Right now I have a 1988 TPI setup...2 pulse VSS on the speedo of course, '165 ECM.

Going to be swapping to a trans with a 40 tooth reluctor, and a "matching" electronic speedometer, from the original cable drive in place now.

It's my understanding that even if you use an ECM with a "built in VSS buffer" ('730 for instance) you can't change the speedo output as easily as you can change the speed the ECM sees, which would cause problems with my electronic speedometer.

As I understand it, (the below makes it pretty clear based on the pulses) the 40 tooth output of the VSS can't just be fed directly to the '730 ECM, for instance.

If anyone isn't quite clear on the DRAC, I was able to find this:

ftp://ftp.diy-efi.org/uploads/DRAC2.pdf

On page 4, it lists the PPM on various pins. Now, if I'm not mistaken, with the optical "2 pulse" VSS that I have now, that translates to 2000PPM. (two pulses per cable revolution, 1000 cable revolutions per mile) So does that mean that if I add a DRAC, and use the 2000PPM DRAC output to the same pin on the '165 that is VSS now (A10 IIRC), and run the speedo off of the right pin on the DRAC, all my problems are solved? Can it be that simple??

I've got a DRAC and the speedometer already, so I'm (hopefully) mostly there.
Don't get me wrong, I'd rather not use the DRAC at all if possible, (more wiring, another component to fail, however unlikely that is) but since I have it, perhaps it would be best to just use it, especially if the '165 will work with it no problem.

 

I'm going to toss an off-the-wall idea out. To change the 40 pulse reluctor to a 4 pulse (or even 2 pulse) unit. Grind some teeth off. If 3 are gone and one left, then 3 gone, one left, that will be a 4 pulse unit.

{edit: lets try this: If 9 are gone and one left, then 9 gone, one left, that will be a 4 pulse unit. Not as I first stated}

You mentioned a matching speedo. Most electronic speedometers that I know of are driven from the ECM. As such the pulse train is 4000 PPM. This may just all work out. . .

RBob.

 

Rbob, thanks for the post.

In http://614streets.com/drac.html
it states "The DRAC receives the AC input from the speed sensor and changes the AC sine-wave signal to a DC square-wave signal."

In a post on this board, "The 165 ECM receives 2000 PPM DC current and the 730 ECM gets 4000 PPM AC current.".

On the DRAC2.pdf I linked earlier, it shows a multitude of "ins/outs" on the DRAC. Pin C11 is a 2000PPM out, for PCM.

If it helps any, the '747 (1991 TBI for sure) I *believe* was what was in the donor of the tranny, speedo, and DRAC that I now have. I tried my best to find what the '747 was looking for as a VSS signal, but no luck.

I'm no electrical whiz, so the "DC square wave" throws me. Would there be something different than DC square wave that the 165 is looking for? I know there is a difference between optical and magnetic, but I *thought* the purpose of the DRAC was to make that "conversion" between optical and magnetic.

I know the DRAC and tranny I have, fed the speedometer I have, so thats not an issue. It's just whether the '747 and 165 look for the same VSS signal? It almost seems that easy, but I just can't find what the '747 is looking for.

 

The '747 is an optical VSS input, which is the DC square wave. Looks like it's a plug and play into the '165.

RBob.

 

To the first, yeah, you can. It's just a "coarse" adjustment so if you want your speedo absolutely right (as accurate as OEM gets anyway) then you may want to go with the Dakota box that everyone uses when doing the T56 swap.

To the second: Yes it can. Roadspeed is prom-changeable, as is intrument panel output. Again, the only downside is the coarse adjust on the speedo.



Given you have that later cluster & the DRAC, I would go that route rather than `730 (for now; nothing wrong with a `730 swap.)