cougar

Single pole double throw switch.
Common to ground side of OD solenoid.
Normally closed side to computer.
Normally open side to pressure switch.
If you use a center off toggle switch, you can have an OD off position.
With a rotary switch, you can get even more creative.

wannadiesel

What's the PCM got to do with it?

This is a very simple and effective way to have automatic OD operation without involving the PCM.

Power to one terminal of the OD solenoid. Run a wire from the other terminal on the solenoid to the pressure switch. From the other terminal on the pressure switch run a wire to a toggle in the cab, from the toggle in the cab run a ground wire. Toggle on = OD controlled by pressure switch. Toggle off = no OD.

cougar

Just in case you want to maintain the OEM set up as an option. Choices, I like choices.

Dean Snow

This mod wont work on the 4wd version without some modding of the 4wd shifter bracket under the truck. Went to do this earlier today and the bracket covers it by about an inch or so and i tried a 1/8 npt short bend 90 and still couldnt seem to get it in there without notching the bracket or removing it

matchew

Anybody get a schematic for this set-up yet? Which plug are we using on the trans? Since I've gotten rid of my PCM, this looks to be the best thing yet.

My only other question...how will this work with 4.10s and a set of 37s?

Fireman20

I swaped a 1992 Cummins into a 1972 4x4 GMC pick-up and did this set up because I have no PCM. What I found is the truck will cut out around 35 MPH and fall on its face with the switch off.... I turn the switch on, the truck runs fine through all gears!.....is this normal?

feets

I've never heard of that happening. Are you sure you tapped into the correct wire?

Exekiel69

Thank You for this thread I really want to have OD on the 92 w250, I see there are 4 different #'s but You only point to #4 for the pressure switch. I ask this bc I don't know which/where is the OD solenoid nor which wire I have to tap into.

I will get the switch from napa after reading this thread seems is the best option, have somebody done this on a 4wd? I see one member pointed out the shifter will interfere with the fitting + switch.
One las question, I installed the control **** from dieselsite that eliminates the TPS thinking the OD would actually work, will this affect that i any way or do I still have control of when the OD will kick in?

Thanks in advance for any help.

Exekiel69

Does someone want to shed some light.

Thanks.

wheelieking71

the plug you are looking for is on the drivers side of the transmission. its receptacle points straight up. it is a 2-wire weather-pack connector. the rear wire is constant 12volts. the front wire is the wire you will be messing with. it goes to ground, through the computer. when certain parameters are met the computer provides the ground and the tranny shifts into overdrive. this pressure switch is taking the place of the computer.

wheelieking71

coincidentally, i just tried this yesterday. i used the napa switch mentioned in the beginning of this thread. it worked, but shifted into overdrive immediately after shifting into 3rd, if not at the same time. has anybody come across a switch with higher pressure points?

GAK30

I have used an Edelbrock #72214 (preset at 50 psi) and a Jegs #555-11207 (also preset at 50 psi) for both overdrive and lockup operation. Both switches are fuel pressure switches normally used in nitrous applications but are simply adjustable, normally open Hobbs switches with 1/8 npt fitting. Other companies may offer the same but I know those two will work. Turn the allen head screw clockwise to increase pressure (raise shift point) and counterclockwise to lower pressure (lower shift point). It takes 2 switches set at different points if you have o/d and lockup.

wheelieking71

cool, thanks for the input. i had been looking at the adj. hobbs switches. but the only thing i am wondering is if the "open" and "close" pressures overlap, or happen at the same pressure? if there is only a pound or two variance between open and closed i can see that being very annoying at certain speeds, and maybe even bad for the tranny.

MatthewH

This mod will work on 4wd trans, you just have to space the 4wd shifter bracket out from the trans. I machined 3 spacers, 3/4" thick, that pushed the shifter outwards, and used a tight 90* elbow. The shifter does hit the floor pan when you turn off the truck, or into 4-low, but I'm okay with it.

I did find however, that using the pressure switch Feets used, it did shift into OD right after it shifts into 3rd. I do know that the trans is running 80-100 psi in OD, so I'm going to find a higher pressure switch and see how it works. When I find something, I'll post a part number.

9453

Hey not to change the subject but do you have any pics or a thread for your build? KB 6s are cool