Two Connections - What for?

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

Scott91370

Full Access Member
Joined
Apr 2, 2018
Posts
924
Reaction score
899
Location
Burleson, Tx
First Name
Scott
Truck Year
1985
Truck Model
Sierra1500
Engine Size
350
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.


You must be registered for see images attach


You must be registered for see images attach


You must be registered for see images attach
 

Bextreme04

Full Access Member
Joined
May 13, 2019
Posts
4,135
Reaction score
4,983
Location
Oregon
First Name
Eric
Truck Year
1980
Truck Model
K25
Engine Size
350-4bbl
Hmmm... I have an 80 Sierra Classic and I don't have any of those wires or plugs.
 

TubeTruck

I'm from Boston. Deal with it.
Joined
Sep 4, 2019
Posts
1,417
Reaction score
2,135
Location
East TN
First Name
Denis
Truck Year
1978
Truck Model
Suburban K10
Engine Size
LQ9
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

Full Access Member
Joined
May 13, 2019
Posts
4,135
Reaction score
4,983
Location
Oregon
First Name
Eric
Truck Year
1980
Truck Model
K25
Engine Size
350-4bbl
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 :shrug:
 

EvilGenius

Full Access Member
Joined
May 12, 2016
Posts
178
Reaction score
116
Location
Metro Detroit
First Name
Darien
Truck Year
1985
Truck Model
K10
Engine Size
305
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.
You must be registered for see images attach
 
Last edited:

Bextreme04

Full Access Member
Joined
May 13, 2019
Posts
4,135
Reaction score
4,983
Location
Oregon
First Name
Eric
Truck Year
1980
Truck Model
K25
Engine Size
350-4bbl
Never mind... choke is just a single wire
 

EvilGenius

Full Access Member
Joined
May 12, 2016
Posts
178
Reaction score
116
Location
Metro Detroit
First Name
Darien
Truck Year
1985
Truck Model
K10
Engine Size
305
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

Full Access Member
Joined
Apr 2, 2018
Posts
924
Reaction score
899
Location
Burleson, Tx
First Name
Scott
Truck Year
1985
Truck Model
Sierra1500
Engine Size
350
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

Full Access Member
Joined
Apr 2, 2018
Posts
924
Reaction score
899
Location
Burleson, Tx
First Name
Scott
Truck Year
1985
Truck Model
Sierra1500
Engine Size
350
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.
You must be registered for see images attach
 

Craig 85

Full Access Member
Joined
Jan 25, 2017
Posts
3,889
Reaction score
4,024
Location
Nashville, TN
First Name
Craig
Truck Year
1985
Truck Model
K30 SRW
Engine Size
454/TH-400/NP205
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.
You must be registered for see images attach

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

You must be registered for see images attach
 

Scott91370

Full Access Member
Joined
Apr 2, 2018
Posts
924
Reaction score
899
Location
Burleson, Tx
First Name
Scott
Truck Year
1985
Truck Model
Sierra1500
Engine Size
350
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. :flame:
 

Scott91370

Full Access Member
Joined
Apr 2, 2018
Posts
924
Reaction score
899
Location
Burleson, Tx
First Name
Scott
Truck Year
1985
Truck Model
Sierra1500
Engine Size
350
****************************** 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.


You must be registered for see images attach


You must be registered for see images attach


You must be registered for see images attach
 

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,121
Reaction score
6,038
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
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

Full Access Member
Joined
May 24, 2019
Posts
11,599
Reaction score
53,648
Location
OH-MI: Just like it sounds
First Name
Chris
Truck Year
1985
Truck Model
K20
Engine Size
5.7
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

Full Access Member
Joined
Sep 8, 2019
Posts
9,121
Reaction score
6,038
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
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
 

Latest posts

Forum statistics

Threads
41,849
Posts
903,449
Members
33,362
Latest member
Dhatch84
Top