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Old 06-23-2019, 06:49 AM   #1
budmanroch
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Default F-6 two speed rear won't downshift

My vacuum actuated 2 speed won't downshift when the truck is running. I checked the valve and it has 15 inches of vacuum when in HIGH range, and 0 in low range. I watched the actuator on the rearend and it moves in and out. The truck will be in low range after it sits for a night. Then I shift to HIGH and its fine. But when I try to go back to low I get a loud grinding noise from the driveshaft. Whatever is supposed to mesh in the rear - isn't as far as I can tell. Any ideas on what to try next? I'm a novice on differentials. Never had one apart.
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Old 06-23-2019, 08:07 AM   #2
jimTN
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Default Re: F-6 two speed rear won't downshift

Usually a vac leak somewhere between the carb and rear end diaphram. If I remember right, high shifts with a spring and low has to be pulled in or maybe the other way. Anyway the shift valve on the firewall will have only one hose going back to the rear.
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Old 06-23-2019, 08:07 AM   #3
big job
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Default Re: F-6 two speed rear won't downshift

Sounds like all is fine. Rule 1, if done for the day always park in 'low range' Not in high.
Rule 2, lets say you are cruising in forth and the axle is in high range and hit a hill and you want to shift axle down to low range - now pay attention; you floor the gas pedal and put button in Low ;;;;; now
quickly pop the clutch - now you are now in low range, get over the hill and pull the
axle button out, let of gas now your into high. simple. Myself I prefer vac over electric its
more friendly. For electric you gotta be quick cause they are so quick you loose it, then
It goes into la la land (grinding) and you must stop start all over again. So bottom line
you must keep pressure on axle gears, then shift, goes for all old or new trucks.
So remember pedal to the medal, pop clutch, and bingo you are now in low pull button
to high let off gas you now in high range: Your F6 with 2 speed equals 8 speeds and
two speeds in reverse. Hope you got it cause I'm no story writer just a truck driver. sam
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Old 06-23-2019, 08:16 AM   #4
big job
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Default Re: F-6 two speed rear won't downshift

Quote:
Originally Posted by jimTN View Post
Usually a vac leak somewhere between the carb and rear end diaphram. If I remember right, high shifts with a spring and low has to be pulled in or maybe the other way. Anyway the shift valve on the firewall will have only one hose going back to the rear.
Hi Jim I say its driver error not vacuum read my reply I drove these since
new and still doing it.......................
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Old 06-23-2019, 07:42 PM   #5
budmanroch
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Default Re: F-6 two speed rear won't downshift

Well, I really don't think its my driving. I drove this all last year and could split shift and downshift on the hills when loaded. This winter I changed the engine and transmission and now I can't downshift the diff. I don't think its a vacuum leak because the way I see it, the the high range is under vacuum and the low range is 0 vacuum and under spring pressure. The reason I think that is because when the engine is off it defaults to 0 vacuum and low range. I watched the actuator when I switched the vacuum on and off and confirmed this. During the engine replacement, I tightened a lot of loose hoses and the threaded connections on the valve and the splitter.

I'm pretty sure something else has gone wrong. I just don't know where else to look.
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Old 06-24-2019, 03:34 AM   #6
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Default Re: F-6 two speed rear won't downshift

I guess I'll retract that reply cause I didn't know that it was working. Does it move at
all in low like in your driveway - if it moves ok in high I should think that rules out a vac..
leak. Almost sounds like the control valve on the fire wall isn't completely exhausting
vac from the actuator. Maybe remove the actuator and see if the lever moves smoothly
you might have to jack it up turn drive shaft and move the lever. Has to be something
simple these axles are pretty rugged. Example of severe vac leak. Have a F6 no high
range when I pull the button to high just about stalls the engine so it has a ruptured
actuator diaphram. Meanwhile I'll keep thinking ? sam
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Old 06-24-2019, 06:39 PM   #7
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Unhappy Re: F-6 two speed rear won't downshift

That's the strange thing. It worked before the engine change.
After the truck sits overnight, It drops back into low range. I can drive it for miles in low range. Works fine. Then I shift up to high range and it works great. The problem is it won't go back to low range no matter what I try. I've done the full throttle thing, I have come to a stop, nothing seems to get it back to low except sitting overnight. I wonder if there is a spring somewhere that is weak and is not forcing the gears back to low range. I hear a lot of grinding when I switch to low and it will not drop in. It used to drop in when I stopped as a last resort, but no more.
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Old 06-25-2019, 08:09 AM   #8
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Default Re: F-6 two speed rear won't downshift

Sits overnight then goes into low? that rules out a weak spring and shows it is working.
This is a symptom that the control valve is not releasing ALL the vacuum in the line to
the actuator. I would make sure it is in high range secure the truck shut engine off
go under there and pull the hose off the actuator should be a good rush of air sound and
watch the lever move. Leave the hose off for now. Put the button in low, start truck it
should now be in low. This is like imitating "sitting overnight". It may have a little
grind as the planetary gears go into mesh. sam


Also have you removed the clevis pin on the actuator and moved the lever by hand should
move easy

Last edited by big job; 06-25-2019 at 08:28 AM.
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Old 06-25-2019, 11:49 AM   #9
Kurt in NJ
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Default Re: F-6 two speed rear won't downshift

Could the hoses on the firewall valve be reversed now after the engine swap
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Old 06-25-2019, 08:52 PM   #10
budmanroch
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Default Re: F-6 two speed rear won't downshift

I'll try this this weekend. I had a gauge on the valve and it went to zero when I went to low. Seemed to react pretty well from vacuum to zero. I don't think its the valve. I haven't tried disengaging the actuator from the arm. I'll see if it moves more without the pin in. Good thought. Maybe it stiffened up from sitting this winter. I know I did.
I traced the hoses and everything looks good. It shifts up fine, so I think they are on correctly.
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Old 06-30-2019, 05:20 PM   #11
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Default Re: F-6 two speed rear won't downshift

OK I tried all the suggestions-nothing worked. I added a helper spring to the arm and that didn't help. Then my son suggested changing the gear lube in the diff. Guess what - there wasn't any in there. I added a gallon and off I went. shifted high to low no problem 4 times. Then it stopped. Pulled the filler plug and I cant feel the oil. Need to get more apparently. Looks like the rear was dry and couldn't make the shift. Maybe a solution is nearby.
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Old 07-01-2019, 05:34 PM   #12
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Default Re: F-6 two speed rear won't downshift

so, if it shifts by itself while we are all sleeping, doesnt that make it possible to not complete the shift, no gears engaged, and roll into the neighbors house?
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Old 07-01-2019, 07:27 PM   #13
budmanroch
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Default Re: F-6 two speed rear won't downshift

Yes it does. That's why I chock the wheels. You would think it would be stuck between gears, but it doesn't. It always drops into low overnight.

I filled the diff all the way up with another 1/2 gallon and took it for a spin. No low range again.......... Frustrating to say the least.....
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Old 07-02-2019, 05:08 AM   #14
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Default Re: F-6 two speed rear won't downshift

Of all things we can not find the shop manual I has to be here-I don't lose books. Now
its looks like something internal as it is creeping into low overnight? How long it ran
with no oil could be a factor. I guess next would be to take the arm and housing off
and see how easy the part the shift arm goes into slides easy back and forth. My parts
book shows no rocket science for this; Something has to be binding I highly doubt its
rust? Something turned to dried mud cause my thinking says high is under power and
low is under spring power thats not enough for something gummed up. Meanwhile I'll
keep digging I been wrenching on these since they built our highways this is a first !!
Also even running without oil, first thing that will go is pinion bearings NOT the
planetary gear set cause its either in or out and is locked to the ring gear. sam
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