87 f250 with 6.9 Trans Issue - Diesel Forum

smokietb · 01-13-2013, 10:14 AM

Hi all. Just picked up a 87 f250 with a 6.9. First one for me so i'll be asking a lot of questions.

I have always been a manual trans guy. Not much exp/knowledge with autos.

The auto Trans will not shift up unless you completely let off the gas. Additionally it will only hit second above 30mph and third above 50mph. second to third it a bit of a thud. Not sure if it has a fourth or overdrive. no issued downshifting.

not sure how tough these trans are. Would you junk it and get a new one or try for a new valve body first?

Thanks,

**Mark Kovalsky** · 01-13-2013, 12:07 PM

Neither. It's a C6, they are about as tough as an automatic can be. It is a three speed, so there is no overdrive and no fourth, and no torque converter lockup.

There is a valve on the injection pump that supplies vacuum to a vacuum modulator on the side of the trans. The valve could be misadjusted, the vacuum line could have a leak, or the modulator could have a leak.

The modulator is easy to check. Find it on the side of the trans. It's the only part that has a vacuum line to it. Pull the line off and see if there is any ATF in it. If the line isn't completely dry the modulator has failed.

As for adjusting the valve, called a VRV, someone else will have to help with that. I don't have the procedure.

smokietb · 01-14-2013, 10:01 AM

Thanks Mark. Sounds like a good start. I'll give that a look tonight.

smokietb · 01-14-2013, 08:09 PM

so it was little gooey down there but the inside of the line looked clean. There is less than a foot of soft line till it hits hard line on the trans side. I found new modulator for $20 ish. I think i'll replace lines and the modulator just for the knowing. Any recommendations on a brand?

The modulator is low on the trans. am i gona dump atf when I pull it?

Is it just something like a set screw on the IP to adjust the vacuum pressure to tell it how quick or slow to shift?

NH2112 · 01-15-2013, 03:01 AM

Going from memory of adjusting the VRV on a GM 6.2l w/TH400, you have to remove the vacuum line going to the modulator, cap its nipple, then apply a certain amount of vacuum to the VRV with a hand pump. Open the accelerator lever enough to insert a gauge block of a certain thickness (0.646" sounds familiar but it's been over 10 years) between it and its stop, and you should see a certain vacuum reading on the hand pump's gauge. Loosen the VRV mounting screws and adjust it till that reading is correct. Reconnect everything and road test. It really is a lot simpler than it sounds LOL

smokietb · 02-17-2013, 02:04 PM

so got a new modulator installed. I didn't know there was a set screw that seems to adjust a diaphragm. I tried the new modulator without the set screw at all and it shifts just as it always had. Then tried running the set screw all the way in an I had to be above 30 to hit 2nd(before was 25) and was unable to even get to third. So it would seem the deeper the set screw the deeper in to the RPMs you have to be to shift. My Chilton is letting me down. I cant find anything about adjusting the modulator or the IP.

I seem to be getting good vacuum. even after sitting for 20min when i pull the line of the trans modulator is hisses for a minute or so pulling air i assume. How much vacuum should i have?

chuckster57 · 02-17-2013, 02:30 PM

Going on memory from my C6 days, the screw on the modulator adjusts how soon ( low vacum) it will shift the transmission. I had a shift kit, and HD convertor, behind a 460 on steroids. Had a heck of a time adjusting it for empty and towing.

Since Diesel engines don't develop manifold vacuum, that's what the pump is for. Since it provides a constant source greater than needed for shifts, the VRV on the IP regulates it to mimic the desired shift point.

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smokietb · 02-17-2013, 04:52 PM

picked up a vac gauge. My pump is pulling 25 at idle. Have 25 at the vrv, and only 4 at the modulator. With any RMP the vac drops from 4 to 0 at the tran modulator. Does that sound right?

smokietb · 02-17-2013, 06:04 PM

So here is what i found. Spec is for the VRV to deliver 13" at idle and 6.5"-7.5", .515" from WOT. There are two screws on the side of the IP you can loosen that allow you to twist the VRV and adjust the vacuum delivered to the modulator.

Mine adjusted to max, only delivers 8" at idle. New VRV it is..