Wiring help on firewall - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Orion 762 · 09-16-2003, 01:33 AM

I've got a '71 Cheyenne, 350 automatic, bought "used" a month ago. After getting the HEI wiring sorted out, and a new wire run from the eng. harn. conn. to the distributor, I thought I'd go poking around to see where "all those other un-plugged wires went. (I downloaded the 67-72 schematics, and 90% of the wires/colors match the original, but it's that last 10% that I'm curious about).

On the driver side firewall, engine side (looking from the front of the truck with hood open, this would be the upper RH side of the firewall, just behind the driver-side hood hinge) there is a "set" of relays mounted to the firewall (with bolts) and a good handfull of (about 6) wires (black, green, light green, blue, brown-white, green-white) coming "from" the relays to a harness that seems to "go no-where". Two of the wires go to what used to be the "backing lamp switch connector" on the old manual tranny, so I've got them figured out (not needed anymore). Two of the other wires (green and green-white) had a "spade connector" simililar to the one on the temp. sensor on the block. Another one "used" to go the an oil sensor gauge on the block, but was disconnected. Not sure what the other wires do, but most thing seem to work okay without any of them.

Question: any idea what the "relays on the firwall" were for?? I can't find them on the 67-72 schematic.

Any hints/help is appreciated . . . Thanks.

Orion 762

Hooter · 09-16-2003, 02:02 AM

those wires are for your TCS (transmssion controlled spark) what they do (or did) is they didnt allow your vacum advance to work unless the transmission was in third gear. The system was also controlled by temperature, hence the thermostat plug. Don't bother get it working again, it is useless and gives you crappy gas milage.

Longhorn Man · 09-16-2003, 11:26 PM

Hooter, you talking about the relays too..or just the wires.
As far as i know, there are no relays under the hood on these rigs...never seen any on any truck as of yet. (nor in the schematics)

miket · 09-16-2003, 11:32 PM

Yes I have them on my 71 as well....they don't look like regualr "bosch" type relays....smaller and metal cased....Now I will have to trace the wires down and see as well...

Longhorn Man · 09-17-2003, 12:22 AM

Hmmm, can you get pics? I've never seen this.

miket · 09-17-2003, 12:27 AM

My truck is on vacation right now....so I looked at the pics I already had and you can't see them.....so I will have to wait until she comes back.

Longhorn Man · 09-17-2003, 12:32 AM

Cool. I'd really like to see this.

miket · 09-17-2003, 12:34 AM

I just removed them about two/three weeks ago and took a wire wheel to the casing to clean em up.....guess they could be fusible links or circuit breakers...

Orion 762 · 09-17-2003, 01:07 AM

Well folks . . . based on Hooters reply, I went ahead and yanked them out with a BFH, then blow-torched the old bolt holes shut so "no one" could ever put them back on again! I trust I was not too hasty? (/sarcasm . . . )

Actually I did a Google search on TCS for these year model chevy's, and many had them installed. The idea was "not to let you have any vacuum advance" until the automatic tx was in 3rd gear. I guess this was early '70's vintage "environmentalism" at it's peak! (Imagine NO vacuum advance until 3rd gear!)

When I got this truck, this "stuff" had already been disconnected, but the wiring harness (and odd relays) was still intact on the firewall. Getting those useless wires and stuff out sure made things look a LOT better under there.

(Actually I took the wires/relays off very carefully: even noted the "change" in my '71 Chevy Truck Log Book.)

BTW - what I "thought" was my "backing lamp switch connector" (unplugged near the tranny down by the header pipes) was apparently the "connector" used by the TCS setup. Earlier threads here had identified this "hanging connector by the tranny" as backup lamp related. With this "new level of confusion" in my mind . . . . okay then, HOW DO you get the backup lights to work?? The "schematic" showed these LG and DG wires going thru the Engine Harness Connector, then on to the fuse box and the "tail and stop lamp harness". On my truck, they went instead to this TCS setup and relays on the firewall. Hmmm . . .

Thanks for your help on this . . . Orion 762

09-16-2003, 01:33 AM	#1
Orion 762 Registered User Join Date: Sep 2003 Location: Mississippi Posts: 36	Wiring help on firewall I've got a '71 Cheyenne, 350 automatic, bought "used" a month ago. After getting the HEI wiring sorted out, and a new wire run from the eng. harn. conn. to the distributor, I thought I'd go poking around to see where "all those other un-plugged wires went. (I downloaded the 67-72 schematics, and 90% of the wires/colors match the original, but it's that last 10% that I'm curious about). On the driver side firewall, engine side (looking from the front of the truck with hood open, this would be the upper RH side of the firewall, just behind the driver-side hood hinge) there is a "set" of relays mounted to the firewall (with bolts) and a good handfull of (about 6) wires (black, green, light green, blue, brown-white, green-white) coming "from" the relays to a harness that seems to "go no-where". Two of the wires go to what used to be the "backing lamp switch connector" on the old manual tranny, so I've got them figured out (not needed anymore). Two of the other wires (green and green-white) had a "spade connector" simililar to the one on the temp. sensor on the block. Another one "used" to go the an oil sensor gauge on the block, but was disconnected. Not sure what the other wires do, but most thing seem to work okay without any of them. Question: any idea what the "relays on the firwall" were for?? I can't find them on the 67-72 schematic. Any hints/help is appreciated . . . Thanks. Orion 762

09-16-2003, 02:02 AM	#2
Hooter My other Love Join Date: Aug 2001 Location: Castlegar B.C. Canada Posts: 4,085	those wires are for your TCS (transmssion controlled spark) what they do (or did) is they didnt allow your vacum advance to work unless the transmission was in third gear. The system was also controlled by temperature, hence the thermostat plug. Don't bother get it working again, it is useless and gives you crappy gas milage. __________________ Castlegar B.C.The great white North (Canada Eh!) Hooter_5@hotmail.com First generation Monte Carlo club pictures of my life

09-16-2003, 11:32 PM	#4
miket Registered User Join Date: Jan 2003 Location: Seattle WA Posts: 1,466	Yes I have them on my 71 as well....they don't look like regualr "bosch" type relays....smaller and metal cased....Now I will have to trace the wires down and see as well... __________________ FocuzTech Performance Evergreen Protective Coatings Gatorhyde...the best kept secret in spray in bed lining & protection

09-17-2003, 12:27 AM	#6
miket Registered User Join Date: Jan 2003 Location: Seattle WA Posts: 1,466	My truck is on vacation right now....so I looked at the pics I already had and you can't see them.....so I will have to wait until she comes back. __________________ FocuzTech Performance Evergreen Protective Coatings Gatorhyde...the best kept secret in spray in bed lining & protection

09-17-2003, 12:34 AM	#8
miket Registered User Join Date: Jan 2003 Location: Seattle WA Posts: 1,466	I just removed them about two/three weeks ago and took a wire wheel to the casing to clean em up.....guess they could be fusible links or circuit breakers... __________________ FocuzTech Performance Evergreen Protective Coatings Gatorhyde...the best kept secret in spray in bed lining & protection

09-16-2003, 11:26 PM	#3
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Hooter, you talking about the relays too..or just the wires. As far as i know, there are no relays under the hood on these rigs...never seen any on any truck as of yet. (nor in the schematics)

09-17-2003, 12:22 AM	#5
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Hmmm, can you get pics? I've never seen this.

09-17-2003, 12:32 AM	#7
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Cool. I'd really like to see this.

09-17-2003, 01:07 AM	#9
Orion 762 Registered User Join Date: Sep 2003 Location: Mississippi Posts: 36	Well folks . . . based on Hooters reply, I went ahead and yanked them out with a BFH, then blow-torched the old bolt holes shut so "no one" could ever put them back on again! I trust I was not too hasty? (/sarcasm . . . ) Actually I did a Google search on TCS for these year model chevy's, and many had them installed. The idea was "not to let you have any vacuum advance" until the automatic tx was in 3rd gear. I guess this was early '70's vintage "environmentalism" at it's peak! (Imagine NO vacuum advance until 3rd gear!) When I got this truck, this "stuff" had already been disconnected, but the wiring harness (and odd relays) was still intact on the firewall. Getting those useless wires and stuff out sure made things look a LOT better under there. (Actually I took the wires/relays off very carefully: even noted the "change" in my '71 Chevy Truck Log Book.) BTW - what I "thought" was my "backing lamp switch connector" (unplugged near the tranny down by the header pipes) was apparently the "connector" used by the TCS setup. Earlier threads here had identified this "hanging connector by the tranny" as backup lamp related. With this "new level of confusion" in my mind . . . . okay then, HOW DO you get the backup lights to work?? The "schematic" showed these LG and DG wires going thru the Engine Harness Connector, then on to the fuse box and the "tail and stop lamp harness". On my truck, they went instead to this TCS setup and relays on the firewall. Hmmm . . . Thanks for your help on this . . . Orion 762



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