[bennett7]

Go easy on me guys, I'm a Brit and new to this auto transmission stuff.
I have a 73 Iso Lele with a Cleveland 351c and a C6. I had the engine rebuilt and it's good however, the C6 was sold as rebuilt but it has problems.
It shifts from 1 into 2 immediately after take off and then "flares" from 2 to 3 instead of changing smoothly. It drives OK but always feels like it should have more power.
I've checked the modulator and it's got vacuum on both pipes and checked engine vacuum...they're all showing 15 inches but I was advised to expect 18 - 21 inches on the vacuum gauge.
Fluid has been changed twice in less than 1000 miles and it's all good colour. Transmission specialist says probably change the valve body as a starting point but C6 parts don't seem to be available here in the UK.
This is not a regular daily ride so thought I will sort it out during the winter months.
Any helpful advice would be appreciated, thanks in advance.

 

[galaxiex]

Quick easy test, disconnect vacuum line(s) at the modulator and plug them.
Shifts should be very late and firm.
Report

 

[bennett7]

Thank you for the advice, I will try that tomorrow as soon as I can get a car lift free and report back.

 

[galaxiex]

Thank you for the advice, I will try that tomorrow as soon as I can get a car lift free and report back.

If you have one, putting a pressure gauge on the mainline pressure tap
would be very helpful to know the trans mainline pressure(s).

Must be at least a 0 - 300 psi gauge.

Mainline pressure tap is on drivers side of the case, couple inches in front of the linkage.
Should be a 7/16" head pipe plug. 1/8" pipe thread

 

[51 Ford F1]

Make sure you’re using the correct auto trans fluid. The C-4’s used type F. I suspect the C-6’s used the same. Using the newer type fluids could cause it to exhibit the systems you indicated. It has to due with the friction modifiers. There’s many an article on the distinction (do internet search). But if the trans was rebuilt with the newer type clutch material, then you need to use the corresponding fluid type. Good luck!
Separate note; make sure your trans is hooked up to an external cooler; either the radiator (if it allows it) or a trans cooler.

 

[bennett7]

Thanks for your input, much appreciated. The transmission was rebuilt and I was advised to use only Dexron which I have done and it has been drained and refilled at around 400 mile intervals but no improvement
The car was built with a seperate transmission fluid radiator/ cooler and it is hooked up and working as it should.
I am still trying to get a car lift free so that I can check on some previous recommendations.

 

[bennett7]

Well, I finally managed to free up a car lift and disconnected the two vacuum lines and plugged them before taking her out on road test. The acceleration in 1 was fantastic but it never changed into 2 ( hence it never changed into 3 ) so I shifted it manually into 2 and it's a good smooth shift and acceleration is excellent however, it would not then shift to 3 in auto mode or with a manual shift.
Directly I reconnected the vacuum lines she demonstrated the same sliding/flaring and very early shift from 1 to 2 at around 17 mph and then almost as quickly into 3 in auto mode ; manual mode was okay although it seemed there was not such strong acceleration as with the vacuum lines plugged at modulator.
Might be significant to know that I removed the EGR valve from the back of the Cleveland carb when I put the engine back in in order to increase air cleaner clearance on hood. I realise that one of the connections on the modulator was from the EGR valve .....maybe I need a modulator with only one connection instead of two ?
Hope somebody can see something obviously wrong here and chime in, thanks.

 

[bennett7]

If you have one, putting a pressure gauge on the mainline pressure tap
would be very helpful to know the trans mainline pressure(s).

Must be at least a 0 - 300 psi gauge.

Mainline pressure tap is on drivers side of the case, couple inches in front of the linkage.
Should be a 7/16" head pipe plug. 1/8" pipe thread

the only 7/16 plug I can see is the band adjuster. When you say it's on the driver's side, I assume you're talking left hand drive ?

 

[PSIG]

couple inches in front of the linkage

 

[bennett7]

thanks for the clarification, I'll revisit it on Monday and report back.

 

[galaxiex]

Plug the vacuum line for the EGR.
Leave the port on the modulator open.
Use only the vacuum line that goes to the center of the modulator. The other end of said line should go to full manifold vacuum.
Re road test.

 

[bennett7]

Genius, absolutely spot on !
I did as suggested and took the old car out for a run...wow , now it runs like a Cleveland motor should.
Great acceleration in 1(auto) around 30 mph change to 2 with no flare and very positive change and around 50mph nice clean shift to 3.....fabtastic.
A million thanks to you, galaxiex, for your input and for getting this oldtimer running smoothly for this oldtimer driver.
With all best wishes for Christmas and the New Year in 2023.