1971 GMC Wiring mystery - The 1947 - Present Chevrolet & GMC Truck Message Board Network

harlyworkr · 05-20-2008, 05:34 AM

I hve a 1971 GMC V8 truck that has a strange harness on the engine and I cant find out what it goes to orwhat it it is for, can someone please shed some light on this mystery?
The harness plugs into a brown wire just above the fuse box inside the engine compartment, it has what looks like a horn relay and another relay attached to it, it travels across the firewall and has three different connectors on it. One looks like it goes to a oil pressure or temp switch, the other one looks like it is a alt plug w/3 wires into the two place plug, and the third plugged into the 350 turbo trans just behind the dipstick tube. I am confused as I have not been able to find this on any schematic.

Mike

tcb-1 · 05-20-2008, 05:43 AM

Mike,
Howdy neighbor! Question: Is the transmission original to the truck? If not, it may be to the original transmission harness.

Maybe some of the automatic guys can shed some light? I'm just a lowly SM465 guy, myself.....

Longhorn Man · 05-20-2008, 05:51 AM

TCS set up (transmision Controlled Spark)
A primitive smog control set up, it would not let the vac advance do anything till the engine was warmed up, and in 3rd gear.

**WorkinLonghorn** · 05-20-2008, 05:52 AM

I've heard of this before and I think the concensus is that it is fron the TCS, Transmission Controlled Spark. I posted a diagram of it a while back. It would prevent diff. vac. advance in lower gears and other conditions.

**WorkinLonghorn** · 05-20-2008, 05:53 AM

That's what I get for nevr learning to type,beat out by LM AGAIN!

Longhorn Man · 05-20-2008, 06:14 AM

LOL! I type with 4 fingers myself.

boundstaffpress · 05-20-2008, 06:25 AM

The three wires should connect to a solenoid on the throttle at the carb. The solenoid will push the throttle while in 3rd gear and allow the throttle to completely close when the engine is off so that it wont diesel.

**WorkinLonghorn** · 05-20-2008, 06:48 AM

Here it is,hope this helps;

harlyworkr · 05-21-2008, 04:36 AM

So the consensus is that I should not hook it back up?

**WorkinLonghorn** · 05-21-2008, 05:02 AM

If you are getting too much mileage then hook it up! You wouldn't want the timing to be correct in the lower gears!!!
(JK) I think it was one of those smog devices that may have cut down emmissions like 2% or something,and helped meet the requirements being enforced,but when the vehicle needed to burn 10% more gas to get from A-B It was in the real world a total bust.
So 86 the junk,(unless you may have to pass original smog equioment test in the future!).

harlyworkr · 05-21-2008, 05:18 AM

Thanx for your help! New question if you will indulge me.... I added Cruise, Tilt, 4 wheel disc brakes, and AC. The ? is...Which cables go to which lever on my heater controll w/ac?

**WorkinLonghorn** · 05-21-2008, 07:17 AM

I've never had to look at that(yet),so I don't know/Start a new thread and you will get the answer for sure.

05-20-2008, 05:34 AM	#1
harlyworkr Registered User Join Date: Sep 2005 Location: Kansas City Kansas Posts: 212	1971 GMC Wiring mystery I hve a 1971 GMC V8 truck that has a strange harness on the engine and I cant find out what it goes to orwhat it it is for, can someone please shed some light on this mystery? The harness plugs into a brown wire just above the fuse box inside the engine compartment, it has what looks like a horn relay and another relay attached to it, it travels across the firewall and has three different connectors on it. One looks like it goes to a oil pressure or temp switch, the other one looks like it is a alt plug w/3 wires into the two place plug, and the third plugged into the 350 turbo trans just behind the dipstick tube. I am confused as I have not been able to find this on any schematic. Mike

05-20-2008, 05:43 AM	#2
tcb-1 Nothing to see here..... Join Date: Jun 2007 Location: Shawnee, KS Posts: 4,625	Re: 1971 GMC Wiring mystery Mike, Howdy neighbor! Question: Is the transmission original to the truck? If not, it may be to the original transmission harness. Maybe some of the automatic guys can shed some light? I'm just a lowly SM465 guy, myself..... __________________ Doug THANK YOU to our American Soldiers & Veterans - POW MIA "You will NEVER be forgotten". The democracy will cease to exist when you take away from those who are willing to work and give to those who would not. Thomas Jefferson

05-20-2008, 05:51 AM	#3
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Re: 1971 GMC Wiring mystery TCS set up (transmision Controlled Spark) A primitive smog control set up, it would not let the vac advance do anything till the engine was warmed up, and in 3rd gear.

05-20-2008, 05:52 AM	#4
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,883	Re: 1971 GMC Wiring mystery I've heard of this before and I think the concensus is that it is fron the TCS, Transmission Controlled Spark. I posted a diagram of it a while back. It would prevent diff. vac. advance in lower gears and other conditions. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.

05-20-2008, 05:53 AM	#5
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,883	Re: 1971 GMC Wiring mystery That's what I get for nevr learning to type,beat out by LM AGAIN! __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.

05-20-2008, 06:14 AM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Re: 1971 GMC Wiring mystery LOL! I type with 4 fingers myself.

05-20-2008, 06:25 AM	#7
boundstaffpress Jamie Hyneman is my twin Join Date: Nov 2005 Location: La Junta, Colorado Posts: 705	Re: 1971 GMC Wiring mystery The three wires should connect to a solenoid on the throttle at the carb. The solenoid will push the throttle while in 3rd gear and allow the throttle to completely close when the engine is off so that it wont diesel. __________________ Justin SOLD 1967 K10 Fleetside, 283, Front and Rear Posi SOLD Color Wiring Diagrams Now in High Resolution V8 and Cab and Cab and Chassis

05-20-2008, 06:48 AM	#8
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,883	Re: 1971 GMC Wiring mystery Here it is,hope this helps; __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.

05-21-2008, 04:36 AM	#9
harlyworkr Registered User Join Date: Sep 2005 Location: Kansas City Kansas Posts: 212	Re: 1971 GMC Wiring mystery So the consensus is that I should not hook it back up?

05-21-2008, 05:02 AM	#10
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,883	Re: 1971 GMC Wiring mystery If you are getting too much mileage then hook it up! You wouldn't want the timing to be correct in the lower gears!!! (JK) I think it was one of those smog devices that may have cut down emmissions like 2% or something,and helped meet the requirements being enforced,but when the vehicle needed to burn 10% more gas to get from A-B It was in the real world a total bust. So 86 the junk,(unless you may have to pass original smog equioment test in the future!). __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme. Last edited by WorkinLonghorn; 05-21-2008 at 05:08 AM.

05-21-2008, 05:18 AM	#11
harlyworkr Registered User Join Date: Sep 2005 Location: Kansas City Kansas Posts: 212	Re: 1971 GMC Wiring mystery Thanx for your help! New question if you will indulge me.... I added Cruise, Tilt, 4 wheel disc brakes, and AC. The ? is...Which cables go to which lever on my heater controll w/ac?

05-21-2008, 07:17 AM	#12
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,883	Re: 1971 GMC Wiring mystery I've never had to look at that(yet),so I don't know/Start a new thread and you will get the answer for sure. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.



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