I am into my 904 to 518 swap. I bought the PATC kit to operate the overdrive and lockup. The kit has a vacuum switch and two pressure switches. The pressure switches seem straight forward. My problem is understanding how the vacuum switch works.

The vacuum switch has two out put tabs. One marked NC (normally closed) and one marked NO (normally open). The wiring diagram from PATC runs a wire from the NO on the switch to the 12v input on the transmission case connector. The instructions that came with the switch show the wire running from the NC tab to a solenoid. Not sure which is right.

I hooked the switch up to vacuum and a hot and tested the tabs with a light at idle (high vacuum).
NO tab is hot when there is vacuum to the switch
NC tab is dead when there is vacuum to the switch

I assume that as the vacuum drops, the power will go off at the NO tab and the power will come on at the NC tab

My understanding is vacuum is high at idle and then drops as speed increases.
So, I would think you'd want the NC tab since you want the power to the transmission solenoids only when you are at higher speed (lower vacuum). Then if you're at speed and slow down, the throtle closes, vacuum increases, power goes off, kicks you out of over drive and lock up. Or am I missing something?

I am totally confused. I am going to call PATC in the AM, but thought maybe somebody here could help me get this straight in my mind.

on my patc setup i couldnt get the vacuum switch adjusted to my liking--i just unplugged it and bypassed with a modern auto fuse---if i was out on the highway about to kickdown i would just toggle off the OD-----someone on this site has a better wiring diagram listed for the patc setup.....

Here's the wiring diagram from the PATC site.

I guess if you use the NO tab on the vacuum switch the circuit will be hot at idle but it won't matter since the pressure switches are closed. Still unclear to me how the kick down would work off vacuum. I guess I'll wire in a toggle switch so I can turn it on and off manually. I was hoping to avoid that.

you understand the hot lead goes into the center of the 3 pin connector and the pressure switches activate OD and lockup by grounding ...

I have this set up on the 518 in the 36. It works very well with no need of a toggle switch. One catch I did not use the adjustable vacuum switch. I used the other switch that was supplied by PATC but is no longer available. It is GM #14014519 and is out of production. If you have a Pull a Part near you check out the GM vans they were used on many of them and are easy to access under the hood. I may have a spare if you are interested. Good luck!

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but it won't matter since the pressure switches are closed

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Maybe your logic is reversed.

36Plym,

What year/model GM van for the discontinued vacuum switch? Where is it located under the hood? Do you have a picture of it? There is a pull-a-part close to me. I may go that route.
thanks in advance.

Yeah, you're right, the pressure switches would be "open", not "closed".



Why is the GM vacuum switch better? Is it adjustable?

Not sure what year vans, maybe nineties. The switch is located in the center near the top of the hood opening towards the windshield. Other vehicles used the switch as well. Get the wires too!

on the patc vacuum switch the adjustment is inside the vacuum port (tube) i think a tiny allen adjusts it---look in the chevy one to see if it is also adjustable.....

Great info guys!!! Thanks for the input.

Hooking up at 518 in a 47 Dodge Pickup as we speak..

Lon

I guess I'll try the PATC supplied switches and see what I get. I am going to wire in a toggle switch just in case since it's easy enough to do.

I should have the 518 in next week so I'll report back what I find.

Thanks for the inputs.

is this a 2 wire a 3 wire or a 5 wire 518 trans ???

Refresh this topic after all these years.

So how did you ended up wiring the PATC vacuum switches? Which tab did you use the NC or the NO?

I've got a 518 46RH with OD and LU behind my 2014 Hemi SRT and am in the process of installing and adjusting the switches.

I was looking at the website of PATC transmission and it looks like they adjusted the page:

It says:

On the adjustable oil pressure switches the “C” post goes to ground, the “NO” post goes to the solenoid and the “NC” post has no wire.

https://transmissioncenter.net/shop...ere-for-727-to-46rh-swap-wiring-diagram/

Problem solved!