A518 swap with 727 question?
 

Im workin on spapping out my 727 with a A518 i picked up. Anyone know what i have to do with the wiring for like the overdrive and such. And what pin is what on the tranny like the ground, power, or over drive. Wound i just run the overdrive with a switch of something or is there a type of power control module that goes with em. I dont have the truck i came out of to go off or the wiring harness yet so some help would be great. Thanks.

Do a search for "Potentiometer" on here.
That's the simplest way to make a controllable overdrive for the 518 without having to get the ECM and wiring from a '91.5 to '93 truck.

Mark.

ok thanks. ill see what i can find. :tu:

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So the potentiometer seems like it is gunna be a great thing to use. The only thing is what should i do about having a o/d off button? Do the wires for that go straight to the tranny or do i need something to control that too.

I did this when I swapped it in my ford. I'm out trucking now so don't remember the part #'s but I'll get em to ya later. Basically all you do is use an old chrysler high psi AC sensor in the 3rd gear test port, screw it in with a 1/8 npt extension I believe it was. It has to prongs on it. The way I ran it was, put a toggle on my dash board ran 12v constant to switch then from switch to one side of AC switch, from AC switch to OD solenoid (which is located on the drivers side above the oil pan on trans. It's a 2 prong connector) power. Then the other wire on OD solenoid goes to ground, I ran mine to frame. It works great if you leave switch off you turn OD off and can hold it out of OD with it turned on it goes through all the gears and out of them. Depending on gearing, mine shifts into OD at bout 27mph with 4:10s with a 3:55 it would be close to 38mph. There is sites that explain this, search 518 OD problem, but they stress that you cannot apply power to it just have to interput the ground. But that's on factory trucks that already get power to it. I've had it this way for 22k miles now and works great.

so you used the ac control as a potentiometer or as an on/off switch for the o/d?

The overdrive is controlled by a single solenoid; when there's 12 volts across the solenoid and you're in 3rd gear, it goes into overdrive. Otherwise you're not in overdrive. All you have to do is ground one pin and put +12 to the other when you want to go into overdrive.

So you could put a pressure sender in the test port along with a potentiometer (and a comparator and a relay?) to have it shift into overdrive automatically... Or you could just put it on a switch and do it when you want it, only when you want it.

I used a switch on my conversion, but I have a manual valve body, and it just wouldn't be right to have it switch into overdrive by itself when I have to shift from 1st through third manually.

it acts as a on/off switch for the OD. yes all the OD needs is constant power to shift but without a tps tellimg it when all it will do is shift into OD a split second after it hits 3rd which is why you need something controlling it. easiest way, toggle switch with 12 volts going to it to AC switch in 3rd gear test port, AC switch to OD solenoid, other wire on OD solenoid to ground. like i said with toggle left on it goes into gear and out just fine. with toggle in off position you keep it in 3rd for pulling etc. its the most simple and actually better than factory way.

so if i put in a switch. and i turned the o/d on before it was in third it wouldnt shitf into o/d until it shifted into 3rd? and what about when i would go to slow down to a stop. would it shift out on its own or would i have to switch it off everytime?
also what would a comparator be for and a relay if i went the potentiometer? im not to great with electical so its hard for me to follow along sometimes.

no it wont hit OD till it hits 3rd gear, it wont skip gears. it will shift on its own the AC switch kicks it in and out according to line pressure depending on rear end ratio, so if you leave it on it does everything it needs to do on its own. if you flip the toggle off it disengages OD. it works just like a factory OD button on the shifter

wow that seems pretty cool:tu:. how does the AC switch get a psi reading though? i really think i will go this route cuz im swapping out my cab now with a different one and that cab has AC so i think i should work. if it gets the reading from the compressor or something like that it wouldnt get it cuz my motor now doesnt have one now to hook up to.