What are these devices mounted to the firewall?

[CheemsK1500]

I've messed with squares for years now, but I have never had a reason to mess with these things. They look like solenoids with vacuum lines attached. I still don't know what they are called or what their actual purpose is. I'm throwing in towel and asking you fine people. Thanks.

 

[Radiohead]

IIRC, those are solenoids for a vacuum dump feature having to do with emissions and/or ignition timing.
Like I said, if I remember correctly....

 

[scrap--metal]

The one by the brake booster is a vacuum switch that will unlock the torque converter on your 700R4 when accelerating in overdrive (when it's all working properly).

If you follow the wires, they will run down to your transmission. Or wherever someone left them hanging if you drive a basket case.

 

[CheemsK1500]

The one by the brake booster is a vacuum switch that will unlock the torque converter on your 700R4 when accelerating in overdrive (when it's all working properly).

If you follow the wires, they will run down to your transmission. Or wherever someone left them hanging if you drive a basket case.

So on a TH350/400 it does nothing? I've seen these on both TH350/400 and 700r4 equipped trucks.

 

[scrap--metal]

I don't think that switch was ever present on a factory equipped TH350/400 truck, only trucks with overdrive edit: lock-up converters (700R4 and maybe TH350c). Someone will probably chime in soon to correct me if I'm wrong.

 

[Goldie Driver]

I don't think that switch was ever present on a factory equipped TH350/400 truck, only trucks with overdrive (700R4 and maybe TH350c). Someone will probably chime in soon to correct me if I'm wrong.

The 1st pic is exactly what Goldie has with a TH350C although mine is spun 180 from your pic.

You may be able to see it here.

I dont have the second- if an 81 up 305 maybe for the ESC.

 

[CheemsK1500]

I don't think that switch was ever present on a factory equipped TH350/400 truck, only trucks with overdrive (700R4 and maybe TH350c). Someone will probably chime in soon to correct me if I'm wrong.

The truck pictured in my first post is an '85 C10. Perhaps it originally had a TH350c that got swapped for a regular th350? The VIN says it started life as th350 truck. And the shift indicator on the collum is for a 350 as well. I honestly have no idea how overdrive systems work on these trucks, so your explanation makes plenty of sense to me.

 

[Turbo4whl]

I don't think that switch was ever present on a factory equipped TH350/400 truck, only trucks with overdrive (700R4 and maybe TH350c). Someone will probably chime in soon to correct me if I'm wrong.

Yes, TH350 has the same convertor locking system as a 700R4. Wiring is slightly different between the two.

You may have the 2 switches mixed up though. The switch with the delay valve is for the trans. The other one is for the Electronic Spark Control (ESC)

You must be registered for see images attach

@CheemsK1500 The whole system for the trans lockup has 2 more controls. Brake pedal defeat switch and a thermal vacuum switch that does not provide vacuum when the engine is stone cold.

The vacuum switch closes at about 13 inches of vacuum (converter locked) and holds down to about 9 inches of vacuum.

 

[CheemsK1500]

Makes sense. I suppose on a truck that's had its ESC deactivated and/or it's transmission swapped, you could omit these and just plug the vacuum ports that the lines originate from.

 

[scrap--metal]

The switch with the delay valve is for the trans.

You're right about that. I believe the vacuum switch for the 700R4 transmission on my '85 K10 is by the booster though. Odd that his would be in the opposite position, or I'm totally off my rocker right now.

Maybe it's a 700R4 vs TH350c thing? Lock-up vacuum switches in different positions?

 

[CheemsK1500]

Thanks for the explanation of the lockup system @Turbo4whl

 

[CheemsK1500]

You're right about that. I believe the vacuum switch for the 700R4 transmission on my '85 K10 is by the booster though. Odd that his would be in the opposite position, or I'm totally off my rocker right now.

Maybe it's a 700R4 vs TH350c thing? Lock-up vacuum switches in different positions?

My 84 K1500 was originally a 700r4 truck (it was converted to a SM465 at some point) I'll take a look tomorrow and see how it's set up.

 

[CheemsK1500]

I looked today, and the my '84 K1500 is set up the same way that my '85 C10 in the first post was. The C10 was a factory th350 and the GMC was a factory 700r4.

 

[ak4life]

Yes, TH350 has the same convertor locking system as a 700R4. Wiring is slightly different between the two.

You may have the 2 switches mixed up though. The switch with the delay valve is for the trans. The other one is for the Electronic Spark Control (ESC)

You must be registered for see images attach

@CheemsK1500 The whole system for the trans lockup has 2 more controls. Brake pedal defeat switch and a thermal vacuum switch that does not provide vacuum when the engine is stone cold.

The vacuum switch closes at about 13 inches of vacuum (converter locked) and holds down to about 9 inches of vacuum.

Hello, you are the first person ive seen that has given some useful info regarding tcc lockup, i am in process of trying to put truck back together and many of my hold ups are regarding the trans and wiring...before i ask my question i will fill you in on vehicle, none of which is rpo...it was an 81 chevy k1500 with esc, and i assume a th350 trans and 208 tcase. It now has a 700r4 with 2500 stall, 383 stroker no esc, and 1989 4 sp column.....so if i have the 700r4 with 89 column, i have enough notches to match trans, so if i have a 4 way plug for trans and one is positve for a seperate manual switch, a ground and ? It only has 3 wires coming from it....then i have the 1981 relay for tcc under driverside dash up on outer frame that covers the column, i even have the original pigtail, it has duplicate wires (i assume because it becomes the middle man to wire through brake pedal, cruise and then to trans. So do i use stock relay with manual aftermarket switch for tcc lockup or eliminate it and i did have the vacuum setup also like in earlier pic hoping i can eliminate as much stock parts, hoses, un neccessary items as possible.....sorry if this is a little scattered and not directly to the point......ill try to refine in the meantime....thank you in advance....