What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

700R4 vacuum switch ??

cuervo

cuervo

Runn'n down a dream!
 Premium
Joined
Jan 2, 2004
Posts
3,800
Reaction score
992
Location
Sweet Home Alabama!
This is on an 83 GMC 1500 2wd with a 700R4.

I thought these were all TV Cables but on my truck there is a vacuum something or other. In this first photo it shows the name.

Trans Signal Conv Hose Vac SW.

I assume it is Transmission Signal Converter Hose Vacuum Switch. Just a guess at this point.

transswitch1.jpg


And this is what it looks like mounted on the fire wall.

transswitch.jpg


So the question is does anyone know the full name of this switch and most importantly, what is its function?

Thanks.
 
I call BS. Maybe your truck didn't originally come with a 700. Either that, or it's some crazy rare first gen 700 :haha: Ohhh, where does the electrical wire go? Do you have a computer under the dash?
 
K85 Octane said:
I call BS. Maybe your truck didn't originally come with a 700. Either that, or it's some crazy rare first gen 700 :haha: Ohhh, where does the electrical wire go? Do you have a computer under the dash?
Click to expand...

It does have some sort or ecm but this is a carbed 305. I'm pretty sure ecm is just for the distributor.

The truck is all factory. The diagram under the hood shows this vac switch is factory.

I haven't a clue where the wires run to.

btw: It does have a TV cable as well.
 
I'm guessing the wire goes back to the computer. Does this truck have the electrical stuff on the carb (CCC)? Or it might just be the knock sensor/ dizzy setup.

I had the older 700 (85) in my truck to start, it was original. Then made the switch to an '89 unit. Neither had inputs like this that I can remember. However, it might be for your lock up converter when the truck is in 3rd or 4rd gear and at the "proper" vacuum range.

My stuff didn't have any of that and survived the 250 mile trip home. Felt good too. Now I have a BTO lock up kit since rebuilding my new trans
 
My '81 G-10 van has the same vacuum switch--but it has a TH350 with a lock up torque converter (MV4),not a 700R4...look on the rear tailshaft for a vacuum modulator,if it has one on the passenger side,you dont have a 700R4...

It might be possible GM used the vacuum switch to control the converter lock up on a 700R4,but I haven't seen one yet,and I've seen a lot of earlier GM's in the boneyards...
 
The "computer" is an ESC electronic spark control box. The truck is an old style large cap HEI.

I was think converter lock up as well but I've never seen this setup before.

There are three wires that come off of the back of that vacuum switch. It runs into a pigtail type deal that must have some splicing in it. The 3 wires run down to the trans, haven't looked where to yet and 2, same colors from the switch, run to the divers side firewall.
 
diesel4me said:
My '81 G-10 van has the same vacuum switch--but it has a TH350 with a lock up torque converter (MV4),not a 700R4...look on the rear tailshaft for a vacuum modulator,if it has one on the passenger side,you dont have a 700R4...

It might be possible GM used the vacuum switch to control the converter lock up on a 700R4,but I haven't seen one yet,and I've seen a lot of earlier GM's in the boneyards...
Click to expand...

It's a 700R4 - the overdrive on the gear selector kind of gives that away. That and the fact that I've had the trans rebuilt before.
 
franks has one on his rig, its to control the converter if you remove it it wont work right
 
I'm not positive, but I think one wire goes to the tranny,and the other two go to the stop light switch,to disengage the converter lock up when you apply the brakes..

Also,just because I have never seen this setup on a 700R4 doesn't mean it never existed...
 
Jump under the truck just now, the wires run down to a plug on the drivers side of the trans.
 
diesel4me said:
I'm not positive, but I think one wire goes to the tranny,and the other two go to the stop light switch,to disengage the converter lock up when you apply the brakes..

Also,just because I have never seen this setup on a 700R4 doesn't mean it never existed...
Click to expand...

It does go through the firewall near the peddles so I'm going say you are correct.


Anyone know what the switch is called?

I had a pretty bad set of vacuum leaks and the trans wasn't shifting right so I'm in the process off replacing ALL of the vacuum lines under the hood. It's a mess!
 
My BTO kit goes to the electrical connector on the dirver's side of the trans. It's just one wire that goes back to the brake light switch as mentioned. It's got a different switch inside the trans to run off the BTO signal and locks 4th gear automatically. With that setup, there's no unlocked 4th.

With my new '89 trans however, the shop put in a different valve body and reused the switch. Now it will unlock 4th.
 
Oh and at the bottom middle of that diagram there is another "something".

EGR/PURGE/TCC
TVS

I got the EGR part but I'm guess on the other TCC (torque converter lockup?) TVS ??
 
"TCC" is the abbreviation for "torque converter clutch"....a Haynes wiring diagram I have calls the switch a "low vacuum switch",but I would ask for a "vacuum operated torque converter clutch switch" if I had to buy one somewhere,to avoid confusing it with some other switch..

When I first bought my van,I "fixed" the TH350 by replacing the stripped plastic govenor gear,that was its only reason it only had first gear and reverse...after that,I noticed the converter refused to lock up,after an 80 mile trip to my brothers place,it smelled like the tranny was overheating..

I soon discovered my brake light switch had "moved" out of adjustment,and half the time the brake lights were staying on continuously...(after I came out one morning to find the battery dead,and the brake lights glowing like candles..:doah:)..thats when I learned the brake switch was tied in with the TCC switch...after a adjustment the converter worked fine again..

I have had my share of vacuum hose "failures"...many of the older vehicles I bought either had them all rotted and cracked,or were connected to the wrong ports--and or were simply plugged off and "eliminated"...

I bought a full spool of 3/16" and 1/4" hose and got a copy of a vacuum hose routing diagram from a local library (Chiltons used to have a book that was just for vacuum hose routings all by itself,that took up a large table at the library!)...after I put them all back where they belonged,and made sure all the devices actually did work (like the vacuum advance,etc),they all ran a lot better...
 
Yeah, I've got a few more the change out. Its the type where all the hoses go into this nice little boot that plugs into some sort of water temp controlled switch on the thermostat housing and a few others. There's also this crazy vacuum controlled flapper valve on the passenger side of the exhaust.
 
Found this - TVS (themal vacuum switch)

It controls quite a bit of stuff and activates when the thermostat opens, signaling that the engine is warm.
 
cuervo said:
Yeah, I've got a few more the change out. Its the type where all the hoses go into this nice little boot that plugs into some sort of water temp controlled switch on the thermostat housing and a few others. There's also this crazy vacuum controlled flapper valve on the passenger side of the exhaust.
Click to expand...

The vacuum switch make the converter go out of lockup when you accelerate . Forces the trans to down shift from over drive to third gear.
 
You must log in or register to reply here.

Latest Posts

Top Bottom