Unknown wire in engine bulkhead connector ???
 

Heres a pic of the engine connector on our 1971 C20. Has 350/350 no AC.

Redoing harness for HEI, electric choke, and tach.

Middle top wire - green - hard to see in pic is temp sender.
Top left light brown wire goes to brake prop valve
Big yellow wire goes to inline fuse and then to HEI (I replaced resitor wire)

Bottom left dark bown wire is the unknown wire, it goes into the wire channel and then is terminated with a plastic male end and hidden inside the plastic wire run. I checked the fuse box side and it is connected and dissappers into the large bundlel of wires going up under the dash. So in must or could have a purpose.

Middle botttom purple wire goes to the starter.
Bottom right blue wire I added for tach connecton (was unused)
(added matching connector to fuse block also)

Does anyone know what that brown wire in the left botttom corner is for ??
I could remove it to clean up the harness a bit, but it is connected on the other side, kind of hate to remove it if it is good for some thing, even if not used now - could that be a factory tach wire ? - truck does not have factory tach.
Thanks for your help.

Re: Unknown wire in engine bulkhead connector ???
 

playing with mine myself, if I'm following you correctly the brown wire as you say, could it be Brn/WT brown with white stripe if so this is as in my bulk head also. it to me appears to be for "TSC" traction control solinoid. but i am sure someone else will respond besides me. mine did not go out to the engine bay but it is in the wire harness into the cab, either going to the starter switch or the plug that goes into the instument cluster

Re: Unknown wire in engine bulkhead connector ???
 

Great timing - I am looking to rebuild my engine harness as well - and can check when I get home...

What terminals did you use to add a wire to the plastic harness? Does a local place carry them or did you find them online?

What substance is used to 'weather-proof' the connection on there?

Re: Unknown wire in engine bulkhead connector ???
 

TRACTION CONTROL SOLENOID??? :lol::lol:

there is however a transmission spark control an early form of lame emissions control devise that was normally disconnected within 5minutes of delivery of the trucks when they were new off the dealers floor

Re: Unknown wire in engine bulkhead connector ???
 

I get the terminals from NAPA , just have to have a guy that knows what you are talking about. Neat trick on removal from the harness side terminal is to cut at the top of the slot with a wire cutter, the "leg" falls off and you don't have to try to squeeze it to get it out.

I use "liquid electrical tape to seal everything back up. And I always solder anything new that I put in for extra assurance that the connection is good.

Now - what is that wire for ????????????

Re: Unknown wire in engine bulkhead connector ???
 

Could it be for a generator dummy light?

Re: Unknown wire in engine bulkhead connector ???
 

Checked my harness(es) - and it doesn't exist on any of the 4 trucks that I have from this era...

To get to the bottom of it - you will have to trace it back inside the cab to be sure - there may not be an easy answer to this - it may just terminate the same way in the dash harness...

Re: Unknown wire in engine bulkhead connector ???
 

Thanks for the info guys - if anyone figures it out let me know. I think for now I am going to remove it and its big terminal end from the engine harness, there is hardly enough room to get the wiper hoses in there now. And I am using a repop plastic harness and it is smaller that the original.

Re: Unknown wire in engine bulkhead connector ???
 

Is the wire Black with Brown? If so it should be going to the junction block that goes to the battery. This wire feeds into your Battery gauge in the dash. You do have gauges right? Does the Battery gauge ever move?

Re: Unknown wire in engine bulkhead connector ???
 

If its 16 guage plain brown it looks to me from the wiring diagrahm it should go to voltage regulator. See this thread, Post number #19.

http://67-72chevytrucks.com/vboard/s...d.php?t=185856

Re: Unknown wire in engine bulkhead connector ???
 

Scratch the last two things I said! I went out and looked at my truck. And I too have the same wire that is run in the cowl wire channel and terminated with a male wire connector that goes to nothing? I will ask GMC Paul. I bet he know!

Re: Unknown wire in engine bulkhead connector ???
 

The puzzle continues - nice to know there are more trucks with this "option".
But what does it do ?

Re: Unknown wire in engine bulkhead connector ???
 

I know it is not the transmission spark control. The TSC wire does not go through that bulk head. That wire is added separately.

I know, not much help. Sorry.

Re: Unknown wire in engine bulkhead connector ???
 

We may have a winner here.

JimKshortstep4X4 wrote in another thread:
"I don't have a picture but if you have the correct harness there should be a red wire and brown wire to be used for hooking up the tach. The red wire goes to the "Unfused ignition" terminal on the fuse panel. The brown wire goes to the terminal on the HEI marked "tach". The tach gets its ground from the case so once you have the two wires hooked up, you should be good to go."

Could it be an unused factory tach wire?

Re: Unknown wire in engine bulkhead connector ???
 

could it be one of the wires you bypass when you hook up an internal reg, alternaor and remove the external regulator?

Re: Unknown wire in engine bulkhead connector ???
 

I've checked into this wire it's brown with a white tracer and isn't shown on any diagrams I've seen on here. Ive been redoing my harnesses and traced every wire on the truck.. that wire is connected to the back side fused blade of the radio fuse holder and is key power only. It does not come up behind the dash for anything, it just goes through the firewall connector and into the harness in the gutter above the engine. My guess would be a power antenna where it runs off the radio fuse. I left it there for exactly that reason.. figured I'd find one and install it someday.

Re: Unknown wire in engine bulkhead connector ???
 

It's on this diagram:
http://67-72chevytrucks.com/vboard/s...d.php?t=290757

It runs from the accessory position of the keyswitch to the bulkhead connector and appears to connect to the brown wire at the connector (that's why it's only on one side). My guess is it supplys power to the other side of the generator light to keep the bulb turned off when the key is in the accessory position. Or something like that anyway.

Re: Unknown wire in engine bulkhead connector ???
 

that diagram is the older trucks that brown/white tracer is the one for the generator dummy light on the dash, it gives the light a ground when you turn the key to the on position before you start.. same as the brake oil press and temp dummy lights (tan/green and blue at the key connector on the 70's). The brown/ white tracer he's talking about is only at the firewall connector but doesn't travel up the harness to the dash.

Re: Unknown wire in engine bulkhead connector ???
 

The wire in question is DEFINATELY for the TCS ( Transmission Control Spark ) solenoid that attached at the carb, the brown wire in question runs from the engine junction block connector to a female wire connector that then splits the wire into 2- 20g brown w/white stripe wires that then ran the rest of the way to the TCS solenoid at the carb.
The easiest method of eliminating this solenoid was unplug at connector you found and remove solenoid & its other harness's which are all separate of any wiring looms.
GMCPaul

Re: Unknown wire in engine bulkhead connector ???
 

Hmm. Thing must have changed from 70 to 71. My TSC wire is red and comes through the firewall separately. Learn something new every day.

Re: Unknown wire in engine bulkhead connector ???
 

ERASER5 the solenoid attached to the carb that has a single PINK wire feeding it is a idle control solenoid not a TCS solenoid, the TCS solenoid has 3 wires that feed connect onto it at carb and 3 vaccuum connections on it. I have the original GM wiring diagrams that are far more comphrensive than the service manual or assembly manual wiring diagrams that break down each separate RPO option and there respective wiring.
GMCPaul

Re: Unknown wire in engine bulkhead connector ???
 

I'm in need of the wiring diagram. Is it on you site? I've seen (but can't find it now) a picture of the bulkhead connector explaining what each connector did.
Anyone have this?
s/t

Re: Unknown wire in engine bulkhead connector ???
 

Not to be argumentative but, mine was a TSC system. Maybe it was dealer installed? See Pics
I even gave some of the parts to another member. I still have the relay, the transmission switch and the vacuum switch. No idle control was on my truck.
The former red wire feed for the TSC is now the electric choke feed.

Re: Unknown wire in engine bulkhead connector ???
 

Heres a scan of the 1970 V8 TCS RPO-Option wiring, the 71 & 72 wiring diagrams are identical to the 1970 shown below
http://www.gmcpauls.com/70-72_TCS_RPO-Option_Wiring.jpg
GMCPaul

Re: Unknown wire in engine bulkhead connector ???
 

Old thread here but helpful in what I'm trying to track down.
It seems that my 72 Blazer (4 speed manual, 4x4, HEI, elec. choke) was wired for this as the pictures show.
4 wires coming out of the firewall (thick red, black, blue, brown/wt stripe), all going across to the passenger side. The red wire went nowhere..was just dangling loose behind the motor. The blue and brown/wt stripe wire went into a connecter and was joined with a green/wt stripe wire that was connected behind the block on the passenger side but went no where. The black wire went behind the engine and is connected to the top of the transmission, right by the 2 wires which I believe are for the back up lights.
Everything seems to be running fine and I'd like to just remove these wires to clean things up, but 2 of them are actually connected. The black wire to the top of the transmission, and then the brown/wt stripe wire about 6 inches where it comes out of the firewall has a connection to a black wire, that runs back into the firewall. Now-since this brown/wt stripe wire wasn't connected to anything but that connecter on the passenger side, perhaps I can remove this? Hopefully my pictures below help..