New member with a question

[Shawn Cullison]

Thanks for having me on the forum, I've been learning a lot from some of the older threads. I got an 84 c10 long bed with the original 305 with a th350 transmission. My question is there is a 2 prong electric plug on the passenger side back of the th350 transmission, it is unplugged right now and runs fine. Does anyone know what this plug is for?

 

[Charlie]

 

[animal]

 

[yevgenievich]

It is th350c then. Plug is for lock up. Ok if unplugged, a bit better highway driving if working
Edit: did not read correctly, lock up would be on driver side 4 prong. Are sure it is not th400?

 

[Shawn Cullison]

Thanks for the info, it looks pretty worn out. Might try to replace it then.

 

[yevgenievich]

Actually passenger side back is not lock up. Th350 should not have a plug there that I can think of. Do you have a picture of it?

 

[Shawn Cullison]

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[Shawn Cullison]

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[Shawn Cullison]

Here are some pics of it. The wire comes up and hooks into what looks like a vacuum controlled switch. Which makes me think its for a lock up, but I don't know

 

[TravisB]

Wish I had the answer to your question but I just hang around for the cool pictures. lol

 

[dvdswan]

Welcome to the site. I also have no answer but enjoy the pics.

 

[Old77]

 

[1987 GMC Jimmy]

That looks like a transmission controlled spark (TCS) switch. It’s a simple pressure switch that’s supposed to change the timing in third gear so more hydrocarbons are burned. Sort of like ESC governed by your transmission. Just smog crap. I’ve never heard of one coming on a truck, but that location is where they were IIRC.

 

[yevgenievich]

Something like tcs is likely it. Definitely not lock up. I originally thought that maybe it was th400 with kick down switch but it does not look like one of those.

 

[1973c10]

Welcome aboard

I believe the switch was usually used for emissions related controls
wire goes to the distributor to advance the timing when the truck is in 3rd gear