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1967 - 1972 F-100 & Larger F-Series Trucks Discuss the Bumpsides Ford Truck

Trans Shift Modulator

Old Oct 8, 2004 | 09:29 AM
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Trans Shift Modulator

I have a 72 F100 with C6 trans. What is the purpose of the shift modulator? I noticed it is vacuum charged. I think my truck may be sucking trans fluid into the carb. Is this possible, and if so how do I test it?
 
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Old Oct 8, 2004 | 11:41 AM
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Modulator sets the shift points - tells the tranny when to upshift. I have heard that people have had this problem with the old C4's, but never seen it. I would look at the inside of the vacuum hose and see if there was any trans fluid in it.
 
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Old Oct 8, 2004 | 12:00 PM
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So I'm guessing the modulator reads the draw of the vacuum and uses that to determine when to shift. Is that right?
 
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Old Oct 9, 2004 | 11:42 AM
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If it goes bad it will suck trans fluid. Pull the hose off it and stick your finger on it were the hose came off. If you get trans fluid on your finger then it is bad.
 
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Old Oct 10, 2004 | 09:57 AM
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Checked it. No fluid in hose. I even disconnected the hose behind the carb and plugged it. Started the truck again and my truck still smokes heavily on 1st start of the day. I've replaced the valve seals. My compression is good I think. One cylinder is 108 and the rest are between 111 and 120. Where is the smoke coming from?
 
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