I cannot figure out these two wires!!! 76 GMC

[OldBlueDually]

Gents, I need some help here. I have used the electrical schematics posted here time, and time again, very helpful. However, I cannot determine what the heck these two wires are for!!!

The first wire is the blue one with a rubber boot. I "thought" it was for the temp gauge....but is it really for that? I ask because I had marked another wire (cloth one, in my next post) as the temp gauge wire....but now I am confused.

What is this blue wire for?

Here is the blue wire;

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Here is the blue wire at the firewall junction;

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[OldBlueDually]

Here is the other wire I am confused about and cannot figure out. It is a white wire with white cloth on it, which I have marked (on that blue tape) as the temp gauge wire, but now I am doubting that. As mentioned above, I cannot find this wire in those wiring diagrams.

What is this wire for?

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Here it is at the firewall junction (red arrow);

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[78C10BigTen]

Rubber boot is oil pressure sensor and the melty lookin second one is the nail head type temp sensor in the head

 

[dvdswan]

the one with the cloth cover is for the coolant sender on the side of the head.

 

[OldBlueDually]

Rubber boot is oil pressure sensor and the melty lookin second one is the nail head type temp sensor in the head

the one with the cloth cover is for the coolant sender on the side of the head.

Thanks guys! So I was partially correct, and so wrong at the same time.

The engine in this truck is an Olds 455, and it has a mechanical oil gauge in the truck with a copper line. So now what do I do about that blue wire with the boot?

As for the cloth wire I will get that plugged into the temp sending unit. The temp sending unit on my Olds has a male spade connector, I will see if it works. If not, I can swap the temp sending unit with the one that has that nail head type plug.

Thanks again, I got down to these two wires and was scratching my head.



Here is the schematic I was looking at, my dark blue wire is near the bottom of my firewall junction plug, not on the upper part like this diagram shows.

Also, would that white cloth covered wire be the 20DG/W-971??? What is TCS? This diagram also shows a Temp SW with wire 20DG-35.....

I was using these schematics here: https://www.gmsquarebody.com/threads/electrical-schematics.3834/

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[Raider L]

That was what I was wondering about, what is the TCS? Temp control Switch? The schematic has that in several places, like "TCS Trans Kickdown Switch". What the heck is that? And there are other "TCS trans" something or other. I figure those designations are for maybe TH400 or 700R4 transmissions that were offered in these years trucks? If your truck had one of those trans, the wiring was already there? Maybe I need to read the manual?

 

[OldBlueDually]

That was what I was wondering about, what is the TCS? Temp control Switch? The schematic has that in several places, like "TCS Trans Kickdown Switch". What the heck is that? And there are other "TCS trans" something or other. I figure those designations are for maybe TH400 or 700R4 transmissions that were offered in these years trucks? If your truck had one of those trans, the wiring was already there? Maybe I need to read the manual?

Whew, glad I am not the only one confused by some of this stuff! Mine does have the TH400 in it. It has a single wire going to the kickdown on the trans, but I do not recall if it goes through the firewall with a rubber grommet, or if through the fuse box/terminal blocks on the firewall...??

 

[Raider L]

@OldBlueDually,

That was the only thing I could think it went to was either a TH400 or a 200-700R4. You know the 700R4 has got the TV cable with electrics for the trans and all that so I figured it had to have been for one of those two. So what does the the switch do to your 400?

Yeah, there are a couple of those wires the schematic says goes through the firewall with a grommet and it's not even there in my truck and never was. They were in the harness going into the cab of my truck through the fuse block. The fuel level wire shows going into the cab through a grommet and it didn't until I reworked my wiring and I put it into the cab with a grommet. It went into my Autometer fuel level gauge I put in. Before that it was connected to the gang plug at the back of the printed circuit board.

 

[Raider L]

@OldBlueDually,

Take that wire with the rubber boot on it and cut it off just past that splice on it in the pic. That wire you can use with a oil pressure gauge you could pick up at most any auto parts place. Put that wire on one post on the back, and I think the other post may be the ground wire. Put that somewhere under the dash. Get one of those little panels and attach it under the edge of the dash somewhere. Don't forget the light for it. You may be interested in my tutorial on fixing up one of the bulb holders for a light.

I may be getting ahead of myself, do you still have that sending unit on the back of the motor? Why was it off to begin with? What instruments do you have in you panel? Don't you have a oil press. gauge? Find that wire on the schematic and look on the back of the gang plug on the back of the printed circuit and that's where it goes to your gauge.

 

[AuroraGirl]

the one with the cloth cover is for the coolant sender on the side of the head.

Thank you. the absolutely soaked and mostly missing cloth cover on my wire explains what it is. I thought it was something strange but I cant reach it so I just sat in wonder.

the blue one almost looks like it can be for an oil pressure thing yes, but also maybe a blade for a quadrajet pulloff or idle kick solenoid. same grommet ive seen before. That may explain TCS wording. Something for a thought. Or if it was for electronic choke maybe it was the other oil sender thingy maybe

 

[AuroraGirl]

@OldBlueDually,

Take that wire with the rubber boot on it and cut it off just past that splice on it in the pic. That wire you can use with a oil pressure gauge you could pick up at most any auto parts place. Put that wire on one post on the back, and I think the other post may be the ground wire. Put that somewhere under the dash. Get one of those little panels and attach it under the edge of the dash somewhere. Don't forget the light for it. You may be interested in my tutorial on fixing up one of the bulb holders for a light.

I may be getting ahead of myself, do you still have that sending unit on the back of the motor? Why was it off to begin with? What instruments do you have in you panel? Don't you have a oil press. gauge? Find that wire on the schematic and look on the back of the gang plug on the back of the printed circuit and that's where it goes to your gauge.

he has a mechanical gauge

 

[AuroraGirl]

@OldBlueDually,

That was the only thing I could think it went to was either a TH400 or a 200-700R4. You know the 700R4 has got the TV cable with electrics for the trans and all that so I figured it had to have been for one of those two. So what does the the switch do to your 400?

Yeah, there are a couple of those wires the schematic says goes through the firewall with a grommet and it's not even there in my truck and never was. They were in the harness going into the cab of my truck through the fuse block. The fuel level wire shows going into the cab through a grommet and it didn't until I reworked my wiring and I put it into the cab with a grommet. It went into my Autometer fuel level gauge I put in. Before that it was connected to the gang plug at the back of the printed circuit board.

he probably doesnt have a 700r4 since hes using an oldsmobile engine.

 

[OldBlueDually]

@Raider L my truck has the TH400 transmission in it, with the kick down wire hooked up from the switch under the dash on the "go fast" pedal

As for that wire with the rubber boot, prior to my engine install I robbed that wire from my parts truck, hence the splice in it. I will probably just nicely loop it back into the rest of the wire protector I have under the hood and leave it. As @AuroraGirl stated I have a mechanical oil gauge in this truck.

The engine I have in it is a Olds 455 from 1969, which I like better than the Chevy engines anyhow just because I am a huge Oldsmobile fan anyhow, not to mention the Olds engines have a deeper/throatier grumble to them AND spark plugs above the exhaust ports!!!

Here is the build if you are curious to see;

https://www.gmsquarebody.com/threads/76-gmc-crew-long-box-dually-gotta-get-it-on-the-road-again.29806/