65Pickup
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• Modesto, CA, USA |
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• Registered on 3/31/2005 |
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• 2 posts |
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 Posted:3/31/2005 17:13 |
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My 68 Ranchero is having trouble shifting from 2nd to 3rd, I fear vacuum is the culprit, but I don't know the actual name of the part I need to replace. the problem is that it wont shift into 3rd gear until 50mph. Any helpful insight would be appreciated. |
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dnevins
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• Subscriber since 12/29/2009 |
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• San Diego, CA, USA |
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• Registered on 12/28/2009 |
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• 228 posts |
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The part you're thinking of is the vacuum modulator, also called the vacuum diaphragm. It sits on the transmission case, and a small hose runs to it from the intake. It tells the transmission how hard the engine is working. With good vacuum you get earlier, softer shifts. If vacuum is lost through a leaky hose or a bad diaphragm, the transmission acts like you're at heavy throttle and holds each gear, which matches your 50 mph 2-3 shift.
What I'd check:
1. The hose. Make sure it's hooked to full manifold vacuum and isn't cracked, collapsed or loose. Tee a vacuum gauge into it and look for steady vacuum at idle and at a steady cruise.
2. Pull the hose off the modulator and look inside the hose and the nipple. Transmission fluid in there means the diaphragm is ruptured, so replace the modulator. A ruptured one also lets the engine pull transmission fluid in, so keep an eye on the fluid level.
3. Put a hand vacuum pump on the modulator. It should hold vacuum. If it bleeds down, replace it.
4. Look at the kickdown rod. It should only come into play near full throttle. If it's bent or binding and not returning, it can upset the shift timing.
If the vacuum supply and the modulator check out, the next suspects are a sticking governor or a dirty valve body, which means work inside the transmission. Some modulators have an adjustment, but owners report the range is only a few mph, so it won't turn a 50 mph shift into a normal one by itself.
Dana
https://partslogic.com |
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