vacuum to electrical switches on the firewall?

[benoit454]

Guys I recently started removing vacuum lines and uneeded emissions junk on the 84 c10. I ran into a couple that gave me questions as to what they go to or work.

They ran off the vac. ports in the thermostat housing, the switch on it had vac. lines that one runs across the intake, valve cover, and up to the firewall, through that sleeve over the brake booster, and back out in the middle right by the sleeve where it connects to this switch or something that has electrical wires on the one side and that vacuum line on the other.

It has the same thing on the other side of the firewall up top had a vac. line going to it and then out the other side is electrical wires, they run down behind the motor and tie in right beside the trans. dipstick, where it's one big switch that has electrical and vac. to it also. then the wires run out and down the back or the motor/bell housing, then over towards the brake booster, and down somewheres.

Now the truck is heat only and th350, not 350c and the trans. HAD the TSC on it, on the pass. side.

Now I unhooked the vac. to electrical switch on the pass. side, that goes down to the big one by the dipstick, unhooked it's vac. line. I also unhooked the vac. lines going to the big thing by the tranny dipstick, and all the ones on the thermostat EXCEPT for the one going to the switch by the brake booster.

Anyone know what they are for? heater? anything important? doesn't seem to have any vac. leaks, but haven't driven it. Only started it and revved it up while listening, seems fine. I can try and get pics if needed.


Thanks for the help. Also I looked in the hayes manual and found nothing at all, there pics were of an a/c truck and didn't have the vac. to electrical things on the firewall.

[James McClure]

Chris: We need some pics. Sounds like some of the wireing\switches are for TCS, but they (84's) didn't have TCS. (Transmission Controled Spark) Mines' an 84 and it doesen't have all that stuff. It may have something to do with AIR (smog pump) system or the evaporative emmissions system. Yup, get us some pics and try if you can to get the wires clean enough so the colors are evident. I have a GM shop manual for 84 for my truck. I'll do some research after you post the pics. jim

[benoit454]

That was one thing I noticed being an 84 it should be a 350c and not have tcs but this one does and the lmc had tsc for like 73-76 or something only a couple of years and it's hooked up so I have no idea.

I have some old pics to show their location but no new ones that are good and up close, I'll have to work on getting some if I can. might take a while though.

[benoit454]

here's pics of the tsc....









these are the old pics to show the vac. to electrical switches on the firewall, might be hard to see....



that;s the one that goes down beside the dipstick and ties into the big plugin vac. source there with electrical out the back of it also.



this one is right by the sleeve over top of the MC and it's line goes from thermostat, to that sleeve, in sleeve, out DS then back out the PS to the switch. Electrical goes down behind motor after the switch or whatever it is.

Sounds like regardless either it's tcs or emissions I can remove them. I wanted to clean it up and do away with them since the vac. was removed and no longer needed.

Also in the thermostat housing with them ports open will they suck in air or something? Do they need plugged off?

[benoit454]

The switch on the driver's side is hard to see but it's next to the booster, to the left of that sleeve over top the booster and below and to the right of the diamond shape junction box for the ignition. It looks exactly like the other one which you can see easy in the first pic next to the dipstick, you can see the vac. line going to it and the wires coming out of it.

[James McClure]

Chris. I think I have enough to work with. Where vac lines are concerned, just trace them back and see where they go. If they go back to the carb or manifold plug them off at thier source. Need more info just e-mail me with your number and I'll call ya. e-mail is "jmcclur2@nycap.rr.com

[James McClure]

Chris. Found it. Look at dist. See if it has a 4 wire connector pigtail comming out of it. If it does it's ESC. In order to change the whole thing away from ESC your going to have to swap out the dist also. The ESC module is located under the dash on the steering colunm support. The reason mine doesen't have it is it's a 350 M engine in a 3/4 ton.

[benoit454]

yea it's got the big plug in on the distributor but what do the firewall vac. to electrical ones have to do with it or tie in? I got new pics I'll post up later on tonight.

I hope none of them affect the dist. that way I can do away with it.

[James McClure]

Chris: Your going to have to swap out the distributor first thing. If you don't and you remove the wireing and the switches and the controler under the dash, she ain't gonna' run.

[benoit454]

well the one in the first pic the vac. is already removed from it and the one big vac to electrical by the dipstick, but the big electrical for the distributor is still hooked up, because if that was unhooked it not even crank, I found that out along time ago.

I wasn't going to remove anything under the dash only those switches or whatever off the firewall and remove their vac. lines.

See we had an 86 with the same distributor setup and after looking back it had the switches too but where they got vac. from I have no idea here's a pic of them...



Now we had all the lines removed from it only ones hooked up was for vac. advance. you can see the big hookup for the distributor in that pic but there's another one on the truck we got now, that last pic there was from the 86. The rest are the 84 in question. The other big one does the same by the dipstick on the 84 has alot of vac. lines and then goes to electrical. I'll look back at more pics and see if I can find anything.

Only thing involved with the electrical I was going to do was unhook them from the switch or whatever on the firewall and then lay then down out of the way, the electrical hookups that is, or tape them up to the wiring harness out of the way to hide them.

[MylilBowTie]

The ESC box is located behind the glove box. There could be something else related on the column but I don't think so.

Here is a link to remove the ESC. http://www.73-87.com/7387garage/drivetrain/escrepl.htm

Those edelbrock foam air filters are really bad for causing fires.

[James McClure]

I got the module location from the GM shop manual for 84 model year. I worked for them for 33 years. I do however remember that module did get moved a couple of times. At one point it was mounted out in the engine compartment on the firewall. I never liked this setup. Some worked flawlessly for years (a rarity) and others broke every week. Now GM's electronics are quite good and failures of a particular part is pretty rare. The biggest problem is the VERY small guage wireing and harnace routing, compounded by high underhood heat and limited space. I got out of the business in '99 so it is rare I stick my head under the hood of a late model, but when I do I see they haven't corrected this particular issue. I hated electrical work on any GM truck built after 87, especially the diesels. Jim

[MylilBowTie]

I am not trying to discredit you. I have had about 5 trucks mostly 83-87 trucks with ESC and they all had the control boxes behind the glove box. I have also parted a lot of 81-87 trucks out. That's the only place I have ever seen them. In the 87 that's where the ECM would be put. My 1981 factory manual also shows that its on the column.

I've thought about using that spot for an msd control box if I ever decide to install one.

[benoit454]

ok guys this is off topic but what I've seen behind the glovebox looks exactly like an ecm and the book I got from hayes even says it's ecm even on early then 87 trucks it's an ecm for the esc, because it says when the ecm loses esc signal then it'll retard timing and what not. It also shows a pictures of the esc module and it's on the steering coloumn you can tell by the picture what's around it.

[benoit454]

back on topic here's more pictures up to date. of the truck in question.....



that'sd the pass. side one in question which vac. is now unhooked from, I unhooked it the other day, before then it was ALL hookd up.

The wires from that switch go down and tie into this one which is on top the manifold right by the trans. dipstick, here it is....



Then the other switch is here by the booster... which is still hooked up and I'll show you were it's vac. hooks up at after this pic.




That is the ported switch in the thermostat housing and I wonder if they need plugged off or not.



Just throwing out a pic of the engine lol.

[benoit454]

well looks like from reading and searching the one on the pass. side is for the esc and the tip in stuff for heavy throttle or whatever and sounds like it be fine if it's unhooked just maybe it might spark knock under hard throttle.


The driver's side one by the booster is for the lockup tc that is supposed to be in an 84 but isn't on ours. So that can be unhooked also, and I need to plug the tvs ports so it doesn't suck air or anything once open.


That all sound correct to you guys?

[James McClure]

Chris: I'm glad you found the ESC module. i was begining to think I lost my last brain cell. I must admit I don't remember the ECM over or behind the glove box, it's been a few years since I fixed these things for a living. More often than not, the failures I encountered were out in the engune compartment. Mostly wireing or module failure brought on by problems in the secondary ign circut. From the best I can figure the ESC system out from wireing diagrams is the ESC module recieves info from engine sensors and sends it to the ECM which processes it farther and sends it's best guess to the ign module, BUT, thats just a guess. Reading farther I see that vehicles destined for Calif had full ECM engine controls (feedback carb) so I guess it would be safe to say mid to late 80's vehicles were transitional between no emmissions and full computer control. Mine is an 84, but it's 3\4 ton and has nothing on it except the AIR system which is LONG gone and is leaded fuel OK. Jim

[mckpaul]

Mine's an 84 and I have both of those switches. They both show up on the vacuum diagram. The one on the passenger side is the tip in switch. Back when I was putting all that stuff back together to pass emissions tests here in Houston (bought my truck from somewhere else in Texas and it was all pulled loose) I did a bunch of research and was up to speed on what it did. I might have some info saved at home.

As for the other switch on the driver side, I have a 700r4 with lockup. The power to your transmission that provides voltage to the lockup converter routes thru 3 or 4 switches, all in series, and all have to be closed for lock up to happen. If any of them open, lock up drops out. One is on the brake pedal so lock up drops out when you step on the brake, one is that switch in question on your fire wall that monitors vacuum, and when you put your foot in the gas pedal to accelerate you loose some vacuum and the switch opens and that makes lock up drop out so your tranny will downshift to pass or whatever. The other one or two switches are inside the transmission, and don't close until you reach 3rd and 4th so lockup can't happen in 1st or 2nd (which would obviously bog you down) Your truck probably originally came with a tranny with a lockup converter. If you don't have a lock up converter now, you don't need that switch. Just follow the vacuum line back to where ever it connects to a vacuum source, pull it off and cap off the port.

If I can find the info on the tip in switch, I'll post it.

[scottsdaleowner75]

I know this is a old thread but i am having a difficult time with my truck right now hooking all this stuff back up. Unfortunately i am in CA and i want to make the truck as stock as i can. So the vacuum switch on the passenger side firewall is the tip in switch and the other is the trans conv vac sw?

[staggerd86c-10]

I thought the vacuum switch on the passenger side was for the lock up, since i changed out the distributor i dont need the one on the passenger side? or that little harness that is just hanging there? awesome!!!

[Smoke_Dog]

I have some serious question now!!! I'm taking pictures tomorrow and posting them! If the 700r is not locking up, what kind of effect do U experience when driving! I get a steady sound of pressure to get a gear change but it want release unitl I back of the gas a little. Then it will go back and forth form over drive to the regular driving gears! If I'm explaining it correctly!!!