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Mystery wire help. Next to temp sender.

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Malathion

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I have what appears to be an extra wire running next to my temp sensor on my 74 Blazer (350). I can't figure out what it is for. I think the temp gauge works but it's hard to tell because the K5 has a 160F thermostat (based on previous owner) along with a flex fan. It doesn't get much above cold on the gauge when running since its below freezing outside and I can't drive the K5 yet, only idle but the gauge does move a little.

The wire has switched power and shows 12 volts. No power when key is off.
Green wire goes to temp sensor. This wire which is yellow or tan (can't tell) runs up behind the distributor and into the main wiring harness.

I want to replace the thermostat and coolant mixture but before I do, I want to make sure everything is working and hooked up properly. Any ideas on what the unhooked wire is for?

IMG_0796.JPG
 
The two brown tan wires I remember one was to the gas Guage and one to the brake prop valve. But those are negative wires not positive. One could have been to the electric choke if I remember correct. It would be key on 12 bolts but can't remember color. My manual is in storage or else I could look it up. If you sign up for a account at autozone.com you can look at wiring diagrams and try to trace it.
 
It probably went to the a/c compressor but should be green, did it have factory a/c, doensn't look like it did from looking at the power steering setup.
My 74 wiring is set up like that with 2 wires from that harness but its not brown.
 
If it doesn't need it in your engine bay you can tape it off for now and if you can find a use for it later you know it's there. Ac compressor wouldn't always be hot if that's what it is. Would come in if you turned the defrost or ac on from the dash switch.
 
Some Gm's around that vintage had transmission controlled spark solenoids and idle stop solenoids,but they usually had green wires....my 70's GM's had a light tan wire to one of the altenator terminals to provide feild current to the coils in order to make it charge,the #1 wire on the two pronged plug...the other wire on the plug was usually red...some trucks had the altenator on the drivers side those years,so maybe thats why you have it located in with the temp sender unit harness...there isn't much of anything else located that needs wires in that area that I can recall..
 
diesel4me said:
Some Gm's around that vintage had transmission controlled spark solenoids and idle stop solenoids,but they usually had green wires....my 70's GM's had a light tan wire to one of the altenator terminals to provide feild current to the coils in order to make it charge,the #1 wire on the two pronged plug...the other wire on the plug was usually red...some trucks had the altenator on the drivers side those years,so maybe thats why you have it located in with the temp sender unit harness...there isn't much of anything else located that needs wires in that area that I can recall..
Click to expand...

I think the idle stop relay is the my best guess.

Edit- looking at the a wiring diagram, the transmission controlled spark (t.c.s) and the idle stop relay are on the same circuit with the color code of (cxl)? The a cxl colored wire goes to the t.c.s solenoid and a dark green with a with white tracer (dg/w) and dark blue (dbl) goes out on the other side of the relay.
I would say the mystery wire goes to idle stop relay.

I going to look up what some of the color codes means.

Edit 2-Doh, I'm a dumb ass, there is a t before the (cxl) so the wire should be TAN. So that mean the mystery wire to me is for sure I.S.R or the t.c.s.
 
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According to my schematics, there are a couple of possibilities. Such as an idle stop solenoid, or a TCS solenoid.

But, given its location, my best guess is the brake warning light. It uses a tan wire, and should be plugged into the single terminal on top of the proportioning valve/brake warning switch.

If you don't mind risking a fuse, you could try turning on the ignition, and have someone briefly ground it to see if the brake warning light comes on in the dash.

If that is what it is, then that is all that will happen. If its a power wire for something, such as the idle stop solenoid, then its going to blow a fuse.

The correct way, would be to put a variable resistance to ground on it and watch the current and the light as you decrease the resistance.

But first, look to see if there is a wire on the brake warning switch.
 
Not to doubt you forum, the brake warning light wire should run along the fender to the core support to the prop valve. It should be tan with a white tracer.
 
Don't worry about doubting me. All I have to go by is the schematic. The only time I see a GM is when one of my friends breaks something and I have to fix it.

I saw it was a tan/white, but figured that age might have obscured the white. From what I read, the most likely was the idle solenoid, but it just did not seem likely it would go that far down the side of the engine and then turn back up to the carb.

The only thing I could think of down there was the brake warning switch. As I vaguely remember, that side of the engine is fairly dead electrically. The bulkhead fittings are there, but most of the wires go elsewhere.

However, where did the wire to the transmission for the TCS go?
 
It is not suppose to go down the block. I think due to the tall aftermarket valve covers it make it look like it does. I think it suppose to split where it does an goes to the front of the carburetor. Do forget, the wire is cut, so it could have been longer.

I had to mess the i.s.r during the early nineties to smog my old 73, so to me if my memory is correct after 20 years, it make sense the mystery wire goes to the i.s.r. It is in the right place.
 
The wire for the T.S.C to a switch (or a pressure switch on a auto) on the top of the transmission. If I recall, it was close to the reverse, (but not the same switch) switch. For what gear is suppose to turn on the switch, I don't know.
 
I should mention, the TCS actually has it own harness. it split off the the main engine harness by the firewall.
 
Thanks for all of the input guys.

blazer74 - I do know that this K5 didn't have factory air so it rules that out. That's what I initially thought too due to the location.

Fordum - My brake warning light does run along the fenders and hooks into the proportioning valve like it should so it isn't that even though the wire is a similar color.

chulisohombre - It could have been for an electric choke but I now have an Edlebrock with an electric choke which uses another wire for power, I guess I should look and make sure that was done correctly.

It could also have been for the ICS as JDNobodi pointed out but I far as I can tell, I don't have that due to the carb conversion. As you mentioned, maybe that's why it's just laying there.

Note that I did ground the wire through my Ohmmeter and it showed 12 volts. I turned on everything, blower, lights, in 4wd (NP203) and the voltage stayed the same. I also tried blowing a fuse but nothing would blow. It would just continue to spark when grounded on the block.

I'll cap the wire for now.:doah: This is the least of my problems with this thing.

Let me know if you have any other ideas.
 
If it is TCS or Idle stop solenoid, It shares its power with the radio, TCS/Radio fuse. Pull the fuse and see if power drops off to that tan wire.
 
The green wire is temp gauge sender. (obviously)

The brown wire goes to the oil pressure sender.


...on my truck.
 
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