Ford C6 transmission - Won't shift into last gear

YELLAHEEP

NAXJA Forum User
I've got an '89 Ford F350 U-haul van with the IDI 7.3 diesel. I bought it with a severe trans fluid leak at the trans cooler line. The previous owner sold it cheap thinking the trans was "shot". It would not go into gear without revving up the engine a bit, then it would engage and do so rather solidly. It would stay in gear until you let off the throttle and you'd have to rev it up again to get it to engage. He said he'd been driving it that way for about a month anywhere from 1 to 3 days in a week, but probably a few hundred miles in that month, and said he'd always added several quarts before driving it anywhere.

I believe the transmission is a C-6. I filled the transmission at idle in neutral like the dipstick says. I had 4 quarts with me when I bought it and after adding this fluid, it would engage better and with less RPM than before, so I drove it home - about 10 miles. It would shift from 1st to 2nd, but never into 3rd. Figured it just needed more fluid.

Got it home and the next day I filled the transmission with another 5 quarts of fluid. It now drives fine, immediately goes into gear and when it shifts into gears, (1st, 2nd and reverse) it's solid.

But.... when driving on the road, it won't go into 3rd at all, no matter the RPM's or speed. It's not like it tries to find 3rd, is simply acts like a 2 speed transmission. It will barely do 50mph and I don't want to push the engine into those RPM's to go faster.

So, with all that said....... Since all of the gears engage nice and solid, no slipping, no burnt smell in the fluid..... Is there something mechanically wrong keeping it from shifting into the final drive gear? I'm not well versed on transmissions, especially Fords so any advice/info would be great!
 
I just e-mailed your description to my brother. He has years of Transmission experience. Hopefully he will respond soon.
 
Thanks brutha!

I PM'd JJacobs as well - he's got lots of tech skills with transmissions too. I'd imagine between the both of them, I should get the info I'm looking for! :thumbup:
 
Put a pressure gauge on it.. rear on jackstands and up to a speed where it should be in 3rd. If the pressure drops out there's an internal leak- cut direct clutch seal or something similar. Air tests are good, with the valve body off use a blow gun at light pressure at the direct apply hole and listen for a thunk or thud.

Your best hope would be a blown valve body gasket or stuck shift valve, but I doubt that's the case.
 
JJacobs said:
Put a pressure gauge on it.. rear on jackstands and up to a speed where it should be in 3rd. If the pressure drops out there's an internal leak- cut direct clutch seal or something similar. Air tests are good, with the valve body off use a blow gun at light pressure at the direct apply hole and listen for a thunk or thud.

Your best hope would be a blown valve body gasket or stuck shift valve, but I doubt that's the case.


How do I find the 3rd gear "direct apply hole"? Under the valve body, there's a ton of worm holes......
 
DaJudge said:
I just e-mailed your description to my brother. He has years of Transmission experience. Hopefully he will respond soon.


Anything back?
 
OK...Finally!

He says it should have a modulator like a GM tranny. Pull the vacum line off it and make sure it doesn't have fluid coming out.

If not, it is adjustable. A small screw driver and some trial and error should get it. The adjuster is inside the vacum line nipple.

Glen
 
DaJudge said:
OK...Finally!

He says it should have a modulator like a GM tranny. Pull the vacum line off it and make sure it doesn't have fluid coming out.

If not, it is adjustable. A small screw driver and some trial and error should get it. The adjuster is inside the vacum line nipple.

Glen

Thanks Glen,

Makes me wonder if the vac. line is just disconnected or leaking.......

Can you ask him if the modulator controls all 3 gears and when they shift or just the shift into 3rd?
 
I will call him today.
 
YELLAHEEP said:
Thanks Glen,

Makes me wonder if the vac. line is just disconnected or leaking.......

Can you ask him if the modulator controls all 3 gears and when they shift or just the shift into 3rd?

all gears, it just 'modulates' main line pressure. Not your problem.
 
JJacobs said:
all gears, it just 'modulates' main line pressure. Not your problem.

I talked to him again and JJacobs is correct. If it is shifting from first to second as it should the modulator is not the problem.
 
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