Two Connections - What for?

[Scott91370]

I have an 85 Sierra Classic cab that is going on an 82 Sierra chassis/engine/trans. These wires com out of the fire wall then magically multiply and end in these 3 plugs which I have no idea were they go (well, the white square 4 pin should go to the transmission). None of the diagrams I have found show this.

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[Bextreme04]

Hmmm... I have an 80 Sierra Classic and I don't have any of those wires or plugs.

 

[TubeTruck]

The big black one looks like maybe it's for an oil sending unit? Not sure if they are single or dual prong though, unless those are the style with the choke that runs off it.

 

[Bextreme04]

The big black one looks like maybe it's for an oil sending unit? Not sure if they are single or dual prong though, unless those are the style with the choke that runs off it.

ooooh thats a good thought, how long are the wires? If the truck had an electric choke the black plug might have gone to the oil pressure switch right above the oil filter, the red one might have been the choke power/ground

 

[EvilGenius]

So those wires have to do with the 700R4 tcc lock-up. The white 4 pin connector plugs in to the "top" of the transmission pan on the driver's side. The red 2 pin connector is for the EGR solenoid. Essentially how the lock-up system wiring works is the transmission's hydraulic line pressure dictates when to lock the tcc in 3rd and 4th gear. To unlock/prevent lock from occurring a voltage signal running to the trans via that 4 pin connector is grounded. The voltage signal is interrupted via your brake pedal and with some models, the EGR solenoid (detects engine load). If you trace the source of those wires in the cab you should see that they run through the brake pedal switch and have a constant power source if ignition is on.

Like others have said, that black rubber connector looks like an oil pressure switch connector. Typically used in conjunction with the electric choke to only have the choke circuit on when there is oil pressure detected.

Keep an eye out for an additional connector as part of the transmission lock-up wiring. On the firewall just left of the brake booster towards the engine there should be a little vacuum modulator (called low vacuum switch) with a vacuum port on one side and a 2 wire connector on the other. If you look at this wiring diagram below (from an 84 model I think) you should see how that lock-up circuit functions.

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[Bextreme04]

Never mind... choke is just a single wire

 

[EvilGenius]

Choke is single wire with 12v on it. However, on many factory setups that single wire is routed through the oil pressure switch so that the 12v only gets passed to the choke (and thus opening it) if there is oil pressure.

 

[Scott91370]

I have the oil pressure switch hooked up and it does look similar but it has 3 wires that come from it and go out to the electric choke and a couple other spots.

@EvilGenius - I do have the vacuum solenoid just haven't gotten far enough to hook it up yet, but that diagram will help!

Thanks for the input so far everyone!!!

 

[Scott91370]

Found another connector. This one comes out directly below the bulkhead connect and has a 1/2" (vacuum) line with it. I think it may be cruise control but the under hood module is not there to compare the plug to.

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[Craig 85]

Found another connector. This one comes out directly below the bulkhead connect and has a 1/2" (vacuum) line with it. I think it may be cruise control but the under hood module is not there to compare the plug to.

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Here's what the cruise control looks like. I know my truck has the same hose.

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[Scott91370]

Still got these two connectors. The only 2 I don't know where they go and cannot find them on a wiring diagram. I'm going to start watching for similar trucks in the area and just ask someone when I find a truck.

Thanks everyone for the input. Next is vacuum connections, which I don't think will be any easier than the wiring.

 

[Scott91370]

****************************** Update ******************************

The round black wire is for TH350C transmission pressure switch on the passenger's side of the transmission. It is used for locking/unlocking the convertor in 3rd gear.

I have an 85 Sierra Classic (TH350C transmission) cab that is going on an 82 Sierra (700R4) chassis/engine/trans. These wires com out of the fire wall then magically multiply and end in these 3 plugs which I have no idea were they go (well, the white square 4 pin should go to the transmission). None of the diagrams I have found show this.

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[AuroraGirl]

the green wire on my truck from that spot apparently runs to fuel switcher under truck... but i dont think its supposed to. i was trying to figure it out myself. 1980

 

[Doppleganger]

This is probably a mile off, but that round black one looks identical to a pressure switch for AC.

I had a similar plug on mine that after dissecting the truck I found it to be related to the tank switching solenoid.

 

[AuroraGirl]

This is probably a mile off, but that round black one looks identical to a pressure switch for AC.

I had a similar plug on mine that after dissecting the truck I found it to be related to the tank switching solenoid.

oh maybe im right then