TH350C Lockup wiring

[kkgp81]

My current project appears to have a th350C with the lockup. It looks like at one point it was wired up but not in a fashion that makes sense as far as color matched wires.

From the fuse block to the throttle pedal there is a slider switch with a wire from the fuse and a wire that was heading out to under the truck. That was wrapped around something and not connected. There are 2 more wires in the connector for the trans that are all old and dirty. My guess is someone was trying to swap in a 350c but not 100%. Is there a diagram that shows the electrical connections on the C unit?

I was going to pull the pan tomorrow to replace the gasket and filter. Mostly just to see if I can stop it from profusely leaking.

 

[Chevy 88]

If you do not know the maintenance history on the trans, then replacing the fluid and filter is very risky. If you do not know why its risky, then just don't do it - don't take the chance.

Others will say that you should replace the fluid and filter without any worry at all, and that if the transmission fails shortly after the new fluid and filter, then your trans was about to die anyway and the new fluid was nothing more than a complete waste of time and money and had nothing to do with the transmission failure.

Take your pick.

 

[idahovette]

^^^^^^If everyone subscribed to this way of thinking we wouldn't have ANY Square Bodies on the road^^^^^^^^^^^^^

 

[kkgp81]

If you do not know the maintenance history on the trans, then replacing the fluid and filter is very risky. If you do not know why its risky, then just don't do it - don't take the chance.

Others will say that you should replace the fluid and filter without any worry at all, and that if the transmission fails shortly after the new fluid and filter, then your trans was about to die anyway and the new fluid was nothing more than a complete waste of time and money and had nothing to do with the transmission failure.

Take your pick.

At the rate this thing is leaking- the pan is coming off….

It runs more of a risk being ran without fluid. Truck has been parked since 2003 just trying to get it street-able. If I need a new transmission after then that’s ok.

Also- related to my original post I found this.

https://www.gmsquarebody.com/threads/th350c-lock-up-wiring.34306/

 

[Matt69olds]

My 81 GMC had a 350C, I wired the converter clutch thru the 3rd gear pressure switch to provide a ground when the trans shifted to 3rd gear. I supplied power thru the brake light switch, the converter automatically unlocked when you touched the brakes.

It works this way, my family drove the truck like that for years. I did have a switch to disable the lockup when it wasn’t needed. But it was a very clunky setup, immediately after a 2-3 shift the converter locked. The big torquey 455 didn’t mind too much, but it definitely sounded strange!!

Last Sumer I finally killed the converter. The trans itself looked great, but the transmission made all kinds of noise and wouldn’t move. I knew the converter was the issue when I turned it upside down and chunks fell out. I installed a 700, and changed the lockup setup using a time delay relay.

Basically, use your pressure switch to ground the relay coil, power the solenoid and relay coil with the power wire from the brake switch. I also used a headlight dimmer switch on the floor to disable lockup when it’s not wanted.

When the trans shifts to high gear, the pressure switch grounds the relay coil. 10 seconds later, the converter locks. Touch the brakes, the converter immediately unlocks. Release the brakes, assuming the trans is still in high gear 10 second later it will relock.

 

[Sad Sack]

I have heard when absolutely needing to pull the pan when not knowing history, consider using Ford Type F to add back. If fluid looks cherry red that's a good thing and can usually be refilled, brown and burnt smelling not good, changing fluid can ruin your day if brown stinky trans fluid is in play.

 

[kkgp81]

I have heard when absolutely needing to pull the pan when not knowing history, consider using Ford Type F to add back. If fluid looks cherry red that's a good thing and can usually be refilled, brown and burnt smelling not good, changing fluid can ruin your day if brown stinky trans fluid is in play.

Fluid looks brand new…. We had to add a lot to move it from its resting spot and it’s a constant add wherever I park it. I’ve been considering parking it on a pan to try to reuse but then my luck all the frogs around here would start turning red from getting into it.

 

[kkgp81]

Pulled the pan off today. Nothing looked alarming. There wasn’t much of anything in the pan. The filter in place didn’t really have anything in it. The bottom side of the plug only has 2 wires going to a solenoid then to another thing. Should have taken a photo but had transmission fluid everywhere. Installed new filter and gasket then put the pan back on. Some bolts were pretty tight on the takeoff so hopefully the pan isn’t all bent wonky.

 

[Sad Sack]

@kkgp81
Should have put a tranny drain plug in for future fluid drains, EZ PZ to do. It's one of those "while you're in there" situations. And was there a magnet in the pan?

 

[Ricko1966]

I've said it many times,if I've got a pan off,it gets a drain plug. Then it gets a fluid change,just fluid drain and fill,every 3 months for a year. Reason is there is so much fluid at the end of the year your fluid is 90 percent all replaced,than a spill and fill every 6 months or even once a year,your transmission has clean fluid with it's additive package still fresh.

 

[kkgp81]

@kkgp81
Should have put a tranny drain plug in for future fluid drains, EZ PZ to do. It's one of those "while you're in there" situations. And was there a magnet in the pan?

Yes- magnet in the pan. Nothing really alarming.

 

[TotalyHucked]

If you do not know the maintenance history on the trans, then replacing the fluid and filter is very risky. If you do not know why its risky, then just don't do it - don't take the chance.

Others will say that you should replace the fluid and filter without any worry at all, and that if the transmission fails shortly after the new fluid and filter, then your trans was about to die anyway and the new fluid was nothing more than a complete waste of time and money and had nothing to do with the transmission failure.

Take your pick.

This is wrong. Dropping the pan for fresh filter and fluid is fine. A flush where you hook up the lines to a machine and completely flush everything out is what kills older unknown transmissions.

 

[legopnuematic]

consider using Ford Type F to add back

Personally the only thing I’d ever consider type F in is a transmission that hasn’t been rebuilt that was intended to be used with type F. Those would use asbestos based frictions instead of the paper type frictions.

Type F is an extremely primitive fluid, it contains no friction modifiers. That is how is creates the ‘firm’ shifts its known for. Dexron 3/6 or atf+4 are far, far superior choices for fluid over type F, with Dex 6 being the ‘best,’ especially for a new build. Type F has been shown to have less holding power on paper clutches and accelerate wear on components over other fluid types.

But for a normal th350/400 Dex 3 is perfectly fine.

 

[Sad Sack]

Personally the only thing I’d ever consider type F in is a transmission that hasn’t been rebuilt that was intended to be used with type F. Those would use asbestos based frictions instead of the paper type frictions.

Type F is an extremely primitive fluid, it contains no friction modifiers. That is how is creates the ‘firm’ shifts its known for. Dexron 3/6 or atf+4 are far, far superior choices for fluid over type F, with Dex 6 being the ‘best,’ especially for a new build. Type F has been shown to have less holding power on paper clutches and accelerate wear on components over other fluid types.

But for a normal th350/400 Dex 3 is perfectly fine.

Good info, B&M states to add it when installing a Transpak on a TH350c hence the mention:

You must be registered for see images attach

 

[Ricko1966]

Good info, B&M states to add it when installing a Transpak on a TH350c hence the mention:

You must be registered for see images attach

I've used type f in my th350s for decades at B&Ms recommendation