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Old 03-27-2016, 05:40 PM   #1
chrismoore701
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Trans won't shift under load

Seems like the Trans will only shift when I take my foot off the gas. I had it gone through and the gaskets redone about 3 years ago. It has been pretty much sitting with little use for that time. It is a turbo 350 with a vacuum line hooked up. Behind a 350. In my 79 c10. Anything I can check before I take it to the shop? I did run the motor a couple times when it was low on fluid. Not dry just low
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Old 03-27-2016, 05:42 PM   #2
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Re: Trans won't shift under load

did you check the modulator and the vacuum line?
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Old 03-27-2016, 05:56 PM   #3
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Re: Trans won't shift under load

Did not. How do you check this? It is hooked up. That's all I know
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Old 03-29-2016, 07:23 AM   #4
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Re: Trans won't shift under load

Anybody know how to test the modulator? Sounds like I disconnect the vacuum line going to the carb. Fire up the truck and put a vacuum gage on the line. see if the truck pulls vacuum, this will determine if my line or hoses are split. Then connect the gauge to the modulator and pump up to 20 inches and see if it holds. sound right?
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Old 03-29-2016, 10:17 AM   #5
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Re: Trans won't shift under load

Yes, verify the vacuum at the modulator with a gauge. If that is good, you can try to test the vacuum can or just replace it ($10). While you have it out, pull the valve out too to make sure it isn't sticking. If it does the same thing after that, then you can look elsewhere.
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Old 03-29-2016, 10:42 AM   #6
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Re: Trans won't shift under load

I thought the valve was part of the modulator, no?
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Old 04-01-2016, 11:07 PM   #7
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Re: Trans won't shift under load

My vacuum hose from the carb to the hard line was not tight on the hard line. Now it will pump up to 20 inches at the carb with the gauge on the Trans line not the carb. So I took it out for a test drive and it winds up pretty good and then slams into gear. I have a B&M shift kit in it. But I bet it gets up to 3000 rpm before shifting. Is this normal. I have a tach I can wire up and check later on. What are you guys thinking?
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Old 04-02-2016, 02:08 AM   #8
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Re: Trans won't shift under load

I would just replace the modulator, they are cheap. Tripple check the line from the carb, to the modulator. If it's shifting ar a real high rpm, and slamming into gear, then it's shifting off the governor. The modulator is most likely adjustable as well. Take the line off the modulator, and look for a small Allen head screw in the nipple . It will adjust how early or late it shifts.
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Old 04-02-2016, 07:59 AM   #9
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Re: Trans won't shift under load

All good advice also check the TV cable and make sure it is adjust properly as well. Also when adjusting the modulator turning it counter clock wise makes it shift sooner and softens the shift turning it clock wise makes it shift later and firmer.
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