I seem to have put myself in an odd position. I'm getting an idea of what I did wrong and where made some mistakes. Which might explain why the PCM is reading so low as well.

In another thread pocket replied:

I had thought all T56 transmissions had the same VSS signal. But then I read the above post by Pocket and it begins to make sense as to why my speedo wiring isn't working correctly.

With the PCM directly hooked up to the Speedo it reads very low, but it wasn't until I used datamaster to read the actual speed the PCM was seeing and it too was low. I'd have to measure it again but I think the speedo is actually displaying the PCM speed signal accurately, and the problem is my VSS signal being too slow as compared to a later T56 VSS.

Is there a way to change the tune in my 95' PCM to account for the different VSS or should I just swap it out for a 94+ VSS and eliminate the SGI-5 box wiring.

I have a Dakota Digital SGI-5 box I suppose I could use to change this. Though it seems like it might be easier to go with OEM components.

 

The SGI-5 is fully tunable and should be able to do what you want. I have a 93 T56 and am using the SGI-5 and even though I'm not using a factory ECU I am using an electronic speedo and it reads dead on. I don't know what my dip switches are set to but I have the car at work today so I could run out and look.

 

I seem to recall a couple page pamphlet that came with the SGI5 box that reconstructed you how to set the dip switches. Did you not use this?

Edit;
http://www.dakotadigital.com/pdf/sgi-5c.pdf

It seems pretty straight forward to set...

 

The '93 LT1 components are different from 94-97 LT1s in various ways.

Speed sensor is not controlled by the pcm on a '93
'93 T-56s have a different gear ratio than 94+ T-56s do.
Doesn't have skip shift solenoid.
Here's the gear ratios
1993 3.36 2.07 1.35 1.00 0.80 0.62 (std)
1993 2.97 2.07 1.43 1.00 0.80 0.62 (G92)
1994+ 2.66 1.78 1.30 1.00 0.74 0.50

 

Also known as M28 (3.36 1st) and M29 (2.97 1st which is what I have)

 

I was using Application# 3 I think. I'll have to look at the instructions again.

I also have a Rev C SGI-5 so my dip switch settings may be different.

The Application I used to set it up specifically mentions newer GM transmissions.

 

I was aware of those differences, just not that the VSS had a different output frequency.

As far as transmission gearing goes though it doesn't matter in the slightest. The VSS doesn't measure the difference between the input and output shafts, it measures the speed of the output shaft. Tire size and axle ratio can affect the speed reading but the internal gears can't.

 

Again I an only running the speedo but it is dead on. I am using output 4 and 1 and 2 dip switches are down and 3 and 4 are up. It was about 15 mph to fast and I held the dn button until it matched my gps. Make sure you hold the adjust buttons and not tap them to adjust.

 

Its actually not straight forward. If it was, there wouldn't be so many questions about it without any real resolutions on these forums.

Dakota Digital's documentation isn't the best, many times you'll find in threads here that Dakota's documentation was incorrect leading to problems when attempting to set it up.

My case is slightly different in that the 93 VSS isn't what my 95 PCM is expecting.

No one has answered my question though, does a 94+ VSS phsyically interchange with a 93 VSS in a T56. The Part numbers are different on the websites, but then again they have to be because the PPR is different.

 

I would imagine the reluctor ring on the output shaft is going to be what's different. It's what actually provides the "teeth" for the sensor to read. If they're different part numbers, I would be surprised if they were interchangeable. It's just a magnet so it wouldn't NEED to be different from the later style unless the reluctor ring was at a different height or something. If you could get the reluctor and sensor it would probably work, but it's just theory from me. I haven't done it.

 

Oh I forgot you have an LT1. Thought you were using an older ECU. It would be a lot of work but I would change the reluctor ring from a 94+ unit. But the SGI-5 box is still required to run the speedo.

 

Well, I was reading through the PCM last night, and it turns out there is a spot to specify the PPR of the VSS. I altered that to 11ppr to match the 93 VSS then recalibrated the tire size and rear end ratio.

As result the PCM and Speedo now read correctly without the use of a SGI-5 box. So I'll be selling my SGI-5 box.