70 SB TCS 4-Speed Trans. Switch

[Pat M.]

Is this switch still available anywhere?
I searched the archives and got GM part number 3961567 from Joe Lucia, but a GM parts search on-line did not produce a current part.
Also, another thread mentioned Zip carries one under ES-475, but a search of it showed only ES-476 available, which is for an auto.
Finally, Dr. Rebuild only shows 72 and up switches. If there are no 70s available, would a 72 switch work?
Thanks

[Doug J.]

Re: 70 SB TCS 4-Speed Trans. Switch

They are on EBay.. Just type in TCS Switch..there is one on there right now:# 260391125391
I bought one from these people for my '71 4 spd...it works perfect. There is also 1 in the EBay stores from the same people with a buy-it-now.

[Pat M.]

Re: 70 SB TCS 4-Speed Trans. Switch

Thanks Doug.

[William C.]

Re: 70 SB TCS 4-Speed Trans. Switch

looking at the part on ebay, the plastic area around the connection is black in color, I believe originals were a natural nylon (light yellow) in color if it makes any difference to anyone.

[Pat M.]

Re: 70 SB TCS 4-Speed Trans. Switch

looking at the part on ebay, the plastic area around the connection is black in color, I believe originals were a natural nylon (light yellow) in color if it makes any difference to anyone.

It does matter to me, but I don't know if I have a choice.

[Doug J.]

Re: 70 SB TCS 4-Speed Trans. Switch

I'm pretty sure someone out there has an original in their '70 or '71 / 4 spd up on a lift with easy access for a picture.

[Pat M.]

Re: 70 SB TCS 4-Speed Trans. Switch

Mine's original, but I'm pretty sure it's failed.

[Dennis D.]

Re: 70 SB TCS 4-Speed Trans. Switch

Think this is the original

[Doug J.]

Re: 70 SB TCS 4-Speed Trans. Switch

Pat,

Go to: www.google.com type in: "3961567 switch" and browse around, there are some there also. Good luck !

[Patrick H.]

Re: 70 SB TCS 4-Speed Trans. Switch

Mine's original, but I'm pretty sure it's failed.

It's hard for them to fail; not much in there to break.
You might want to double check it first.

[Pat M.]

Re: 70 SB TCS 4-Speed Trans. Switch

It's hard for them to fail; not much in there to break. You might want to double check it first.

I realize that but I don't know what else to check.
What's happening is there are times I'm getting vacuum advance when I shouldn't; in other words, the solenoid is de-energized when it shouldn't be. But the problem comes and goes, and sometimes it works fine.
I pulled the temp. override switch wiring while getting full vacuum and nothing changed, so I don't think that's it.
And I can energize and de-energize the solenoid manually while still on the car, so I don't think it's that either.
Finally, using a test light at the solenoid it almost seems like there's a short in the wiring, which a friend told me may indicate a bad trans. switch.
If not the trans. switch, what else? The relay on the firewall? How do I test that?
Thanks

[William C.]

Re: 70 SB TCS 4-Speed Trans. Switch

Well, here is how it works, where the switch is screwed into the side cover, there is a ball on the end of the switch that has a spring inside the switch pushing it down an insulated tube to where it solidly meets the bottom of it's travel and provides a ground path from the pin on the side cover to the trans ground. It is activated by (depending on the year) one or two notches in the shaft of the arm that closes the switch in either 4th gear or the 3 and 4 shift engagement on the side cover. Pull the switch out, clean it with solvent and check for continuity with it out of the car. If you have continuity from the connection outside the switch to the ball, push the ball back into the housing and the continuity should end, showing an open circuit. If this simple check shows the switch is ok, yo can look elsewhere for your problem. You are always checking for vacuum with the trans in fourth gear, correct? By design, with the trans in neutral, if my memory is correct, you will have no vacuum at the distributor, unless the overtemp switch has turned the full time vacuum system on, which is another item to consider..

[Joel A.]

Re: 70 SB TCS 4-Speed Trans. Switch

Is there not also a "time delay relay" on that year model? It delays the vacuum solenoid opening for about 20-30 seconds after the shift into 3rd/4th gear?

[Jim T.]

Re: 70 SB TCS 4-Speed Trans. Switch

Pat I don't have a 4-speed 70, but I do have a 70 with a turbo 400,
Your switch on the transmission and mine operate the same in reguards to the TCS system.
The switches are nothing but a ground when the transmission operates in high gear.
If I was testing your switch I would have the car at normal operating temp in neutral and disconnect the terminal end going to the transmission switch. If the rpms stayed the same that would indicate that the switch was doing its job and not active as a ground.
I would then connect a test wire to the disconnected wire terminal and ground the other end and check the engine rpms. They rpms should increase if the wiring to this point was functioning as designed.
I have not used my TCS since my car was new. It is inactive. I have full time vacuum always. Best benefit is greater gas mileage when in the lower gears and a cooler runing engine in heavy traffic. The engine idles faster with full time vacuum with no more applied gas pedal.
You could also just sitting in your car shift into 4th keeping the clutch in and check the rpms since the switch is supposed to work in high only according to the owners manual.
Ever wonder why the owners manual depicts a 3-speed transmission?

[Pat M.]

Re: 70 SB TCS 4-Speed Trans. Switch

Thanks guys.
Bill, yes, I was only checking for vacuum in 3rd and 4th gear, which is correct for my car. Thanks for the idea of how to check the workings of the switch.
Joel, I don't know if my car is supposed to have a delay, but it never has. When it was working correctly, I got instant vacuum in the upper gears.
Jim, great idea of how to check the switch and wiring before I even pull the switch itself. And yes, I usually check the vacuum just sitting in place.
I also know of the benefits of full-time vacuum, but it was a bitch to get this working right in the first place. Also, I want it to appear correct, and it was an additional chore to jury-rig it to look right and not function. So I just came to the decision that I'd just leave it working.
Thanks again, and I'll keep you all posted.