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Old 07-07-2021, 08:41 AM   #1
sick472
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What does this vacuum pod do?

My 85 K20 has a vacuum pod just above the brake booster. I have owned it for 7 years now and the vacuum line (going to the right) has never been hooked up to anything. I'm swapping motors and thought, now would be the time to fix it or eliminate it.

What does this do?
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Old 07-07-2021, 04:10 PM   #2
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Re: What does this vacuum pod do?

Quote:
Originally Posted by sick472 View Post
My 85 K20 has a vacuum pod just above the brake booster. I have owned it for 7 years now and the vacuum line (going to the right) has never been hooked up to anything. I'm swapping motors and thought, now would be the time to fix it or eliminate it.

What does this do?
That is the low vacuum pressure switch for the 700R4 transmission. It releases the lock up torque converter when you stomp your foot into the gas, in connection with the TV cable for passing gear. Should have a blue wire coming off the brake light switch, and a wire from the pod down to the transmission.
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Old 07-07-2021, 05:13 PM   #3
kwmech
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Re: What does this vacuum pod do?

Ya know i thought it was a transmission control for the 700 but him saying it is in K20 kinda threw me off as I don't know of any 3/4 tons with that trans
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Old 07-07-2021, 05:16 PM   #4
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Re: What does this vacuum pod do?

Thanks for the reponse Dan! I'll have to check the wiring to see if it is hooked up.

Where should the vacuum hose be hooked to? Currently it is hanging loose in the driver's side fender. I doubt there is any appreciable vacuum in there to help out the transmission...lol
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Old 07-08-2021, 05:33 AM   #5
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Re: What does this vacuum pod do?

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Originally Posted by kwmech View Post
Ya know i thought it was a transmission control for the 700 but him saying it is in K20 kinda threw me off as I don't know of any 3/4 tons with that trans
I had a lite duty K20 (didn't have a full floating 8 lug rear) that had one. Apparently not very common.
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Old 07-08-2021, 08:12 AM   #6
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Re: What does this vacuum pod do?

I'm sure it's a 700R4 and the fact that the switch is not hooked up to vacuum... it makes sense as the lock-up wiring at the transmission has been unhooked in the past as well (I checked last night). So, it appears that neither the wire at the transmission is connected nor the vacuum hose at the low vacuum pressure switch. Something must be up with the lockup function of the convertor/transmission.

I'm skeptical to hook it all back up, but (should I feel punchy) where should the vac hose off the switch be connected?
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Old 07-14-2021, 11:03 AM   #7
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Re: What does this vacuum pod do?

I have decided to have the transmission rebuilt since the motor is out an also needing a rebuild. My rebuilder has commented that the transmission will overheat on a long trip without the lock-up functioning. I have made several 2 hour trips with it like this and have not burned it up, but I would like to get the lock-up working since I will end up with a rebuilt motor and transmission.

Where does the vacuum line hook to for the vacuum source?
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Old 07-14-2021, 01:24 PM   #8
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Re: What does this vacuum pod do?

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Originally Posted by GM72K10 View Post
I had a lite duty K20 (didn't have a full floating 8 lug rear) that had one. Apparently not very common.
The semi-float 14-bolt was not just a light-duty option. It was installed even in the heavier "c6p" 8600-gvwr 3/4-ton trucks. Evidently, GM considered it heavy enough for large payloads.

GM did install the full-float 14-bolt in the big-block c20 trucks. My theory is they assumed that combo was being purchased to tow trailers with. The larger ring gear on the full-float axle would hold up better to towing.
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Old 07-21-2021, 05:07 PM   #9
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Re: What does this vacuum pod do?

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Originally Posted by sick472 View Post
I have decided to have the transmission rebuilt since the motor is out an also needing a rebuild. My rebuilder has commented that the transmission will overheat on a long trip without the lock-up functioning. I have made several 2 hour trips with it like this and have not burned it up, but I would like to get the lock-up working since I will end up with a rebuilt motor and transmission.

Where does the vacuum line hook to for the vacuum source?
Sorry it took so long to get back with you, I was in the hospital, thought the Covid had gotten hold of me, thank god not. Anyway that vacuum line just connects to manifold vacuum, so than when you stomp into the motor and vacuum drops, it causes the lock up torque converter to release. One wire from it goes through a blue wire into the firewall to a switch on the brake pedal, normally part of the brake light switch unless it has cruise control, then it uses that switch for the power. The other wire goes down to a square 4 wire plug on the side, halfway between the shift linkage, and the governor dome on the driver side. Normally it's just one wire from factory, or in my case I ran a seperate ground wire to a solid ground on the engine. Factory used a 1 wire pressure switch inside the transmission, using the transmission as a ground to the engine.
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Old 07-22-2021, 08:13 AM   #10
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Re: What does this vacuum pod do?

Thanks a million Danny! No worries about the post timing as the transmission is still in the truck, but only a couple hours away from hitting the floor and going to get rebuilt....glad your back and good to go! Thanks again, your response is appreciated.
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