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About a month ago I purchased a 1988 Ford F250 Diesel Truck.
Everything seemed great. It was running smooth and shifting great.
Yesterday the transmission started acting up. It seems to be shifting late into gears. Where my rpms will reach around 3000 and then when I let off the gas as the rpms go down the truck will shift into the next higher gear.
And the shift isn't as smooth as it was the previous in the month, but I am assuming that is a result of the higher rpms.
The only other symptom I notice is that while in 3rd gear even on level road, the truck seems to downshift into second gear early. This sympton resembles the same feeling you get if you are towing a heavy item and start driving up a hill while also hitting the gas pedal hard, because it will downshift but then the rpms shoot up.
Any tips or suggestions are appreciated or if you have any questions to ask that might help clarify further what is occcuring...ask away
I assume your trans is a C6 so the shift points are vacuum controlled. As your truck is a diesel the vacuum source is a vacuum pump driven off the crank pulley. So have you got plenty of vacuum? A good indication is if the power brakes work good. If you have good vacuum this is supplied to the C6 via a modulator valve on your injector pump. The vacuum finally ends up at a modulator diaphram on the C6. So check all the vacuum pipe work. The pipe going to the injector pump and leaving the injector pump and running down the frame rail to the trans. If this all checks out pull the vacuul pipe at the trans and see if any red fluid is present, this would indicate a faulty diaphram. Good luck. I should add that if you have had any work done on the injector pump the vacuum modulator valve may be out of adjustment. Good luck.
anything less than 17 inches of vacuum, and your trans will start to act up.
preferably, you want more like 20 or 21 inches of vacuum
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Well, I did a visual inspection of all the vacuum lines and all looked good except one which was cracked at its end. So I cut it a little shorter and reconnected it.
I also pulled the vacuum modulator at the transmission and inspected it. I didn't see any fluid, but to replace it was only $10.00 so I went ahead and purchased a new one.
After installation I noticed a major improvement.
It still seems to shift at a high RPM and between 33-35 mph if my foot isn't on the excelerator the truck downshifts...enough so that you hear the engine rev up.
The truck still has a semi-hard shift unless I am heavy on the excelerator. If I am heavy on the excelerator the shifting seems to go smoother.
The instructions for the vacuum modulator said it has an adjustment screw on it to compesate for older vehicles. On quick inspection I wasn't able to locate this screw......it must be a really small screw. But this will be my next modification....
Well, I backed out the screw first a half turn (counter clockwise) and didn't notice any real change.
So I did an additional full turn counter clockwise and it seems to have made a difference in the shifting not kicking in as hard.
I don't know if its just me (which is possible) but it still seems to be late in shifting and adjusting the screw didn't seem to have any affect as far as that goes.
I know I read somewhere that mentioned not to turn the screw in (clockwise) more than 4 turns from the original factory settings. If I keep backing out the screw (counter clockwise) another turn or two will that affect the late shifting or will the trans just stop shifting all together?
and how many rotations is it safe to turn the screw (counter clockwise) from the original factory settings
I think you are going to have to check what vacuum you actually have, as suggested by TJC T. If you have around 20in. then you will have to adjust the modulator valve on the injector pump. However first establish your vacuum reading.
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