Transmission will not shift into 3rd
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Mountain Pass
Joined: Oct 2017
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Mark, So unplug the hose from the modulator part at the transmission, keep it plugged in up top, start the engine and test for suction coming from that hose? Am I understanding that correctly? In regards to suction(vacuum), how much Vacuum should I expect or is any kind of vacuum enough?
Thank you!
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Mountain Pass
Joined: Oct 2017
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James
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Joined: Apr 2009
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From: Stanley, VA
The correct way is to use a vacuum gauge (cheap at Harbor Freight) but I think a "kinda, so-so way" to check would be put your finger over the intake manifold port (the uncapped nipple in your picture) with the engine running make a mental note of how the vacuum feels with your finger over the port. Plug the hose back in and get underneath and pull the hose off the modulator. With the engine running, put your finger over the hose, it should feel the same,
Like I said, kinda backwoods but you'll at least see that there's vacuum at the modulator.
Like I said, kinda backwoods but you'll at least see that there's vacuum at the modulator.
Senior User
Joined: Mar 2017
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From: Everton MO
when my modulator was disconected after the hose fell off the manifold the truck would go into first but never shift after that, so its most likely the modulator being bad, i replaced mine when i did my tranny fluid and pan gasket. i would replace the hose too, mine was dry rotted.
Thread Starter
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Mountain Pass
Joined: Oct 2017
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The correct way is to use a vacuum gauge (cheap at Harbor Freight) but I think a "kinda, so-so way" to check would be put your finger over the intake manifold port (the uncapped nipple in your picture) with the engine running make a mental note of how the vacuum feels with your finger over the port. Plug the hose back in and get underneath and pull the hose off the modulator. With the engine running, put your finger over the hose, it should feel the same,
Like I said, kinda backwoods but you'll at least see that there's vacuum at the modulator.
Like I said, kinda backwoods but you'll at least see that there's vacuum at the modulator.
James
Thread Starter
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Mountain Pass
Joined: Oct 2017
Posts: 101
Likes: 2
The correct way is to use a vacuum gauge (cheap at Harbor Freight) but I think a "kinda, so-so way" to check would be put your finger over the intake manifold port (the uncapped nipple in your picture) with the engine running make a mental note of how the vacuum feels with your finger over the port. Plug the hose back in and get underneath and pull the hose off the modulator. With the engine running, put your finger over the hose, it should feel the same,
Like I said, kinda backwoods but you'll at least see that there's vacuum at the modulator.
Like I said, kinda backwoods but you'll at least see that there's vacuum at the modulator.
"Remove the rubber hose from the modulator nipple by pulling it straight back by hand.
Insert a narrow shank flat head screwdriver into the nipple until it seats into the slot of the adjustment screw.
Turn the modulator adjustment screw in a clockwise direction with the screwdriver to firm up and delay the up-shift of the transmission. Do not adjust the screw more than four turns in either direction.
Turn the adjustment screw in a counterclockwise direction to speed up and soften the up-shift.
Adjust the modulator screw one turn at a time. Replace the vacuum hose onto the modulator nipple until it is fully seated."
I have that tube off and was thinking if the vacuum is good, this might be a next step before completely replacing it. Not that a $30 part is going to kill me but hey I thought I would ask.
James
Turn the modulator adjustment screw in a clockwise direction with the screwdriver to firm up and delay the up-shift of the transmission. Do not adjust the screw more than four turns in either direction.
Turn the adjustment screw in a counterclockwise direction to speed up and soften the up-shift.
Adjust the modulator screw one turn at a time. Replace the vacuum hose onto the modulator nipple until it is fully seated."
I have that tube off and was thinking if the vacuum is good, this might be a next step before completely replacing it. Not that a $30 part is going to kill me but hey I thought I would ask.
James
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Joined: Apr 2009
Posts: 14,333
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From: Stanley, VA
That adjustment is to sorta "fine tune" the shifts. I don't think adjusting it will make any difference. But then again, it doesn't cost anything either.
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Mountain Pass
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James
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From: Stanley, VA
Ok so I hooked the hose back up to the portion on top by the carburetor, and I get what seems like a pretty good amount of suction. So even though the Modulator did not leak when I unplugged it, could it still be the problem? I am getting suction to the modulator though. Any other steps to test?
James
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I am dealing with a simalar issue with my C4 but mine issue is most likely cuased by me rebuilding the transmission.
I would go ahead and change the modulator to see if that fixes your issue. If not drop the pan and valve body and do air checks to see if the issue is the valve body or in the governor or clutch packs.
I would go ahead and change the modulator to see if that fixes your issue. If not drop the pan and valve body and do air checks to see if the issue is the valve body or in the governor or clutch packs.
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Mountain Pass
Joined: Oct 2017
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MikeoOoOoO So I replaced the transmission Modulator and it appears as though that did not fix the problem, the shifts from Park to reverse and drive seem better and it’s shifting to second, but it does not seem to still shift into third. I am NOT a mechanic so I have a feeling it might be time to take it to a transmission shop, any things I might try?
james
james
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Pull the vacuum hose off of the modulator, and off of the manifold. Plug one end and try sucking on the other. Make sure it's not leaking. Test every other part of the circuit as well - I'm thinking it might be a rubber hose into a metal line, and then back to rubber at the other end.
When doing some engine work on my old '68 Cougar with a 289/302 and C4, I knocked that vacuum line off, and it would shift into second gear as I remember, but there was no engine braking in at least one of the two forward gears I did have. Major relief when I found the disconnected line, and fixed that bug...
When doing some engine work on my old '68 Cougar with a 289/302 and C4, I knocked that vacuum line off, and it would shift into second gear as I remember, but there was no engine braking in at least one of the two forward gears I did have. Major relief when I found the disconnected line, and fixed that bug...
Hotshot
Joined: Apr 2009
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From: Stanley, VA
Seeing as it looks like you have vacuum to the modulator and it still isn't shifting to third my next suspicion would be possibly something to do with the governor.
Since you've said that you're not mechanically inclined, I would say that it's time to visit the transmission shop
Since you've said that you're not mechanically inclined, I would say that it's time to visit the transmission shop
Laughing Gas
Joined: Jun 2016
Posts: 855
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From: Des Moines, Washington
Just a quick sanity check on my part.
Are you basing it not shifting into 3rd on engine RPM or by counting shifts? The C4 and C6 transmissions are 3 speeds with 3rd being 1:1. That means the engine will spin much faster then you might be used to if most of your experience is with over drive transmissions. My truck turns 3000+ RPM at 60 MPH.
If you are counting shifts, it's safe to ignore me...
Michael
Are you basing it not shifting into 3rd on engine RPM or by counting shifts? The C4 and C6 transmissions are 3 speeds with 3rd being 1:1. That means the engine will spin much faster then you might be used to if most of your experience is with over drive transmissions. My truck turns 3000+ RPM at 60 MPH.
If you are counting shifts, it's safe to ignore me...
Michael