Help with underhood harness on a 71 C10 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

AASmedic · 02-15-2020, 01:32 AM

Can anyone tell me what these harnesses and relays go to and what they are used for?? Thanks in advance.

54Caddy · 02-15-2020, 03:18 AM

Emissions stuff. Should go down to the transmission

**Steeveedee** · 02-15-2020, 11:40 AM

Right- Transmission Controlled Spark. It prevented vacuum advance until the engine was warm enough and the trans was in high gear. It looks like the harness has been chopped off. You can remove it or just ignore it. How is your vacuum advance connected to the vacuum source?

AASmedic · 02-15-2020, 10:40 PM

Quote:

Originally Posted by Steeveedee

Right- Transmission Controlled Spark. It prevented vacuum advance until the engine was warm enough and the trans was in high gear. It looks like the harness has been chopped off. You can remove it or just ignore it. How is your vacuum advance connected to the vacuum source?

Makes me wonder if this truck was originally automatic. The connections were just laying behind the engine and not connected to anything. The engine was replaced 20 years ago. Vacuum advance is hooked up to the port on the carb. It has a 4 speed manual transmission. Do I just need to remove this part of the harness?

**Steeveedee** · 02-17-2020, 11:23 PM

You can just toss it if you want. The system also worked on manual transmissions, in which case there was a manual switch on the shift linkage (at the transmission) instead of a pressure switch in the valve body of an automatic transmission.

02-15-2020, 01:32 AM	#1
AASmedic Registered User Join Date: Dec 2015 Location: Albuquerque Posts: 773	Help with underhood harness on a 71 C10 Can anyone tell me what these harnesses and relays go to and what they are used for?? Thanks in advance. Attached Images __________________ My Paypal address is: http://paypal.me/KFerguson66 72 K20 Cheyenne Super My "re-build" thread http://67-72chevytrucks.com/vboard/s...?t=710221&goto Thread for my Reproduction Jack Bags http://67-72chevytrucks.com/vboard/s...d.php?t=715455

02-15-2020, 03:18 AM	#2
54Caddy GM Addicted Join Date: Jan 2009 Location: Visalia, Ca Posts: 2,291	Re: Help with underhood harness on a 71 C10 Emissions stuff. Should go down to the transmission __________________ ..Mac’s Garage.. 73 GMC, 54 Caddy, 2010 Silverado, 68 C10 RIP DAd My 73 GMC Truck Build Pic 2010 Silverado 4/6 on 24s IG:MacsGarage

02-15-2020, 11:40 AM	#3
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,088	Re: Help with underhood harness on a 71 C10 Right- Transmission Controlled Spark. It prevented vacuum advance until the engine was warm enough and the trans was in high gear. It looks like the harness has been chopped off. You can remove it or just ignore it. How is your vacuum advance connected to the vacuum source? __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

02-17-2020, 11:23 PM	#5
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,088	Re: Help with underhood harness on a 71 C10 You can just toss it if you want. The system also worked on manual transmissions, in which case there was a manual switch on the shift linkage (at the transmission) instead of a pressure switch in the valve body of an automatic transmission. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA



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