OBD1 PCM interchange/compatibility, 5.2 vs 5.9

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fshd4it

fshd4it

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Hey all. I'm wondering about the negative effects (I've read fuel curve differences) of running a 5.2 PCM with a 5.9 engine. '92 Dakota 5.2 donor w/a 518. Motor was junk, hence the 5.9, and I do have a 94-95 5.9 PCM. I'm trying to figure this out because of the wiring change between the early 2-wire VSS, which also runs a speedo cable, and the later '93(?) up 3-wire no cable version. The combo unit works off sensor ground and speed signal to the PCM. The later unit, however, is wired to the 8v supply (in series w/the cam and crank sensors), the sensor ground, and the speed signal. Nutshell- if the newer 5.9 PCM won't recognize the 2-wire VSS, what issues am I going to see with the 5.2 unit?
 
Likely the speedo won’t work and shifting will be abnormal.
 
fshd4it said:
Hey all. I'm wondering about the negative effects (I've read fuel curve differences) of running a 5.2 PCM with a 5.9 engine. '92 Dakota 5.2 donor w/a 518. Motor was junk, hence the 5.9, and I do have a 94-95 5.9 PCM. I'm trying to figure this out because of the wiring change between the early 2-wire VSS, which also runs a speedo cable, and the later '93(?) up 3-wire no cable version. The combo unit works off sensor ground and speed signal to the PCM. The later unit, however, is wired to the 8v supply (in series w/the cam and crank sensors), the sensor ground, and the speed signal. Nutshell- if the newer 5.9 PCM won't recognize the 2-wire VSS, what issues am I going to see with the 5.2 unit?
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I am doing something similar to my 91 Dakota, but rewiring the engine compartment to use the Magnum EFI.

Recently ran across a post on a different forum where the guy ran into the speed sensor issue and had a good fix.

He found that if you use the standard 2 pin sensor with the speedometer cable hookup, and run one pin to ground and the other pin to the PCM, it all works just fine.

Here is a direct quote: "What nobody ever mentions is that is it a Reed Switch sensor. Not a hall sensor nor a VR sensor as you might imply from the two wires. It's basically a little magnetic switch that switches between connecting the two inputs and open circuit 8 times per revolution. What this means is that with the VSS pin on the PCM pulled high internally, all you need to do is run ground to one pin and the other to the VSS wire in the C2 connector and you will get an 8K ppm digital signal. Speedometers are designed to turn 1000 times per mile. If you buy the longer speedometer cable to fit a 4x4 Dakota, it's the right size with the right ends for your older 4x4. You can also order the correct sealed plug from standard motor as well --- S727 or grab one from a junk Dakota."

https://ramchargercentral.com/proje...D=fea67c3529ae967c0d0f1f71aa9ed241#msg3502670
 
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Thanks for finding that, some really useful info there. Searches are funny, I've been looking into this all week (before I posted about it), and ended up on R/C central several times but never came across this one. Looks like he's doing an OBD2 swap, leads me to believe if it works for that, it should work for the end-of-the-run OBD1 I've got. One question: do you think he's refering to a body ground, or the "sensor ground" identified in the '92 pinout? Just to make things more confusing, the '95 pinout calls it a "sensor return"... what fun.
 
Something else to be aware of (in case you didn't already know) is that the fuel system is different.

The Dakota would have a return style fuel system with a varying fuel pressure due to the regulator and vacuum connection on the fuel rail. When the 5.9 came out, they changed the fuel system to a returnless style and put the FPR on the pump at the tank, so the fuel pressure is a constant reading instead of varying and might even be a different base pressure. So far, I haven't found a 5.9 PCM that would run a return style fuel system so I think there is no option to run a 5.9 PCM with the stock Dakota fuel system.

The simple fix is to use the 5.2 fuel rail from the Dakota and don't connect the vacuum plug on the FPR. But that might still require upping the pressure some if it doesn't meet the spec.

As I have planned for this, I have had several people tell me that the 5.9 runs fine on a 5.2 PCM, so you could try that as well.
 
fshd4it said:
One question: do you think he's refering to a body ground, or the "sensor ground" identified in the '92 pinout? Just to make things more confusing, the '95 pinout calls it a "sensor return"... what fun.
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I will have to go back and re-read it. Seems like he talked about what he spliced where, but I don't remember exactly.
 
I've got the original '92 Dakota fuel tank and pump, as well as the return style fuel rail. The truck is up and running (not driving yet though) with the newer 5.9 pcm, Slowly moving through the next steps.
 
I haven't tried running the motor with the '92 5.2 PCM yet, but I will soon, at least so I know that much. I'm not using the 5.2 injectors, but ones that are supposed to flow a bit better from what I've read. My fuel pressure's in the low 40s according to my gauge, but I was going to try it on my '95 5.9 truck to see how accurate it is. I also have an Innovate A/F gauge that I may tap in near the O2 sender, just to see where I am. Not sure what I'll be able do with that info though...
 
fshd4it said:
I've got the original '92 Dakota fuel tank and pump, as well as the return style fuel rail. The truck is up and running (not driving yet though) with the newer 5.9 pcm, Slowly moving through the next steps.
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If you hooked up the vacuum port on the FPR, I think you will find that you will have drivability issues down the road. The PCM expects a steady pressure but the vacuum port will cause it to fluctuate with will in turn cause the PCM to mod the tables (as much as it can). And I think it becomes a merry go round of changes that the PCM can't fix and makes worse. It probably idles and drives fine, but I think the more you drive it the worse it will get.

Guy I follow used a truck PCM in his Barracuda when he swapped the '94 GC 5.2 into it because he needed a stick PCM. The '94 GC still used a return style fuel system but I think the truck PCM he used didn't. In the end he tossed the PCM because he figured it was faulty. It very well might have been, but I suspect it was (at least in part) because the PCM couldn't adapt to the varying fuel pressure. In the end, he said it ran significantly better with the earlier PCM.

Not telling you the story to try and support my idea, only to say that if his issue were related to the fuel system mismatch, it wasn't just a surge once in a while, it kept him chasing gremlins until he tossed the PCM.
 
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The original IAC motor didn't work at all, wouldn't idle. I replaced the IAC, mostly idles like it's supposed to, but it has been having random surges. I assumed it's because the PCM sometimes has to "retrain" itself after an IAC change, but it certainly could be a fuel issue. The truck doesn't die, which is what I've experienced previously after changing them. I need to put the gauge back on and run the motor a bit more, and I'll try it with the vacuum line removed. Lots left to stumble my way through, including taking some hella-better notes.
 
fshd4it said:
... including taking some hella-better notes.
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If it weren't for my notes, there is no way I could have pointed you to the thread I did. All I could remember was that the guy fixed it, but I couldn't have said how. :D
 
I found a post somewhere last night where they talked about using the return style fuel rail with an adjustable regulator. At least I think that's what was mentioned, it's all getting to be a blur.
 
I think I saved a note about a guy screwing a screw into the vacuum port on the fuel rail mount FPR to adjust the pressure. Might have been where I first started to understand the difference. But that was years ago.
 
My friend and I are slowly stripping a '99 1500 that had a 5.2. I've got the fuel rail (as well as the motor) already, I can go back and get the pump and lines. Semi-big job, but I could swap all that out. As long as there aren't too many other hurdles.
 
fshd4it said:
My friend and I are slowly stripping a '99 1500 that had a 5.2. I've got the fuel rail (as well as the motor) already, I can go back and get the pump and lines. Semi-big job, but I could swap all that out. As long as there aren't too many other hurdles.
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Don't think the fuel pump would swap. I could be wrong, but seems like I looked at that. I would also guess the line would be iffy since the Dakota and Ram are pretty different in size.

Then again, maybe I thought about it and dropped the idea because I didn't want to try.

I really think all you need to do is pull the vacuum line off the FPR and only worry about adjusting the pressure if it is below the spec for whatever PCM you are running. And I would match the injectors to the PCM as well.
 
Well, here's the newest snafu... in a long line of. I dug out the PCM from the Dakota, and it looks like it's been replaced at some time. No factory sticker, little ones that say "warranty void if removed" and an illegible paper label. So I'm not even sure this is a '92 5.2 computer.
 
fshd4it said:
Well, here's the newest snafu... in a long line of. I dug out the PCM from the Dakota, and it looks like it's been replaced at some time. No factory sticker, little ones that say "warranty void if removed" and an illegible paper label. So I'm not even sure this is a '92 5.2 computer.
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Hmm...at first I would guess it is a match if you took it off the Dakota yourself, but if the motor was junk I wonder if the PCM was wrong for it and helped it along to the grave.
 
I think it was just worn out, didn't look like a lot of maintenance, and almost 250k on the truck. PO parked it when the trans went south, so evidently it was still running. I've been told by a couple mechanics I trust that the PCMs are rarely bad, but folks change them out while chasing engine problems. I've had a slew of issues with my trucks over the years, never been the computer.
 
I did not read all the above so forgive me if this has already been covered. My 95 Dakota has been running on a Mopar Performance 5.2 module for 16 years on a mild 5.9. It runs great for what it is and gets good fuel mileage. I do have a 5 speed though so I cannot help with the trans. questions. I run my 5.9 on 24lb injectors and yes the 40psi fuel pressure is correct. I believe the return system was threw 93.
 
Hopefully i'll get a chance to test my fuel pressure today, planning on bypassing the relay and running the pump with the engine off. I fgure that will show max pressure from the pump. I read that the MP PCMs do well adjusting to/learning fuel curves, but again, this is just one of many things I've "read". The 24# injectors you're running, are they factory pieces? Grey or black tops, perhaps? And thanks for sharing how it's worked for you.
 
fshd4it said:
Hopefully i'll get a chance to test my fuel pressure today, planning on bypassing the relay and running the pump with the engine off. I fgure that will show max pressure from the pump. I read that the MP PCMs do well adjusting to/learning fuel curves, but again, this is just one of many things I've "read". The 24# injectors you're running, are they factory pieces? Grey or black tops, perhaps? And thanks for sharing how it's worked for you.
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They are actually from Ford Motorsport. Cheapest option when I bought them as the whole set was $215. This was 15 years ago though. 5.0 Mustang uses the same injector so buying a Mustang part is cheaper of course.
 
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Gotcha. I guess that's a pretty popular swap. I've been looking at flow numbers from a few vendors/rebuilders, as I've got 3 different sets of magnum injectors to choose from.
 
Just a thought, but why not run the 5.9 long block with the 5.2 intake/electronics? I don't think it will make it run poorly, and I've heard of many people running stock/very mild 5.9s on 5.2 computers. I have a mild 5.9 running on a 5.2 computer, but it has been tunes via SCT Flash which you unfortunately don't have that option unless you swap to OBD2.
 
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