Question about relays and this mystery wire - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**71CHEVYSHORTBED402** · 09-10-2016, 06:42 PM

1. There are two (relays I'm guessing) on the firewall. Proper names for these would be great. Seems these work in combination. Can anyone give a brief description for these? What are the odds these are still good? My electrical was hooked up (less possibly mystery wire (below)). The vacuums were not. Can't test, because there's only half a truck at this point.

2. Reference the wire with the wire nut in the first picture. Do you know what it's for?

3. See the "prong" behind the top terminal on the LH (truck side) relay in the second picture. It nearly looks like a prong, but I can't imagine it's used for anything. Do you know what this is? Perhaps it's precautionary to ensure the plug stays put.

I've dug into the assembly manual some. The schematic is a little hard to read, but I'm a work in progress.

Don't know if the following will add any value, but here are some notes from what I've identified to this point. THANK YOU:

Idle Control Solenoid (a.k.a. Idle stop, a.k.a. Combination Emission Control)

There are two relays on the firewall, one on the LH truck side (TS) the other on the RH (TS). These instructions are for the relays in a mounted position:

One brown 18 AWG wire comes in from the RH (TS) of the firewall plug. Approx. 16" from the firewall a splice plug routes the wire to two places. The shortest wire plugs into the bottom terminal on the RH (TS) relay. The longest wire routes to the idle control solenoid.

Two blue 18 AWG wires plug into the top terminal on the RH (TS) relay. The shortest wire plugs into the bottom terminal on the LH (TS) relay. The longest wire routes to the idle control solenoid.

One brown/white striped wire 24 AWG plugs into the top terminal on the LH (TS) relay. The wire routes to the idle control solenoid. Note the plug at the terminal includes a green wire that's been cut. What is it?

The wires to the solenoid route through the firewall gutter and on the retainers on the LH valve cover. The valve cover retainers don't seem to provide much stability. Perhaps I'm missing something.

Note: The blue wire, the brown/white striped wire and the mystery wire use the same plug at the LH (TS) relay.

**71CHEVYSHORTBED402** · 09-10-2016, 09:32 PM

I was such in a hurry to beat my drums I forget the picture. Very sorry about that.

**71CHEVYSHORTBED402** · 09-11-2016, 12:47 AM

Making some headway, assuming I'm at the right spot in the book:

One reads: RLY ASM TRANS CONT SW DELAY. Appears this is the smaller relay that mounts closest the LH fender. I find this schematic very difficult to read. Perhaps the cut wire with the nut (OP), which appears brown (colored blind too), ran to the transmission kick-down? I don't have the kick down unit.

One reads: RELAY ASM CONT SW. I'm guessing that black wire goes to the transmission kick-down too. I'll locate the end at some point.

**VetteVet** · 09-11-2016, 01:20 PM

Everything you are describing is part of the TCS system (transmission controlled spark)
which Gm designed to comply with the EPA emissions regulations beginning in 1970 for our trucks.
Very few of these TCS systems survived more than a year before they were removed.
The theory was that the transmission worked in conjunction with the engine vacuum to retard the ignition spark to prevent unburned gas from going out the exhaust.

When the owners discovered that the engines ran better with this stuff disconnected, it
didn't take long to see it gone.

Here is some more info.https://www.google.com/search?domain...D%3A1%3B&hl=en

**71CHEVYSHORTBED402** · 09-12-2016, 11:12 PM

Quote:

Originally Posted by VetteVet

Everything you are describing is part of the TCS system (transmission controlled spark)
which Gm designed to comply with the EPA emissions regulations beginning in 1970 for our trucks.
Very few of these TCS systems survived more than a year before they were removed.
The theory was that the transmission worked in conjunction with the engine vacuum to retard the ignition spark to prevent unburned gas from going out the exhaust.

When the owners discovered that the engines ran better with this stuff disconnected, it
didn't take long to see it gone.

Here is some more info.https://www.google.com/search?domain...D%3A1%3B&hl=en

Thank you Vette. I'm a "purest" but have my limits. I won't be reinstalling these.

09-10-2016, 06:42 PM	#1
71CHEVYSHORTBED402 Senior Member Join Date: Feb 2009 Location: Northern Nevada Posts: 7,118	Question about relays and this mystery wire 1. There are two (relays I'm guessing) on the firewall. Proper names for these would be great. Seems these work in combination. Can anyone give a brief description for these? What are the odds these are still good? My electrical was hooked up (less possibly mystery wire (below)). The vacuums were not. Can't test, because there's only half a truck at this point. 2. Reference the wire with the wire nut in the first picture. Do you know what it's for? 3. See the "prong" behind the top terminal on the LH (truck side) relay in the second picture. It nearly looks like a prong, but I can't imagine it's used for anything. Do you know what this is? Perhaps it's precautionary to ensure the plug stays put. I've dug into the assembly manual some. The schematic is a little hard to read, but I'm a work in progress. Don't know if the following will add any value, but here are some notes from what I've identified to this point. THANK YOU: Idle Control Solenoid (a.k.a. Idle stop, a.k.a. Combination Emission Control) There are two relays on the firewall, one on the LH truck side (TS) the other on the RH (TS). These instructions are for the relays in a mounted position: One brown 18 AWG wire comes in from the RH (TS) of the firewall plug. Approx. 16" from the firewall a splice plug routes the wire to two places. The shortest wire plugs into the bottom terminal on the RH (TS) relay. The longest wire routes to the idle control solenoid. Two blue 18 AWG wires plug into the top terminal on the RH (TS) relay. The shortest wire plugs into the bottom terminal on the LH (TS) relay. The longest wire routes to the idle control solenoid. One brown/white striped wire 24 AWG plugs into the top terminal on the LH (TS) relay. The wire routes to the idle control solenoid. Note the plug at the terminal includes a green wire that's been cut. What is it? The wires to the solenoid route through the firewall gutter and on the retainers on the LH valve cover. The valve cover retainers don't seem to provide much stability. Perhaps I'm missing something. Note: The blue wire, the brown/white striped wire and the mystery wire use the same plug at the LH (TS) relay. __________________ Tony 71 Custom Deluxe, SWB, 2WD, 402, A/C. I developed an assm. guide "kit" for restoring it from ground up. With assys, the guide accts for 1000s of OEM identifications and part numbers, all written in short order. 700+ images include assm, illust., charts, and points of interest. Much of the info. applies to all 67-72 GM trucks, and to a lessor degree all 67-72 GM vehicles. My build thread, and more on the guide https://67-72chevytrucks.com/vboard/...d.php?t=730025 Last edited by 71CHEVYSHORTBED402; 09-11-2016 at 12:50 AM.

09-10-2016, 09:32 PM	#2
71CHEVYSHORTBED402 Senior Member Join Date: Feb 2009 Location: Northern Nevada Posts: 7,118	Re: Question about relays and this mystery wire I was such in a hurry to beat my drums I forget the picture. Very sorry about that. __________________ Tony 71 Custom Deluxe, SWB, 2WD, 402, A/C. I developed an assm. guide "kit" for restoring it from ground up. With assys, the guide accts for 1000s of OEM identifications and part numbers, all written in short order. 700+ images include assm, illust., charts, and points of interest. Much of the info. applies to all 67-72 GM trucks, and to a lessor degree all 67-72 GM vehicles. My build thread, and more on the guide https://67-72chevytrucks.com/vboard/...d.php?t=730025

09-11-2016, 12:47 AM	#3
71CHEVYSHORTBED402 Senior Member Join Date: Feb 2009 Location: Northern Nevada Posts: 7,118	Re: Question about relays and this mystery wire Making some headway, assuming I'm at the right spot in the book: One reads: RLY ASM TRANS CONT SW DELAY. Appears this is the smaller relay that mounts closest the LH fender. I find this schematic very difficult to read. Perhaps the cut wire with the nut (OP), which appears brown (colored blind too), ran to the transmission kick-down? I don't have the kick down unit. One reads: RELAY ASM CONT SW. I'm guessing that black wire goes to the transmission kick-down too. I'll locate the end at some point. __________________ Tony 71 Custom Deluxe, SWB, 2WD, 402, A/C. I developed an assm. guide "kit" for restoring it from ground up. With assys, the guide accts for 1000s of OEM identifications and part numbers, all written in short order. 700+ images include assm, illust., charts, and points of interest. Much of the info. applies to all 67-72 GM trucks, and to a lessor degree all 67-72 GM vehicles. My build thread, and more on the guide https://67-72chevytrucks.com/vboard/...d.php?t=730025

09-11-2016, 01:20 PM	#4
VetteVet Msgt USAF Ret Join Date: Jan 2005 Location: Kalamazoo, Michigan Posts: 8,703	Re: Question about relays and this mystery wire Everything you are describing is part of the TCS system (transmission controlled spark) which Gm designed to comply with the EPA emissions regulations beginning in 1970 for our trucks. Very few of these TCS systems survived more than a year before they were removed. The theory was that the transmission worked in conjunction with the engine vacuum to retard the ignition spark to prevent unburned gas from going out the exhaust. When the owners discovered that the engines ran better with this stuff disconnected, it didn't take long to see it gone. Here is some more info.https://www.google.com/search?domain...D%3A1%3B&hl=en __________________ VetteVet metallic green 67 stepside 74 corvette convertible 1965 Harley sportster 1995 Harley wide glide Growing old is hell, but it beats the alternative.



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