LT1 and 700R4 - VSS solve (No dakota box needed) - Third Generation F-Body Message Boards

FBLT4 · 09-23-2009, 07:31 PM

For awhile I've had a 700R4 behind my LT1 in a thirdgen firebird. I used a digital dakota SGI-5 box to convert the VSS to the appropriate signal so the speedometer reads correctly, however, the PCM has had no VSS reference.

Just recently I was able to fix this. I got out my voltmeter and measured DC voltage across the VSS in my transmission and AC. DC didn't seem to do much but AC voltage I saw that it read according to my speed. Since I knew the LT1 PCM (16188051) expects an AC signal, I assumed this could be compensated.

Knowing the signal was the correct type, but just not the correct pulses, I connected it directly to the PCM. My MPH read between 0,1,2 and 3. I never made it to 4. That was good, it means the computer is seeing the signal and it is fluctuant based on my speed.

Looking at tunercat (www.tunercat.com), I was able to make a change to the input pulses. On the TOOLS menu, there is a SPEED CALIBRATION section. You can input your tire/wheel size and rear gear ratio (which you can find out from comparing wheel turns to driveshaft revolutions - search for this if you need to know how) and you can put in how many pulses your VSS is putting out. The VSS I have I got from a wrecker yard. I don't remember the particular vehicle but I'm pretty sure it was a firebird or a camaro. Don't forget the software wants to know how many pulses per DRIVESHAFT REVOLUTION, not pulses per mile. I programmed 4, put in my tire size and rear gear ratio, and it's running about half the actual speed that I'm going. Tomorrow I am going to program a 2 in there, because I am assuming I have a 2 pulse per driveshaft revolution sensor in my 700R4. I will post again confirming that this fixed everything.

Bottom line - There is a VSS out there that mounts into the 700R4 that outputs an AC signal which will talk to an LT1 PCM - and If you get one, then you can wire it directly to the LT1 PCM and program it's ratio correctly. I assume the PCM will output 4000 PPM for the speedometer correctly if it's correctly indicating the speed of the car. I haven't tried this yet and I will confirm this as well when I get to this.

I searched for hours and days and could not find a good clear explanation of how to correctly use a 700R4 with an LT1 PCM aside from posts talking about dismantling the entire transmission to get the reluctor ring on from a 4L60E. So I made this post, hopefully it will help others out there who are trying to do the same thing. I really want a T-56, but I need to find one and a good place to work to swap it in... In the meantime, I'll be tooling around with the 700R4, but at least now the computer knows how fast it's going and I will be able to see my speed as well without the extra digital dakota box

ALSO: Let me add if you have ABS and/or cruise control, it's best to leave the high settings and satisfy the PCM with the correct signal (40 pulses per driveshaft revolution for auto and 17 pulses per driveshaft resolution for manual). This higher resolution signal is required for the quick refresh rate and responsiveness of the ABS computer. Using a slower signal can be very dangerous and can cause the ABS system to operate incorrectly. Since I don't have ABS on this car it wasn't an issue for me. I just wanted the PCM to know the speed so it can be optimal for drivability and also so my speedometer shows correctly.

For the googlers: LT1 PCM ECU VSS 700R4 reprogram tunercat PPM calibrate speedometer