What is this on the firewall on 85 GMC?

[72farmer]

I just picked up this 85 GMC at a auction last sat.What is this on the firewall with wires on one end and a vaccumn line on the other end.

[kwmech]

I think that is part of the torque converter lock-up for the Turbo 350

[100%Chevy]

Originally Posted by kwmech

I think that is part of the torque converter lock-up for the Turbo 350

Don't you mean 700-R-4?
That's what it's for on my 84 GMC.
Mike.

[natedogg]

It's the vacuum switch for the electronic spark control (ESC) distributor. The vacuum switch for the turbo 350c or 700r4 is located next to the brake booster.

[56weav]

really? I have had my 86 for 1 year. That switch is also on mine ,however, it isn’t hooked up to anything. The vacuum on the distributor was / is hooked up to a vacuum off the carb

I would guess that plumbing it to the distributor would be a good idea ����
I have been surprised that it ran at all with all the vacuum leaked I found .

[natedogg]

I bet someone swapped out your ESC distributor for a regular non computer controlled HEI distributor. You no longer need that switch, if that's the case? The ESC distributor has an extra little harness that connects to the computer behind the glove box. There is also a knock sensor that works with this system. The vacuum hose from that switch doesn't connect to the distributor.

[56weav]

Possible that the previous owner swapped things out but I have learned that he or they did most things poorly or as cheap as possible . The distributor appears to be original by it aged appearance. What would I look for to determine if the distributor was swapped out ?
Thank you

[gezn2]

Originally Posted by 56weav

Possible that the previous owner swapped things out but I have learned that he or they did most things poorly or as cheap as possible . The distributor appears to be original by it aged appearance. What would I look for to determine if the distributor was swapped out ?
Thank you

An ESC distributor will have four wires coming out of the case below the cap going to a flat connector that fits one coming out of the firewall.

[95 S_Trucker]

Originally Posted by 56weav

Possible that the previous owner swapped things out but I have learned that he or they did most things poorly or as cheap as possible . The distributor appears to be original by it aged appearance. What would I look for to determine if the distributor was swapped out ?
Thank you

Your truck came from the factory with ESC. Someone swapped distributors. It could have had a used distributor swapped in, or the it was changed when the truck was fairly new. Vacuum advance is definitely not original on an 86. Look at the emissions hose diagram.

[56weav]

Thanks for the information. I’ll take a look tomorrow
Other than a couple of basic vacuum driven items, all of the vacuum systems had been removed. I was able to get the cruise back up and running , I also removed the smog system it was very apparent that it wasn’t doing anything except taking up space.
Fortunately I live in an area with no emission tests

[Desert1957]

It definitely is the vacuum/ electric control for the torque converter lockup.

GM Part# 14032087 still available from GM or on Ebay ACDELCO# 212331 Rockauto

[56weav]

1st , my apologies for hijacking this thread 72 farmer.
I went out and looked at the distributor there is a 4 wire connectors with wires coming out of the base of the distributor that goes to the computer. the electric / vacuum switch on the fire wall has the electrical hook up but the vacuum is connected to nothing. The distributor has what I would consider to be a standard vacuum pot at its base .the distributor is gm issue btw. Where do I go from here ? Nothing would surprise me the previous ownership
Was questionable with thier mechanical skills and or knowledge
Btw I have the restoration of this truck backwards in order. The cosmetic stuff is happening first, I had the opportunity to have it repainted as a retirement gift from a former customer I couldn’t pass up ,they liked me . Thank you for your responses

[natedogg]

First of all, the picture in original post is not for the lock up converter on the 350c or 700r4. The switch for the lock up converter is next to brake booster. Follow the wires and you will see. The ESC worked with a knock sensor to retard the timing. The vacuum line coming out of the switch on firewall needs connection to any vacuum source. Your truck will still run even with some system components not connected. Follow the harness out of the firewall to see if everything is connected. Knock sensor is just above oil pan on passenger side.

[56weav]

Thank you
I’ll hook up that switch to a vacuum source and see what happens,
The knock sensor is hooked up when I flushed the cooling system. I removed the knock sensor and cleaned it up etc, btw the hole it fit into was plugged with rust. I laughed when I was doing that the previous owner had figured out that it was needed they had cut the wire but then had “ fixed it “ not very well I might add. I’ll get back to this thread in a few days
My wife and I are headed to Wisconsin to see our grand daughter Retirement allows that ��again thank you

[72farmer]

No problem hijacking my post 56.Helped with identifying it.I guess I need to trace wires on it to see where they go.I did not think a TH350 could have a lock up converter.But I don't know for sure.

[Dead Parrot]

Originally Posted by 72farmer

No problem hijacking my post 56.Helped with identifying it.I guess I need to trace wires on it to see where they go.I did not think a TH350 could have a lock up converter.But I don't know for sure.

A normal th-350 doesn't. A th-350C does.

[natedogg]

Starting in 1980 or 1981 the THM 350C was introduced. It has a lock up torque converter. At about 40 mph the converter locks up and drops engine rpm. 1986 was the last year this trans was put into a GM truck. However, Someone could have swapped in a older THM 350 in your 85. A THM 350C has a plastic square shaped electrical connector on driver side just above the pan.

[WyattTX]

That is a nice survivor you found. It looks just like my '85 (aside from the GMC badging.) My truck lacked that wiring when I got it. Due to problems associated with them, people often ditch ESC distributors in favor of the pre ESC design. Survivor squares that have not been molestesd by shade tree mechanics, abused to horribly, or involved in a botched restoration or botched customization are getting rarer, and rarer. Enjoy your truck.

[72farmer]

It really is a survivor Wyatt.Only rust I can find is in the bed behind the left rear tire.I believe all the paint is original.Only problem is it is a 305 yuck!!And the freeze plug above the starter is leaking a couple of drops of antifreeze each night.
Thanks everybody for straighting me out on the lock up converter.The vaccumn line hooked up to the switch now does not go to the engine it heads down towards transmission.I sucked on it and heard something move.I have to investigate the vaccumn line when I get it on the lift.

[56weav]

Hello , upon my return both vacuum over electric switches on the firewall were not hooked up to vacuum , so both now are hooked up and receiving vacuum from the carb.
The distributor has the 4 wires coming out of the bottom and a vacuum pull on the distributor along with a knock sensor. The vacuum diagram does show a vacuum line from the carb to the distributor btw.
I drove the truck and I can’t say I tell any difference . I did fine anothervacuum leak and the cruise control is working better ( cruise was non functional when I bought the truck )