c6 transmission will not shift up to 3rd gear

I just recently got my truck running down the road... and have come to find out my newly rebuilt transmission will not upshift into 3rd gear...

I have read around and heard similar stories... but never a real definitive answer.

The modulator valve is plugged into the manifold vacuum port on my edelbrock carb... it shifts from 1 to 2 with ease...2 to 3 has yet to happen.

I have a vacuum port tree that I can put on the manifold.. but dont see how its differnt than the manifold vacuum port on the eddie.

What is the correct procedure to diagnose this problem and the correct way to fix it.

Thanks for your help.. by the way the 5.0 launchs that truck rather well... kickdown into p*** gear and hold on. I was surprised.

 

Does it up shift if you manually shift it? I have had some govenor sticking problems on Ford rebuilds that usually cure themselves in a day or so.

 

I tried that... I pulled it down into first... up to second.. fine.. up to third... nothing...I havent driven it but about 10 miles... its not very pleasant to be at 2000 rpm cruising at a slow pace... no speedo so dont know how fast but I would guess 25 to 30.

 

I had the same problem with my c6 it was the rubber hose that goes from the metal line to the modulator valve. It must have gotten some trans fluid on it and got all spongy and actually grew on the inside diameter. It has done it a few times over 2 years,as a matter of fact. Try a new piece of vacuum line and hopefully you will be all set.

 

O mine did shift into 3 manualy, but check it out maybe youll get lucky.

 

You KNOW for fact that this vacuum source has full vacuum at idle that drops off as RPM increases? Carbs usually have ported and non-ported vacuum locations...

 

lol well I read the instructions on the carb... and I do have it on the non ported... but I tried it on the other one as well no difference. but do I know if it is full or not no. Tomorrow I will clean up the old original vacuum tree and put it in and see if it makes a diff.

 

what about modulator adjustment?

 

Check the va***e with a guage on the steel line at the modulator,should be 18 to 22 ,but will work with as little as 12 to 14.Sometimes the steel line can get plugged with carbon,also with engine running remove the rubber line at the modulator off the mod,the engine must run like it has a va***e leak.In some instances it will show correct sorta low vac but not enough volume of va***e due to clogged steel line...

 

Well, Im not running any steel line...its a thick wall vacuum rubber line...
If I am to run steel line which thickness should be used? or does it make a difference with the steel vs the rubber?

 

Are you positive it's a C6? You said it is behind a 5.0 and they are not as common for a small block. It may well be a C6, but I was just considering the probabilities.

One thing to check is if the modulator will hold a vacuum. Put a mitivac on it and see if it holds any vacuum. It it doesn't, then replace it. They are pretty cheap and super easy to change. Depending on how busy you are and what you consider your troubleshooting time to be worth, it may make sense to replace it and eliminate that question.

Hey, isn't there a mechanical kick-down adjustment on the C6 as well?

Also, check the basics. Make sure the fluid is the right kind. Fresh filter. Correct torques and all that.

 

positive its a c6, just had it rebuilt about 6 months ago.
it had a brand new modulator valve on it... however... it did sit for 6 or so months open... and it got painted... possible clogging? dunno...

the kickdown to my knowlege has no adjustment?? just a rod that connects to the carb... then to the bolt on the trans...

Fluid... type f... new filter the trans came from a 67 merc. that was originally on a 390... the trans shop took the guts and placed into the small block c6 trans case... retaining all the internals and tailshaft. These guys do hundreds of these so i know they know what they are doing... plus several guys here on the hamb have used them. If all else fails I take it to their shop on monday.

 

Make sure the kickdown rod has a return spring on it to pull it forward and it isn't binding. At WOT you should still be able to stick .100 of feeler gauges between the lever and adjuster screw. At closed throttle the lever should be full forward.

The governor can cause it to stick. Sometimes the fluid p***age from the valve body gets trash in from the old cooler line carrying crud. Clearing it will do the the trick unless the governor is put together wrong.

BTW, how did the brakes turn out?

 

okk.. well im looking online for a new modulator valve... and in doing so just got confused. their are a lot of multiples here...

Which modulator valve do I get...
do I get one for the old merc engine...
do I get one for a 1984 ish ford f150 which is essentially the way the truck was built... only using a 5.0 engine. with flat tappet cam... with carb intake and pulleys and balancers.... anyway. or do or I use one from maybe the mid 70s when the 302 was in use on fully carburated vehicles.

Then which stripe do I use. green purple blue.
ahhhhh good grief.. this is conusing!?!?!?!?!?!

 

Most of them are adjustable and its not a big difference anyway. Just stick a small screwdriver into the vac line port. Right turn is later shift left is quicker shift.

 

yeah all that is good... dunno about the p***age ways.. but the truck has a new radiator and coolant lines...

The brake finally bled out... it took me about 3 times all around the truck for it to finally bleed out... the truck stops great! now... despite the 3rd gear thing... Ive made leaps and bounds of momentum on the truck... its running, driving, lights work after an hours work today, got a new exhaust sytem today, it STOPS!, I got the kickdown installed, and when it kicks down... shish... might as well be in a mustang... lol. well ok maybe not that far... next up after 3rd gear is brake lights!! I am going to pull the pedal ***embly from a mid 70s truck.... as thats where I got the brakes from. and it has a bracket with a switch for the lights... or I could plumb one into the brake lines... but doesnt seem to safe...

Thanks for askin. and helping

 

i take it that is for all gears? or is that for just 3rd?
1 to 2 comes around fairly quick.

 

All it just changes the pressure on the diaphram to make it use more or less vac to operate it.

 

If I replace the mod valve which one should I get? green purple blue.
Should I get it for the current motor or the engine the ****** came out of? I think that might be what happend??? maybe?? dunno.

 

Just a thought,but are you SURE it is VACUUM hose that you have on there?

Just checking,especially with no hard line,the hose could be collapsing.

 

My C6 is set up just like yours,vacuum line the whiole way to the trans. The ****** shop I took it to, to adjust it for me ,and SHOW me how to do it,told me to change it to the hard line for best results over time. He also told me not to have more than 3" of rubber line at the connection to the carb and ******.

 

Which side of the Edelbrock carb are you plugging into for vacuum?

Edelbrocks instructions indicate the right side bib is for vacuum advance, that one is ported vacuum - which doesn't work well for the majority of engines.
Left side carb bib (both of them in front) is full time vacuum.


Does the C6 operate like the C4 in that you need proper vacuum to shift?

My pal just got his SBF & C4 combo shifting right.
He had a low vacuum problem that wasn't rectified until he got the camshaft in proper time.

Once that was squared away, the trans shifted into high just fine.

 

Well if you are standing in front of the vehicle... the full time port is on the right... and the ported is on the left...

What size hard line did they recommend you use?... I do have a mild cam in the engine.. I can tweak on the timing some but i am far far from a timing guru... more like novice. Do you think I would have better luck with a vacuum tree on the manifold?

Would a vacuum leak cause this issue?

 

Yes,vacuum leak will cause a problem with shifting.
The line he said to use was 1/4".

 

hmm well my dad told me to get a spray bottle and mist gas around the carb to see if the engine surged... Im still green on all this so i was a bit timid to pefrom this action... for some reason.. I have to think to myself now adays if this blows up what will my wife say?? ya dummy what did you expect when you spray g*** on a hot suface...

Any other methods for checking for leaks or is this the best option. I know there is a gauge... which I dont have... my dad does 3 hours away. lol does me no good here. I guess i can go purchase one from the parts store.

 

When you disconnect the vacuum line the engine RPM should change. If it doesn't...you have a leak!

 

spray carb cleaner instead of gas. Use the plastic straw to direct the flow. Keeps it off the hot surfaces

 

I had very similar issues with mine after I rebuilt it and it ended up being a stuck govenor. I would take it to the shop that rebuilt it and let them check it out. It will take a shop about 10 min. to check that out on a lift.

 

I should have emphasized I was looking from the drivers seat.
You're right and so am I.

Yes, if the C6 operates similar to the C4.

Get yourself a vacuum gauge and see how much vacuum you do have at idle.

My pal's engine wouldn't pull over 12" regardless of RPM until the cam timing was squared away.


Do not mist gasoline onto the engine.
A recipe for disaster.

I used to date a girl whose brother was badly burned while pouring gasoline into the carb from a can.
Nice kid, sad story.

 

YIkES... yeahhh. dad is one of thos ol' pros... he can do it and be fine.. if I did it I would look like a extra long chili cheese coney... that stayed on the grill too long.... I will have to pick up a gauge before I drive it cross town..