Okay, I'm just about done with my harness swap. I have the harness re-loomed, re-located, and all done up, the knock sensor has been replaced, I'm running a MEMCAL from a 91 Grand Prix for my VATS delete, and pretty much all I have left to do is to get my speedo working so I can get back to the yard so I can nab the wiper motor from the 91.

I'm stuck with the speedo. Currently, I have the purple and yellow wires coming off of the VSS in the trans tailshaft running to BOTH the ECM and my stock speed sensor buffer. Both the ECM and the buffer see the same voltage signal from the VSS (VAC pulses). However, with both modules connected (I have a second set of wires spliced into the ECM harness for the signal run to the buffer box), I have no outputs at all. If I disconnect either one of the 2 modules, the other works. This leaves me with 2 choices--either I run without knowing how fast I'm going, or I run with a code 24. I'd just like it if everything worked, primarily because I have an intense dislike for idiot lights shining in my face when it's unnecessary.

Any thoughts as to how I can run the VSS signal to both? I CANNOT use the speedo output from the ECM because the speedo doesn't work if I connect it to the green/black output wire from the buffer box (without the box) for some reason.

 

Try reversing the polarity on one VSS input.
Maybe GM changed the polarity between the '730 ECM and the buffer box (which is normally fine since nobody has used them together so far) which would short out the signal if both were connected.

If that doesn't help, I'd recommend going the "right" way. I.e. removing the buffer box completely and using the outputs from the ECM (which is what they're for) to run the signal to the cruise control module and the instrument cluster (thought C207).

Here is the schematic: http://www.austinthirdgen.org/mkport...F_and_vin8.jpg

Hope this helps.
Lou

 

Enough said. That was the first thing I tried (yes, I completely removed the buffer box for that), and now my odometer is half a mile behind (work and back for one day). The gray wire coming out of C207 terminal "D" is the speedo output for the 91, which does the same job for the 91 as the green/black wire for the 87, supposedly. I had the cruise hooked up as well, but I wouldn't know if that worked or not because I don't have a working cruise switch at the moment to turn it on.

Yellow to yellow and purple to purple (VSS inputs to the buffer and ECM, either harness) should be the same, right, seeing as how I still get the same VAC response out of the buffer VSS input whether it is connected through the 87 harness or the 91? If I connect up the 730, the signal doesn't change, either. But, when I connect the C2 connector to the 730 (the VSS input wires are in C2), the buffer stops sending the signal to the speedo head, and that is what makes no sense.

 

It is entirely possible that the VSS signal is not strong enough to hold when you connect the ECM and the buffer box to it at the same time. What does the voltmeter show at that point? Normally, the VAC reading on the yellow and purple wires is a few volts, correct? What do you read on the same wires when you connect both, the ECM and the buffer box?
Lower voltage? Or plain zero? That would tell us whether it's a weak sensor or internal wiring problem.

Lou

 

:edit: Tried reversing the wires between the ECM and the buffer as you suggested (buffer yellow to ECM purple and so on) and now both are hooked up and working. I owe you a ton, man. Now, if I could get my alternator fixed and see if it runs, I'll be sitting pretty.

 

Why does that not surprise me??

Good old GM, always screwing things up. No wonder they're going under.

Glad you found it, I'll make a mental note to myself never to trust GM diagrams again.

Lou

 

Well, I figured I'd try it, just for grins. Turns out it works.

Another thought: Maybe that's why the ECM wasn't sending out a speedo signal, either. GM switched to a different VSS PM generator somewhere in there, I guess (I know the thing itself looks different, has a different connector, and has a different part number from the one for my 87). Maybe that one is wired backwards from the old one, similar to the pickup coil in the dizzy on some engines and some years? Either way, I couldn't use the one from the donor car, seeing as how the 700R4 uses a VSS that's around twice the size of the one in the T5.

 

Does anyone know if my speedometer and cruise control will work if I removed the ECM? I swapped the TBI for a carbureted engine. There is a brown wire that runs from the VSS to the ECM. Why does the ECM need this wire? Any help would be appreciated. Thanks.

 

I replied in your own thread.

Lou