nothing in it (no wire on the other side) Move Grey wire so it joins the brown wire when you push both sides of the connector together. You will understand when you look at it. Basically you are moving the grey wire to a different pin slot. So when the connector is joined together the brown wire and grey can connect and communicate. Grey wire is speed output from ecm to the speedo. Now you can
ditch the yellow buffer box altogether.

 

Good info.... but if the trans that has always been there is going back, and the speedo worked before, then nothing at all has to be done to keep it working.

The buffer box isn't hurting anything.

The VSS doesn't care what engine is in front of the trans.

 

correct if you're using the same trans you already have there's no need to change anything, even if you were to swap the tranny's all you need to do is swap the vss and speedo gear's from your current trans into the 87's in place of the adaptor that the cable goes into, they both go into the tailshafts in the exact same way and are held in by a single 10mm bolt, just make sure if you are using a differen't rear gear ratio from the donor car or are unsure u compare the number of teeth on the 2 gears and adjust or keep the same accordingly (Basically what I'm trying to say in a less confusing way is they are exactly the same except one has a hole to bolt a cable to and the other has a small sensor with a place to hook up a connector to so don't overcomplicate things)

 

This information is incorrect. An '87 MAF TPI ECM can not buffer the VSS and send the buffered signal on to the speedometer.

The MAF TPI cars with the 4-pole magnetic VSS in the transmission used that same yellow (sometimes white) buffer box that your stock '91 TBI car has.

RBob.

 

the problem is on the 91 the speedo is electronic and goes to the ecm them to the cluster, where the 87 ecm dont support a electronic speedo and uses a cable

 

the speedo will work. Your ECM wont
you will have to convert the speedo signal from 4000ppm to 2,000ppm (or the other way correct me if wrong) and run it to the older ECM.
or you could use your current TBI ECM and use a SD TPI set up. There used to be a company that made a TBI to TPI adapter harness for trucks. Street and performance?
or you could use your computer and splice in the maf harness injs and lengthen the IAC TPS, a few other things and swap chip/pump to TPI
I think your over thinking the problem, I do it to sometimes ..
GM service manuals for both years will help ya tons.

 

thats interesting, so your saying have 2 ecms in the car, run power to the original ecm from the tbi setup but let it only work the speedometer?

 

You may have missed my post above. The MAF TPI ECMs that used the magnetic VSS in the transmission used the same 4-OUT BUFFER box that is stock in your car. And yes, there are MAF TPI f-bodys that used the mag VSS in the transmission.

No need for two ECMs. Neither the stock one (1228746) or the MAF one (1227165) will buffer a magnetic VSS. Hence the use of the 4-OUT BUFFER box.

(note that I upper cased the 4-out buffer as that is how GM printed it on the label).

RBob.

 

not magnetic but my 1991 has a gear and a electrical connector where the 87 uses a speedo cable

 

How did this thread even get past this succinct post?

 

No, your '91 does have a magnetic VSS.

And, it doesn't matter what the '87 has for a VSS. The 4-OUT BUFFER supplies the correct VSS PPM signal to the ECM (2K PPM digital).

And if this isn't catching on, then swap your whole system out for a cable driven speedo and leave us alone... Because you obviously know more than all of the other posters in this thread.

RBob.

 

um, i pulled it out a couple of weeks ago to look at it. it has a gear at the bottom and a electrical connector ontop...

i held it in my hand


thanks for the attitude tho appreciate it

now take a hike

 

> ...and a electrical connector on top...

Correct, which makes it a 4-pole magnetic VSS.

Best of luck with your swap. Bye.

RBob.