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Old 12-06-2023, 03:46 PM   #1
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Engine: 8.3 cummins mt643 tranny
Manual Lockup Switch

Hello. I would like to install a manual lockup switch on my mt643 so i can lock up 1st and second when going up or down a steep grade. I would only do that when i manually shift it down. I have experienced high tranny temps when going slow up long steep and windy roads. I have already installed a auxiliary tranny cooler. If someone has a schematic/instructions on this i would appreciate it.

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Old 12-06-2023, 07:30 PM   #2
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Location: NM USA KD6WJG
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Year: 1991
Coachwork: Bluebird
Chassis: All American RE 40 FEET
Engine: Cummins 8.3
My converter locks in manual second. I don't know about first lock. I have never used it. Mine may be different because it has the retarder. There was a kit at one time that used an electric over hydraulic solenoid to direct line pressure to the lock up gauge port. I have installed them many years ago on PTO installations. Otherwise the PTO would stall. The same solenoid was used on the HT 700 series for compression brake installs and exhaust brakes.
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Old 12-07-2023, 10:18 AM   #3
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Year: 1984
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If you're overheating the trans while it's in 1st gear, you're either slipping clutches, or you're way overgeared. The correct course of action is to rebuild the trans or re-gear the axle, not locking up the converter. What s2mikon mentions is for engine retarder use, as an unlocked converter makes an engine retarder less effective. It's not to lockup the clutch to prevent fluid overheat.

I'm gonna guess your trans is worn and you're slipping. A manual lock up switch won't fix that.
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Old 12-07-2023, 10:30 AM   #4
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Year: 1991
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Engine: Cummins 8.3
The first place the OP needs to look at is his oil cooler. I have seen more than a few that were plugged up. A previous owner does a quick overhaul and doesn't replace the cooler, then the next owner has problems overheating.
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