I have some questions as to what all I’ll need to change to do a wholesale drivetrain swap in my 1988 D150. I am aware that starting 88 they changed the motor mounts to prepare for the upcoming overdrives for 89. Currently I have an all stock 318/904 combo. I just bought a 318/A500 (or 518 not quite sure yet) that came out of a 1989 3/4 ton chassis. It had been body swapped to a 49 Chevy so idk if it was a D250 or B250 originally. I’m sure it’s an 89 because it had the Dodge cluster still with a 5 digit odometer. I have combed through my stock 1988 harness and to my surprise I already have the connector for the “O/D Off” button as well as wiring up to the connector on the engine harness for the overdrive solenoid on the A5xx transmission. So far the ONLY difference I can find is that my 88 harness uses a 3 pin oxygen sensor connector and the 89 engine has a 4 pin sensor. That’s an easy swap.
Will my 1988 computer just happily run an overdrive trans or are they factory programmed to one transmission? Seems they were preparing for it at least with the provisions in the harness for an O/D trans. I know I need the A5xx transmission mount. Will I need to swap computers? If I put the 1989 computer in will I need the change my whole engine harness just for the 4 pin O2 sensor or will the 89 computer accept the 3 pin O2? I know I’ll need to shorten the driveshaft.

So I know at a minimum I need:
shorten driveshaft, A5xx transmission crossmember, the A5xx harness up to the connector on the passenger side of the firewall, and the O/D button plugged in.
My question is: will I need to modify the column shift linkage or is it the same, do I need to swap ECUs and engine harness as well, or will an 88 computer happily run an A5xx transmission?

 

You could find a pcm with od or just use the kit from patc. They changed their site and the link i had for doesn't work but i think this is it.
https://transmissioncenter.net/shop/...iring-diagram/

 

I have a PCM from the overdrive truck but I’m hesitant to use it because it uses a 4 pin O2 sensor and my harness uses a 3 pin. So I would also need to change the whole engine bay harness. I’ve figured out a way to test if my pcm has overdrive. I’ll report back.

 

Ok so I’ve tested my harness and I have odd results. With my 1988 or the 1989 computer which came from the O/D truck, I get the same results. The harness has one orange which grounds (circuit shows 13 volts) at 45mph in drive and disconnects (0volts) when coasting down to 40. I believe this is the Torque Converter lockup.
The other wire responds to the the O/D off switch so I assume it’s overdrive, but it responds wrong. When OFF it grounds to 8 volts. When on it shows 1.5 volts. In the “on” position it never grounds when driving. In the “off” position it’s always energized at 8 volts. Stranger still they both ground with the key off, and if it probe across both orange wires which are both supposed to be grounds, I get 6.6 volts! The overdrive wire is positive 6.6 and the TC wire is ground. I get all of these same results with either of the computers. I entertained the idea that the harness is different and the wires are ending up at the wrong pin, but rhe TC works right, and the Overdrive wire responds to the button, so they have to be pinned out correctly.

 

Od wasn't available in 88 and both(lock up /od) will ground to come on. I have a print for 93 and lock up the pcm grounds a relay for lock up. You can check the # on the pcm that will tell you what it is for. I do have wiring for a 89 i can look it up and tell you what pcm pin is what.

 

If you’re able to bring up 1989 schematics I would really appreciate it! Ive been unable to find them and 1989 uses an 8 pin connector at the transmission harness junction, whereas the 1991 schematic which is widely referenced uses a 10 pin connector.

Knowing that overdrive wasn’t available in 1988 I was surprised to find that the 88 computer seems to be coded for it anyway. At least the lockup function works as expected. It’s just the overdrive wire that’s behaving opposite of how it should. Like I said though, I get the same results with the 88 or 89 computer plugged in, and the 1989 computer was running the a500 it came with just fine so I know it works.

 

So I found wiring diagrams that mostly match my vehicle. These are for a 1989 Ramcharger and the locations colors and quantities of wires all mostly match up. The only difference is the junction marked “center of dash” doesn’t exist, and I think that may have been an error in the first place because that is past the bulkhead connector, and you can’t have a “center of dash” connection under hood.
I now have a reasonable understanding of how the overdrive system operates but I am no closer to understanding why mine is outputting 12 volts at pin 55 when OD is on and grounding when OD is off. My plan at this stage is to swap the computer and harness that came with the transmission in, and see what I get. If it doesn’t work, I’ll just power the overdrive off of the TC lockup wire because that is working correctly, and I’ll put the TC lockup on a switch.

 

The prints i have only go back to 90 but the pins on the pcm are,54 is lock up and 55 is od. The print you have says they are different. So they must have changed between 89 and 90.

 

Yes, my understanding at this time is that the 1989 circuit works as follows: both the overdrive and lockup are provided +12 volts on the center wire at all times.
For the lockup, there is a relay on the drivers fender which comes on and off with key power and connects the lockup wire to pin 55 on the computer. Pin 55 normally supplies +12 volts making the circuit live on both sides, but incomplete. When conditions are met pin 55 switches to ground and the circuit comes alive.
For the overdrive, pin 38 is connected to one side of the overdrive off button and the overdrive wire is connected to the other. The overdrive off button simply connects and disconnect pin 38 to the overdrive solenoid wire. Pin 38 normally supplies +12 volts and when conditions are met switches to ground and the circuit comes alive.
If the OD off switch is engaged theoretically the circuit is incomplete, however there is actually a diode I see on the diagram that bypasses the switch for reasons I’m not sure of.

My issue basically is that my computer is operating backwards, and/or my off switch is bad. When I toggle it to off that does somehow prompt the computer to switch to ground, which shouldn’t be an issue, but the circuit isn’t disconnecting so that ground is reaching the solenoid wire. It suggests the OD off switch is bad. It would seem I could simply install a regular switch across U4 and U40 and that would restore normal operation. There are (untested no guarantees) switches on eBay for $80 but the only thing I would lose with a regular switch is an automatic return to “on” function each restart.

 

The pcm doesn't switch between positive and negative, Positive is supplied from either the ignition switch or the asd relay. The pcm supplies a ground to pin 55 for lock up and 38 for od ( i go the #'s from the print you supplied). On the prints i have there is another wire that goes to the pcm to switch off/on the od. I don't have one for 89 so it may be different and i didn't see anything on the #diagram for it(it's hard to read so i may have missed it). Really need a complete wiring diagram for a 89.