Electrical Mystery for 700R4?

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Spookee

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oregon
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hannah and pat
Truck Year
1984
Truck Model
k10
Engine Size
350
Hi! We just got our 84 k10 metal baby back from the transmission shop. The 700R4 transmission was rebuilt after the 3-4 clutches went out. They also installed a transgo shift kit and replaced the rear planet, pump slide, new plates and input drum. It has a 24 month / 24k miles warranty now :)

However we now have a new mystery - when we picked up the truck the mechanic said that there is something going on electrically with the torque converter lockup and they weren't able to figure it out. I don't really understand what it is so I'm hoping to share it here and see what you guys think. Here are the notes I took from our conversation with the mechanic -

The voltage that should be going to the transmission through the brake switch doesn't seem to be the full battery amount (?). they tested the voltage and couldn't find the root cause. Without that working the truck won't go into lockup correctly. So to troubleshoot they ran the electrical straight from the battery/back of the firewall to the transmission. it is internally wired so that lockup will come on in 4th gear. The mechanic said that this should be fine for daily driving. his only concern is that the truck will go into 4th and lockup and that will lug the motor down. He showed us how they wired it in case we want to take it somewhere to undo it in the future (pictures included below).

I'm wondering 1) what could be the potential cause of this electrical issue and 2) could anything go wrong keeping it wired like this. Thanks again for any help I truly appreciate all the knowledge on this site!

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Jgonick

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Joel
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1985
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There are a few different setups. First of all lets look at the brake switch. It is exactly opposite from the tail lights. When you press the brake it allows current to the tail lights. When it comes to the TCC when you press the brake it needs to cut the power to TCC. Most trucks originally used the cruise control "combination vacuum release/valve/converter clutch switch". see attachment. Many people will still use that switch if it is still in the truck. You can get a brake switch with cruise control (4 terminals) and use that instead. The cruise control portion cuts power when the brake is pressed -exactly what you need.
Example link
Brake switches are notorious for malfunctioning (breaking or shorting out randomly & not being consistent) That would be the first thing I checked or replaced if there seems to be a voltage issue.

There are a few different setups depending if it had California emission, egr or not -- Attached is an example that will at least let you see the basic wiring.

Hopefully this helps a little.
 

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Last edited:

Jgonick

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Joel
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1985
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K10
Engine Size
350
Also - Some just wired a toggle switch in the cab to manually control the TCC.
 

gmbellew

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you can just run D around town and it will be fine. But I don't think locking every time you hit 4th gear will be a great setup on the highway with those tires unless you are running some 4.56 gears.
 

Spookee

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oregon
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hannah and pat
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1984
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k10
Engine Size
350
Thanks guys for the information I really appreciate it! For now we will just drive around town in D
 

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