Does anyone have a simple method to overcome the loss of overdrive when a carb conversion has been done with these transmissions? I'm assuming from looking at the schematic that the PCM monitors the speed sensor, TPS, and coolant temp and grounds the pin for the overdrive solenoid based on those sensor inputs. Obviously, with the PCM not getting those inputs the truck only has 1,2,3 gear and the overdrive button (although lighting up) does nothing.

The truck is a 91 that I swapped a magnum 5.2 into. It has a edelbrock carb on a airgap intake and a vac advance aftermarket distributor.

 

Overdrive and lockup both. Don't forget the lockup! That helps gas mileage also!

It's actually pretty darn easy ... some folk local to me sell a conversion kit, but if you hunt you can do it all yourself with this information: https://transmissioncenter.net/shop/...iring-diagram/

RwP

 

I used the PATC kit that Ralph linked. I integrated it into the existing overdrive circuitry so I was able to maintain the override switch in the cab.

The kit consists of three parts....A VAC switch that kills the 12VDC to both overdrive and lockup solenoids on Wide Open Throttle, and two Pressure Switches that switch ground to the lockup and overdrive solenoids.

The PATC uses the ground of the casing of the transmission as the ground source (no extra ground wire), so make sure that your tranny is good and grounded and don't use any pipe dope on the fittings from the transmission to the pressure switches in order to maintain the ground.

The basic wiring is, you'll have a wire (ground) from each pressure switch to one side of the lockup solenoid and one side of the overdrive solenoid. Then the 12VDC will come from your Vacuum switch that you connect to your intake manifold (manifold pressure). The VAC switch is normally open held closed by Vacuum (PATC's diagram is backwards). So when you are idling or under light load and Vacuum is high, the switch is closed and sends 12VDC to the other side of the overdrive/lockup solenoid. When you go WOT and the Vacuum drops, the switch opens up and unlocks the converter and downshifts.

Where you derive that 12VDC from is whether you want to maintain the override switch in the cab. The circuitry for the override switch is a TRIAC switching transistor, so I tied that to a relay. I reused my AC relay since I removed AC, but you can use any relay. The cab switch will control the relay, and you can run ignition 12VDC through the relay to supply the overdrive solenoid. That way, you can still turn off overdrive if you want.

I integrated all this into the factory wiring harness which required sending the ground signals up the transmission harness to the engine connector and then back down again. You can just hook them up locally if you prefer but I wanted a factory look.

I have some hand drawn diagrams if it will help.

 

This is one of the many things that is not mentioned when people say "just convert it to a carb".

I bought this truck for 500 with the LA318 and 240+ miles assuming I would have to put a few bucks into it. (always liked the body style)

I posted up my desire to put the Magnum 318 into the truck and got about 50 replies to the thread about how "easy" it was with just a few little modifications. Off of that "advice" I grabbed a low miles magnum 318 off craigslist that had been in a fire expecting to use the TBI setup off the original truck and keep the computer...... Yeah, that didn't work out.

I plan on writing a long write-up on this project (when it's all finished) but MAN, talk about leaving some stuff out. Even though I've been on the internet for YEARS and YEARS I have never seen so many "never did it before but I'm an expert" posts in my life.

My write-up is going to include the very, very long list of things that must be done to keep all elements of the vehicle in working order to swap to the magnum engine into the pre 92' truck and converting it to a carb...

Short list being:
Intake manifold
oil pan
serpentine belt setup (going from 5 to 6 rib or hybrid)
EXHAUST PROBLEMS (major problem lucky I know how to weld and have the money to screw around with MAKING and exhaust)
Aftermarket distributor
Air conditioning...... Still to be completed
external voltage regulator
Getting trans to work without the computer
Getting tach to work (pretty easy actually)
Throttle linkage (unless you want to spend a lot of money)

Those are just some of the things that come to mind right now. Again, I'm lucky in the sense that I am very experienced and have a two car garage full of expensive tools. I would feel very bad for a novice or young kid trying to do something like this themselves.

It's not that is "that bad" it's only that it certainly isn't presented as the intermediate level engine swap that that it is on the internet. LOTS of people who clearly have never done it pipe up on the internet giving people advice as if they have.

Even that old OBDI PCM had it's tentacles into a LOT of different stuff which ALL has to be figured out.

Thanks for the knowledgeable advice.

 

well if you had read my build thread I covered all those and more consider yourself lucky that you went serpentine and not v belt...I had to swap the whole front end to make that work. The ODBI isn't that bad....Cruise Control, Fuel management, AC and the transmission....you have to rewire the ground side of the ASD relay but that wasn't bad. I literally went through the FSM and highlighted every circuit....yellow to remove, blue in cab, orange outside of cab. I removed AC and cruise so that made the job easier. In the end the factory harness was maybe 20 wires left....three main circuits I had to improvise....the transmission, the tach and the distributor...I went with an HEI setup so the ignition was easy...no coil and no VR.

I agree misinformation abounds....I heard the same thing about swapping a carb magnum...easy. But when I really searched it only one other person had done my set up with vbelts. And it required a radiator modification. So I swapped the front end...I read threads on that too that were completely wrong from reality. But I'm not blaming anyone...it happens when info gets passed along....the finer details are left out. I wasn't terribly experienced but I'm a research junkie so I was able to figure it all out....and you learn who's knowledge to trust and who's to take with a grain of salt. I will say the active members on here are immensely helpful...and most will be the first to tell you they don't know for sure if it's something they've not personally done.

 

Quote:

Originally Posted by ojcool

This is one of the many things that is not mentioned when people say "just convert it to a carb".



I posted up my desire to put the Magnum 318 into the truck and got about 50 replies to the thread about how "easy" it was with just a few little modifications. Off of that "advice" I grabbed a low miles magnum 318 off craigslist that had been in a fire expecting to use the TBI setup off the original truck and keep the computer...... Yeah, that didn't work out.

I plan on writing a long write-up on this project (when it's all finished) but MAN, talk about leaving some stuff out. Even though I've been on the internet for YEARS and YEARS I have never seen so many "never did it before but I'm an expert" posts in my life.

My write-up is going to include the very, very long list of things that must be done to keep all elements of the vehicle in working order to swap to the magnum engine into the pre 92' truck and converting it to a carb...


Those are just some of the things that come to mind right now. Again, I'm lucky in the sense that I am very experienced and have a two car garage full of expensive tools. I would feel very bad for a novice or young kid trying to do something like this themselves.

It's not that is "that bad" it's only that it certainly isn't presented as the intermediate level engine swap that that it is on the internet. LOTS of people who clearly have never done it pipe up on the internet giving people advice as if they have.

Even that old OBDI PCM had it's tentacles into a LOT of different stuff which ALL has to be figured out.

Thanks for the knowledgeable advice.

OJ, I think you are confused. On this forum, you have posted 15 times, and of those, none are for advise on a magnum conversion, or for that fact, any engine conversion. The closest thing I could find, was a post about trying to put ram headers on a dakota.
No doubt, people sometimes give bad information, but for the most part, we all try to help each other.