Question: What is needed to make the 140 MPH cluster(1988 GTA) work?
I seen mention of a VSS buffer and some converter box......???
I dont have the mentioned yellow buffer box, as my car was originally cable drive.

My platform(originally): 1985 T/A LB9, auto. 85MPH cable speedo. 6k tach
Current platform: 5.7 LS1/T56 140MPH 8k tach

I have pulled and repinned the cluster connectors to accommodate the '88 cluster. All the gauges work correctly except for the tach and speedo(neither are reading anything).

I was planning on running a wire from..... C1 connector Pin 14(speedometer Pin), Dont quote me because I dont have it in front of me. Anyhow, Run that wire to the VSS output from the LS1 ECU.

Then reprogram the parameters as mentioned in five7kid's sticky.

Sound good, in the ballpark, or not even the same sport?

As long as you hook the speedo wire to the LS1 PCM, then you should be in business. That's what I did with the LT1 when I put it in the GTA. As for the tach, from what I remember reading the LS1 tach goes on a 4cyl signal and is probably why the tach isn't working but I believe there is some way to change that in the PCM.

Hawks sells a box that should get your stock gauge working. You should hook up the VSS to the PCM if its not already its affects some tuning tables. It will probably run okay without it but you might pull codes

SO without the box from hawks , there is no way to get the tach and speed signal ?

I thought I read somewhere, the speed and tach signal vis vss comes directly from the LS1 PCM. wire that to the correct pin on whatever gauge cluster ur using it should work.

I also read that the earlier LS1's , there was already a wire for these 2 but on the later ones these signals were combined in a databus going to the LS1 cluster.

But regardless, the speedo and tach pins should be present on the LS1 PCM.......right ?

No other option currently available that I'm aware of

Its not so cut and dry. The stock fourth gen speedo does take a signal from the PCM but most aftermarket electronic speedos dont seem to work this way. In my case I run Pro Comp ultra lights and the speedo signal had to come directly from the VSS off the trans in order for the signal to read properly. I tried it from the speed out wire from the PCM but nothing happened. Easy fix though I just bi-wire spiced into the VSS out at the trans meaning I turned the wire into two leads and fed one to the PCM and one into my speedo and all works well. Tach signal again can be tricky depending on what gauge you use. You know now that the pulse is 4cyl but the signal from the LS1 PCM out is often again not strong enough to power many tachs so you need to add a boost feed. This is easy you just splice into the tach out signal a 12v+ routed through a 10K resistor and voila magic happens you have a working tach. Running a stock third gen gauge you will need to put a buffer box from Baker electronics ( or? ) to change the output to match your tach or you can also use HPtuners or equivalent to adjust the signal frequency via tuning it into the PCM itself.

The wires are there yes. No idea about a data bus cluster?

Yes they are there check the pinouts for your year PCM and you'll see them easy.

I know u explained a few option here. I just want to clarify I understand this correctly....

Speedo:
Does this work with only your Pro Comp ultra lights speedo or can the same concept be applied to Stock 3rd gen electronic speedo ?

Tach signal :
Are u also using another Pro Comp Tach ? I will be having a guy tune the PCM to get the car running anyway. Can I just wire the Tach out pin from the LS1 PCM to the Tach wire on my stock gauge cluster connector and tell the guy to adjust the signal to a 4 cycle ? (Anywhere I can read more about adj. the tach signal in the LS1 PCM )




On the side note
I read some where in my 82 service manual that the stock 82 tach signal is also applied a +12V switched supply. I may be wrong . so dont quote me on it.

And ya appriciate your patience in explaining this.

I'd like to know more about this signal frequency as well. My tach isn't working (yet).

Hmmm....maybe this (http://www.ls1tech.com/forums/conver...-tach-yes.html) is why. Might be worth the 99 cents to try I'll be trying this first before buying any aftermarket tach or box.

The stock GTA cluster will read at a different rate that the output of the M6 VSS so although this will make your speedo read it wont be accurate. IIRC this too can be corrected in the tune with HP ( efilive ) etc. or you can run the converter box.

Yes Pro Comp Tach. If you read that link posted below you'll see that some gauges work and some dont its hit and miss and seeing as I havent run a GTA tach I cant say for sure if the stock LS1 PCM signal will be enough to power it but regardless the pulse will not be correct and this must be changed again with HP or the extra box.

I think your on the right track now but if you get the stock GTA tach working without the boost feed post up about it.

Thanks for the replies. Good Info.

BTW, my factory 7k tach(incorrectly mentioned as 6k) worked great with the LS1 PCM. A little bit off but usable. IDLE: the tach read 1050 rpms

I did hook the 140 speedo to the PCM. Still no reading. I have a guy in Columbus that does PCM programming(I think HP Tuners) for a reasonable price, out of his home.

What should I tell him to double the pulse width for tach, and what for the speedo?

Thanks again.

Yeah tach should work aces if you double it as I know LS1 pulse is same as 4cyl so that reasoning sounds sensible to me. I'm sure if you search you'll find the VSS pulse counts for your stock third gen and the LS1 vss I know I've seen posts with those numbers somewhere. Then input the differences again using HP

After some research, it looks like 4000 Pulses Per Minute (PPM) is the magic number for the 140MPH Speedo to read. Next time I get a chance, I'll try some PCM adjustments and report back.

The output on the LS1 PCM should be already set to 4000ppm.

Are you sure that you have it repinned right? My '86 cluster didn't have the provisions for an electric speedo but swapping in a dash harness and cluster from an '89 it worked like a charm, and worked when I put my LT1 in my GTA which has an electric speedo.

I just wanted to add that on a +99 LS1 harness, the there is no pin on the C105 conn pin# G ckt#121 on the male side, its only for M6 cars.

There is another white wire on pin # H ckt#85 on the A4 LS1 harness thats not required.

I popped the rubber stopper off, moved the white wire over to pin # G and connected it to the white tach wire on the 3rd gen harness for Tach.

I have not tested it, but from what I am reading I should get some signal.