Thanks for the links to the PCM's, I'll check them out. Side note, I was scammed on a Mazda tech forum looking for a one year only PCM for a 1991 B2600i 4x4 that I had. After what seemed like a genuine conversation over the forum, I sent paid him for the computer to only hear excuses.
I don't often buy from forums or online groups for that very reason. When I do take that risk I use Paypal goods and services and pay the extra fee. So far I have been lucky and haven't been burned trying to buy Mopar parts or parts for my 85 Supra. Yet.
I haven't hooked up the speed sensor to the 904 yet. The Dakota used a gear driven VSS that fit in the place of a regular mechanical speedometer drive. I would like to keep my car's original speedometer, but I also need the VSS signal for the PCM and RWAL module. I have an idea of what I want to make for an adapter to drive both, but haven't got there yet. I am open to ideas and suggestions.
I was asking because I didn't use a speed sensor when I was running a 904, and the engine use to backfire (afterfire?) through the exhaust on deceleration. Not like a modern performance tuned BMW, but it was bad enough that I didn't like it. Then I upgraded to a 46rh, but I wanted to keep my mechanical speedo so rather than figure out a solution I initially used pressure switches to control the OD and lockup.
Before long I found my answer: late 80s / early 90s mechanical speed sensor, like on a 91 Dakota (for example). As a bonus to having the PCM control OD and LU, and still having my mechanical speedo, I found as soon as I connected a speed sensor the afterfiring out the exhaust stopped happening on deceleration. Apparently the PCM will give the engine a bit of idle air on deceleration and that prevents the afterfiring. I have heard this problem doesn't exist with later OBD2 setups that aren't using a speed sensor, so maybe the engineers figured out a different way to prevent it from happening.
Here's the sensor I used:
More Information for STANDARD MOTOR PRODUCTS SC108
You can see one side screws into the speedo gear output on the transmission, there's a mechanical speedo output, and a 2 wire sensor connection. A few notes...
- I replaced my speedo cable with one from a 91 Dakota. It has a 5/8" nut as opposed to the 7/8" that would be used on the 904, and it has the push-on connector on the speedo side. At 80 inches it's a couple inches long even for my 46rh (but manageable), so it might be too much for a 904. I'm not sure if they make a shorter 68 inch cable with the 5/8" nut and push-on speedo end.
When I put in the 46rh I had one of
these cables to extend my stock speedo cable to reach the 46rh speedo gear housing. I really wanted to find one with the 5/8" nut on the transmission end because that would have been perfect, but I couldn't find one anywhere.
I also looked into getting
this guy's HM105T adapter, but he wanted some insane amount of money for what is essentially a nut with threads on the inside and outside, so I opted not to go with that. In hindsight, I should have just spent that money or had one machined locally. I didn't have fun replacing my speedo cable, and this would have made that unnecessary.
- You'll note that I mentioned it's a 2 wire mechanical sensor. One wire is the signal to the PCM, the other wire is a ground. The Magnum setup uses an electronic sensor, which has a 3 wire connector to the speed sensor: signal, ground, and power. With my setup I simply don't use the power wire and it works to control OD and lockup perfectly. I think the 2 wire connector is the same one from the OBD1 EVAP or EGR, because I had one in my pile of left over wiring that fit perfectly.
Also note that the later OBD2 setups don't use a speed sensor on the transmission, it's on the differential somewhere (I think - not on the trans, anyway). I think the last year for it on the trans is 1998, so if you do your OBD2 upgrade and use a later PCM and you need a speed sensor for one reason or another, you will have to consider this.
I think that covers everything, and should hopefully give you some options to consider.
