C-6 shift problem

I recently bought a 1988 F-250 4WD with a C-6 transmission. Today it would not upshift until 3,000 RPM (approx 25 MPH and 45 MPH) no matter what the load. Any ideas?

Reply to
CSA722
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Re: "I recently bought a 1988 F-250 4WD with a C-6 transmission. Today it would not upshift until 3,000 RPM (approx 25 MPH and 45 MPH) no matter what the load. Any ideas?"

--------------------------------------------------------------- I don't know about '88 year C-6 but if it's like earlier models you have a leak in the vacumn hose between the manifold to the modulator.If no leak then the modulator itself can be unscrewed and replaced.It's in the back of the trans.

Reply to
Brian Orion

Either a bad modulator or a bad vacuum supply to it. Be sure the vacuum line is properly connected to the intake and that the rubber elbow connection to the modulator are good. If you disconnect the elbow at the modulator and fluid is present there or at the intake manifold connection, the modulator is definitely bad. The modulator only control the

1-2 upshift but, it will influence the 2-3 if you are pressing the throttle harder to get it to upshift. Be sure to get the correct modulator for your trans calibration. It will need to be adjusted when installed. Be careful not to lose the actuator pin when you remove the old one. You will need it in the new one. BTW, the modulator is located on the right side of the rear of the case with a vacuum hose going to it. This would be a good time to remove the pan and service the trans if you have not already done so. The converter should also be drained to get all the old fluid out. The modulator is a simple repair. You may be able to get a good price to replace the modulator and service the trans from an independant shop.
Reply to
lugnut

Thanks for the replies about the vacuum modulator. I was heading in that direction but it took a while for me to remember things I used to know and nearly forgot.

Reply to
CSA722

A little update on the shifting problem. After checking vacuum to the modulator (itwas OK) I replaced the modulator and it didn't help. I started looking a little further into it (as I should have in the first place) and found out the kickdown lever was stuck in the full throttle position. It would not return to the normal position even after disconnecting the cable. After stopping to buy a pan gasket and some fluid so I could take a look inside, it started to shift normally again. Just got back from a trip into the mountains and it behaved perfectly. Any ideas?

Reply to
CSA722

snipped-for-privacy@aol.com (CSA722) wrote in news: snipped-for-privacy@mb-m27.aol.com:

maby hadn`t been full detent for a while and the varnish stuck it for a while in the valve body. changing the oil is about all you can do for that and hope it doesn`t get any worse. KB

Reply to
Kevin Bottorff

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