my speedo died after my swap from a 91 camaro rs tbi to 89 maf tpi, i think it worked for my first test drive then died, wasnt really watching it though.

i put tuner pro on it and found that it still is reading mph and tested all the leads from the buffer box but found that im not getting any signal from the output on the buffer to the speedo.

is this a normal thing to die on the buffer? just figured if one output died the whole thing would be dead not half working.

doesnt look like theres many parts inside i cant decide if i should try to fix it or just buy new, hate to buy somthing thats not the problem.

thanks

 

okay so found i had another buffer box so i changed it, theres no change, still no signal on the brown wire to the speedo. i think it would be a strange coincidence for both boxes to do the same thing.

i also pulled the cluster to check on the speedo there and tested for continuity on the signal from the buffer - it checks out

the small pc board inside the cluster for driving the speedo smells like its burnt and there is corosion on the ic pins not sure if that could be a problem too nothing looks fried/burnt just coroded.

im still stumped on the fact theres no signal from the buffer box, debating on buying a new one going to try another used one when i can find one as well as a new cluster hopefully tomorrow.

if anyone has any ideas to what is up with the buffer please feel free to chime in your $.02

 

you probly already fixed it but if not theres an artical on that swap you done tbi to tpi on here somwhere I think and it talks about you having to change some stuff. Your car has an elictric speedo and the 89 used the one with the vss mounted behind the cluster. The artical might be under swapping in an electric speedo. That one might help but you need to try to find the one where they swapped the tbi to tpi. Good luck...

 

The buffer box has its own VSS wiring from the trans so original VSS drive wont make any difference

Just a random thought, but somewhere in the earlier TPIs the VSS wires switched positions. Odd though, a signal error should null all 3 signals, not just the speedo one

 

I've seen stranger things happen !

I assume that you already installed the proper speed sending into your tranny already. If the Buffer box is receiving the signal from the electric sending unit on the tranny, but is not sending a signal; than the Buffer is still suspect IMO.

 

Can I assume that since you mention MAF instead of SD, you pulled the 730 ECM and put in the 89 165 ECM? If so, as I understand it, the 165 ECM was not designed to work with the electric speedos. If you still have the 730 in there, I don't believe that the 90-92 TPI systems used a buffer box.

Not an expert, have just dug around working on a similar issue, so may want to wait for one of the more seasoned folks to verify the above. Just thought it might help.

Good luck!


References:
No electric in 89 - https://www.thirdgen.org/forums/tran...echanical.html
No buffer for 90-92 TPI - https://www.thirdgen.org/forums/elec...em-1991-z.html

 

In my case the car is a 91 RS so it was a 305 TBI equipped car. From my understanding all the TBI models use a electronic speedo to a buffer and the buffer splits the signal 2k and 4k pulse - 3 signals are outputted cruise, speedo, and signal input for ecm.

The 89 '165 ecm MAF I used uses the vss speedo input from the buffer, if i went with the '730 ecm SD then the buffer is not required as the ecm has direct input from the vss/trans and output from the ecm for cruise and speedo.

My problem was as I eventually figured out was the actuall speedometer, the IC that interprets the signal fom the buffer was corroded and I think it may have back fed the buffer basically shorting it - its just a thought though. So i changed out the speedo section of the cluster and changed the buffer and walla it works.

-3rdgengmachine

89 MAF system uses yellow buffer box located next to the ecm same as any TBI system

-Pocket

I was on the same train of thought, after replacing cluster and buffer everything is working - the buffers are the same PN for both the TBI and TPI
still strange how just the one output was dead.

 

just cuz we all like pictures

 

Nice looking engine!

 

The 165 ECM was used with both speedo types. Birds were electric, camaros cable. 90-92 730 ECMs (V6 and V8) did not use the buffer, only TBI use it in those years

Ive found buffer boxes as far back as 86

Strange that it only cooked one output, glad you got it fixed

 

I have a mutt project where I will be keeping a mech speedo with a tbi 305/200r4 combo what I need to know is what is the output of the vss before the buffer 2000 or 4000. I was curious if the buffer is an amp, providing the 4000 output for the speedo or is it spliting the signal
Thanks guy, lot of great info here
sorry that would be 91 bird

 

FYI: 1984 thru 86 Camaro Berlinetta was the first Thirdgen to use the Speedo Buffer assembly and electric speedo sending unit.

 

interesting topic cuz I have a similar problem.

I have a 92 RS Vert Dropped in a 383 with a 92 TPI harness and 7730 ecm.
Different 700r4 with the correct sender.

I can get a MPH reading via data logging thru the aldl but my speedo doesnt work.
If Im able to log mph wouldnt that mean the vss buffer is ok? and the speedo itself would be bad?

OR does the aldl data pull right from the ecm?

 

I would think that the data you are seeing is from the ecm and you may have either a bad output to the speedo or a bad speedo

 

I cant remember if the input is 2000 or 4000ppm. It doesnt really matter because you cannot change it

Outputs are 4000ppm to the speedo and 2000ppm to the ECM and cruise module. The 730 ECM doesnt need a buffer box because it does that function internally. The VSS is wired directly to it and it outputs directly to the speedo and cruise module from the ECM

http://www.austinthirdgen.org/mkport...F_and_vin8.jpg

Cool, I did not know that. Then again, berlinettas are not so common in JYs

 

I though the 90/91/92 vss buffer was right on the back of the cluster.

how can I test the ecm output? Can it be done with a DMM? to read the pulses?

time to just out my flow charts and get a wire hunting.

 

in response to tpi383.. i don't belive the buffer is an amp. on your ecm pocket is correct you don't have a buffer for it just straight into the ecm and straight out from the ecm to the gauges, if your data logging/can see speed read out from the aldl the ecm and aldl are functioning correct sounds like you may have a bad speedo. do as i did and just change out the board with the ic on it inside the cluster, you will need another speedo to swap the parts from.

 

just re-read your original post. was your engine a tbi engine i believe the rs should have been, if so did you swap all the wiring including the wiring under the dash because the tbi engine would have had the buffer box - if you re-pinned the existing interior harness than you buffer will still be inline and would be causeing confusion on the speed pulses, does the cruise work? the cruise would also be connected to the buffer and on your 730 ecm the cruise should be connected to the ecm. the aldl would be giving the correct speed readout because its connected direct to the ecm.

if you try to use a dmm the readout looks like jibberish just an fyi, even when using the pulse function on the dmm.

 

Not sure what tool to read a VSS signal with. It puts out ALOT of pulses each revolution

His RS could have been V6 or TBI. V6 will run the speedo correctly from the ECM as it uses the 730 just like the TPI. TBI will have to be bypassed

http://www.austinthirdgen.org/mkport...SS_V8_vinE.jpg

Connect buffer pin F to pin C. If the speedo functions correctly, then run the cruise wire accordingly

 

It was a 92 RS Vin E 305 tbi.

No i didnt repin the under dash harness.

Just plugged in my 92Z harness.

Looks like I'll jumping F-----C , putting my 3bar map and 50lb injectors in and going for a test ride this weekend. Time for some boost. !!

 

Unplug the buffer, insert a jumper wire and test drive. If the speedo works, solder it in

 

question. My car is a 91 firebird, originally a 305 TBI. The guy I bought it from put in a 350 TPI. My speedometer doesn't work, the tach works fine though. I know that i have no buffer box in this car. Where should i start testing or looking? I have a multimeter. I thought that my vss may have gone bad, but how do i test it? at the actual vss on the trans? or should i do the wire test at the ecm. (B9, B10, and B11) ?

 

91 TBI will have a buffer box from the factory

Also, the speedo connection in the C207 is different between TBI and TPI. TBI will be on pin K while TPI will be on pin D. You will also have to bypass the buffer box by connecting the pins C and F. This will give you a strait shot from the ECM to your speedo

The buffer box is located on your dash above the pass side hush panel. Once the panel is removed, drop the front trim panel from the dash and the buffer box will be the left most module, either white or yellow

 

i dont see a buffer box. cant i connect a wire from the ecu to the speedo? if so explain

 

sorry, i think i need you to explain in more detail. if im looking at the ecm then exactly which wires do you want me to try and connect.?

 

You can strait wire it from the ECM yes. The effort was to run it through the existing wires to minimalize intrusion

Changes are in the C207 and buffer box connector

 

ok got it but can u tell me again exactly which wire on the ecm and where or what color is the wire to the speedo unit that i am going to wire it to? http://tpiparts.net/1227730_pinout_diagram check that out and tell me where to wire it to

 

forgot to add i am using the 350 tpi ecm & prom w/ my 350 tpi motor. not sure what wiring harness is in the car. Maybe the 350 tpi one or could be a tbi harness

 

C1 B11 drives the speedo

C2 C1 drives the cruise module

 

Speedo not working either.How do i test for a bad vss or test for a signal?

 

 

Hey Pocket, I know it has been like a year since i attempted to do this , but could you explain again how to wire my speedometer to the ecm directly again. My car is a 91 tpi with a mechanical looking speedometer and no speedo cable.LOL You were saying to connect pin C to F? or something like that and it would be a straight shot to the speedometer? Please let me know

 

Pocket, removed buffer box connector and put a jumper in between the two brown wires(pins C and F). Still no speedometer.Any ideas?

 

Figured it out! forgot to also move gray wire on pin D in the c207 to pin K.