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Old 08-20-2006, 11:42 AM
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TH400 shifting high rpms

I did a tuneup on a friend's 90 GMC van 305 TH400 trans,trans rebuilt 3 months ago. He asked if I would check the trans it shifts at high rpms. When I started on the van I noticed that it was running rough. So I changed wires(two in bad shape) and plugs(one was oil fouled) others wide gap. I thought it might be the modulator acting up or not at all. But I thought better get it running right then deal with trans since the trans needs vacuum to shift. Sure enough worked fine when I road tested it. Now weeks(3) later he says its shifting high rpms again. My question is: because one spark plug is fouled again(guessing) is the vacuum down enough to not shift properly or is the modulator leak and the engine running poorly lacks enough vacuum as to not power the modulator?

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Old 08-20-2006, 11:56 AM
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High vacuum makes the trans shift sooner. I would T in a vacuum gauge at the modulator and drive it.
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