TH350 Leak

[fireball517]

I've recently bought a '79 GMC Sierra Grande and have noticed quite a leak of ATF. The person I bought it from told me it was there but a slow drip. I'm not sure what his definition of slow is, but it runs!

Either way, I'm not really sure where to start on this, I thought it may just be the gasket for the pan, but it seems to be leaking from above it as well. From a blue plug? It has no wires running to it but in the photos you can see it running down.

Also what is this plug for?

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[firebane]

1) Drain the fluid and replace the gasket. A moving vehicle will blow fluid EVERYWHERE.

2) That plug looks like you have a TH350C transmission not a normal TH350

 

[fireball517]

Alright, I'll get to that asap.

 

[89Suburban]

That blue plug socket looks like a lockup TC or 700R4 plug socket to me.

 

[fireball517]

Thanks for the info, this is my first GM truck. I've wanted one for a while but finally found one at a decent price. Grew up in one and I love this truck.

 

[HotRodPC]

That is a Th350C. It's just an O Ring around the plug. Could also be the shift selector leaking but not a common place to leak.

 

[fireball517]

Ok, I'm going to clean it off in the morning. See where it leaks from there.

Also should there be something connected to that plug or is it alright to run it like it is? Where would it run from? I understand it is a lockup for the TC, to improve mileage?

I'm not too sure about transmissions but I'm willing to learn.

EDIT: I was looking under the truck again today and noticed something, where some wires plug into, but there wasn't any plugged into it.

I stole the image, but whatever this wire is for, it isn't on my truck..

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[HotRodPC]

Ok, I'm going to clean it off in the morning. See where it leaks from there.

Also should there be something connected to that plug or is it alright to run it like it is? Where would it run from? I understand it is a lockup for the TC, to improve mileage?

I'm not too sure about transmissions but I'm willing to learn.

EDIT: I was looking under the truck again today and noticed something, where some wires plug into, but there wasn't any plugged into it.

I stole the image, but whatever this wire is for, it isn't on my truck..

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That there appears to be a smog device switch. What? Transmission has a smog device switch??? Yep sure does. In some year models and like for Cali emssions. It's a pressure switch that could prevent timing advance through the igntion module unless the transmission was in 2nd or 3rd gear. I forget what all was controlled that way now, but yes, there were smog control switches in automatic transmissions. This is also sometimes why there were different tuning specs for auto or manual transmissions.

 

[HotRodPC]

Oh and some cruise control systems also used a similar switch so that cruise control could only be used in top gear. That would keep someone from setting cruising control while in 1st or 2nd gear.