feets

Fixing my OD problem was a no brainer!

I used an $8 A/C low pressure switch for old Chrysler K-cars, 1/8 NPT extension, cheapie rocker switch, 18 gauge wire, and some spade connectors.

Here's the pressure switch:
Napa #207616 or ACDELCO Part #1550078



Here's the rocker switch:



Here's the 1/8 National Pipe Thread extension:



Here's the 3rd gear pressure test port on the passenger side of the tranny. It's number 4 (highest plug marked) :



I removed the 3rd gear presure test port plug on the tranny and screwed in the 1/8 NPT extension. Be sure to use thread tape or other form of thread sealer so it does not leak. Then, I screwed the switch into the extension. Without the extension, the pressure switch hit the side of the tranny and would not screw in all the way.

I tapped into the ground wire for the OD solenoid and connected it to the pressure switch. I then hit the other side of the switch and ran it up beside some other wires and in through the large round grommet in the firewall. It poked though the middle where the other wires pass through.
The rocker switch has a 1/2" shaft so I popped a hole in the metal bracket that holds the hood and brake release handles. I plugged the wire from the pressure switch into the rocker and grounded the rocker using the screw for the parking brake handle. The switch is hidden from view when sitting in the driver's seat.

The tranny automatically kicks into OD close to 40 mph in my truck (3.54 gears and 265/75-16 tires) and drops out of OD at 35 mph.
Using this on a 4.10 ratio would turn on OD around 35 mph and would drop out of OD around 30 mph.

The pressure switch is nice because the turn-on pressure is higher than the turn-off pressure. That way it won't bounce on and off if you're driving close to the cycle speed.
If these shift points are too high or low for your taste, find a switch that activates at different pressures. Your local parts house should let you dig through the catalogs without any complaints. This one turns on at 38 psi and off at 34 psi. Higher pressure means higher shift points.

The toggle switch is optional and will disable the OD when thrown.
If you use one of the cheapie 3 pole lighted switches like the one shown above, you can't use the light. The switch connects the positive terminal to the load terminal. You've got to use that terminal to control the ground on the solenoid. You won't use the ground terminal on the switch. It remains unconnected.
Putting power to the switch is now a bad thing.

In all, it took me about 20 minutes to slide under the truck and put everything together. Unless you're large or the truck is low, you won't even need to jack it up.

rbrettctd

Sounds like youve got a pretty good idea. Youve got a vote for the sticky from me .

feets

I posted a wee bit early. I got to thinking about why the shift point was 50 mph. I goobered and put the pressure switch on the overdrive pressure port.


I swapped it over to the passenger side where the governor pressure port is and it dropped the shift points down to 40 mph on and 35 mph off like I wanted originally.

I did have to drop a 1/8 NPT extension in there to keep the pressure sensor from hitting the tranny case.

wannadiesel

Now that's thinkin'!

jughead

one could then add a central ac unit adjustable high pressure switch in the wire to keep od from shifting at 50 at full throttle. tapped in to the turbo boost pressure. has worked well for me for several years.

feets

It can be used with a vacuum switch to do the same thing with a gasser. I was planning on doing that with my hot rod but I need to rethink it. Since I'm running turbos, it would shift out of OD whenever I built boost. That would get annoying on a hilly highway.

DanMan

jughead, you mentioned exactly what I'd like to do. That is keeping the tranny from shifting out of OD at highway speeds at WOT. So how then could one do that, how did you do it?


btw jughead, what all have you done to yours to get it that fast? And what do you get for mileage?

JD730

Thats actually the governor pressure test port. The pressure there will be approximately 1psi per mph, may vary slight depending upon you rear end ratios.
Your on at 38psi, off at 34psi corresponds with you 40mph into OD and 35mph out of OD.

feets

Yep. That's what makes it a reliable fix.

As for staying in OD at WOT, you simply need to add a switch to ground the solenoid. You can keep teh fully automatic function and simply override it when you feel the need.

jughead

danman i have a schematic. i will have to get my son to scan it. then we will post it. will take a week or 2 because he is out of town. on a scale of 1-10 i am less than zero with computers. will get it done as quick as possible.

Thomtek5

OK You can call me stupid....
This is a fix for the really annoying habit of my truck to shift in and out of OD between 35 and 50 mph?

jughead

was it waldo emerson that said "i never met a man that didnt know more about SOMETHING than i do. i dont know how one would install this on a factory wired od. my 90 came with a 3 speed. i have never met anyone i would call stupid.

feets

Another benefit is that the throttle input will change the shift point. A light around town throttle application will result in a 35 mph shift. Wide open throttle will give you a shift into OD at 52 mph.

I'm really happy with this one.

wannadiesel

Yes, it would cure that. I'd adjust the TPS first, though. It's free and less time consuming.

01sbc

Okay so here's my story, I bought this truck kinda cobbled together from the previous owner. He said that when he had the transmission rebuilt he didn't have o/d put back in. Well I went mudding one time and got on the interstate shortly after, sure enough, shifted into o/d at about sixty. Well the thing is, the "rebuilt pump" he put on doesn't have a TPS. But like I said, every time I take it mudding at this certain place it works until I get to school. I come out from school and it doesn't work anymore. So I'm thinking about doing this over Christmas break along with a few other projects and gremlins I need to chase down. Do you guys think this will work for me? Thanks in advance