Thread: 87 GMC issues
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Old 11-18-2017, 07:52 PM   #5
foamypirate
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Join Date: Apr 2009
Location: Pflugerville, TX
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Re: 87 GMC issues

Quote:
Originally Posted by Captain J View Post
Thanks Sabaka454
Maybe you’re right, my question
When driving, the gear will not shift from 1st to second at low speed (as normal), it shifts at higher speed 30-40mph, otherwise it will stay in 1st gear.
What causes the gear to shift at higher speed? Is it the MAP sensor which I recently changed? Or something else?
Nothing was done to the transmission.
Only the TBI, was overhauled
I hope it’s clear.
You are possibly missing the vacuum line to the vacuum modulator. With the line disconnected, the modular sees no vacuum, and assumes you have the throttle wide open. I'd check there, first. This is assuming you have a TH400 (my guess, due to the 7.4L big block).

Reverse lights, as mentioned, are operated by the switch at the base of the column, inside of the cab. It's adjustable by loosening the screws and moving it around. It may take some tinkering to get it just right.
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