Sorry for the length of this post. I’m sure I’m making this all harder than it needs to be…
Everything I’m writing here is based on my present understanding of information I’ve read from several forums so far. PLEASE correct me if I am wrong! I really want to understand this stuff...
My project is based on a modified 7427 PCM for MPFI use. The engine for this project is a 489 BBC with a 4L80e automatic transmission. I will be building my own intake manifold for this project inspired by the look and feel of the earlier TPI systems. It’s all getting installed in a 1948 GMC 2-Ton flatbed truck.
I need some help getting clearer in my understanding between what some call the DRAC (1992-1993?), and what some call the VSSB (1994-1995?). It’s important for me to also say my questions concern only GM OBD1, the DRAC/VSSB modules that are a little white plastic stand alone box, and NOT something that’s built into the back of the speedometer. I’m also only concerned with the model of DRAC/VSSB that receive a 40 pulses per revolution output from the VSS commonly found in the 4L60e/4L80e automatic transmissions.
It’s my understanding the earlier DRAC’s have 7 possible jumper locations on the circuit board for calibrating the speedometer with a known tire diameter and rear axle ratio. The later VSSB’s have 14 possible jumper locations on the circuit board for calibrating the speedometer with a known tire diameter and rear axle ratio.
I’ve also read the later 14 pin VSSB’s can be used in place of the earlier 7 pin DRAC’s, but not the other way around. What I don’t seem to be able to find or understand is why. I’m guessing it has something to do with the word “Buffer.” It’s possible the earlier DRAC’s do not have a buffered output? If it’s that simple, I could possible create an optically buffered output for the earlier DRAC outputs?
You might be thinking, “Why not just use a later VSSB for your project David?” That would be great! Except I’m not sure what I’m looking at when making purchases on ebay. So far all the DRAC/VSSB’s I’ve purchased are turning out to be the earlier 7 pin DRAC’s. It’s not practical for me to visit a Pull-A-Part yard because of where I live. If possible, I try to get everything I need from an automotive parts supplier, or places like ebay and Car-Parts dot com.
What I would love to understand is if the earlier 7 pin DRAC will work with the later 7427 PCM? If not, can anyone help me find a published list by GM (or?) that I can lookup the three letter codes commonly found on these DRAC/VSSB modules. That way I can get an idea of what type and year of vehicle they came out of before I hit the “Buy it Now” button.
Anywho, thanks in advance for your thoughts and help with this.
David
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