500 SEC Transmission drags engine idle down

[Gear1275]

gents,
As per title, in idle and neutral my 500 sec idles fine (now I’ve addressed a vac leak)
But now if I engage drive or reverse the engine sounds like it’s under load and the revs drop to about 550 from an idle of 650ish.
Also, when I engage drive and reverse it is very clunky. The shifts are smooth and the box kicks down fine and pulls very well but it’s just the clunk on engagement and the fact that it drags the engine down.
If it was a manual car it would be like you had engaged a gear and pulled the clutch out past biting point to the point that the car feels like it would stall if you didn’t release the brake.
Hope the description helps in diagnosis.
The fluid level in the box is good.
Are there any adjustments to be made?
James.

 

[grober]

Your engine is under load-before engaging drive your torque convertor was freewheeling now its trying to drive the gearbox- the gearbox is effectively locked in a gear but not moving so slippage/drag has to occur- like your clutch analogy.
the clunks are more likely to be coming from the engine/gearbox/propshaftcentre bearing mounts being soft or worn- or possibly the flexible prop shaft joints moving on drive takeup as the engine torque is applied? Most of these early boxes are vacuum controlled so make sure there are no vacuum leaks [ the rubber joins/branches are prone to perish and crack. Here's a link to fault finding the 722 box- diffferent car but same design of box
http://www.w124performance.com/docs/mb/other/tranny_722_Mitchell.pdf

 

[alabbasi]

It sounds like your vacuum modulator is not connected or out of adjustment. There's a line that goes from the back of the engine to the side of the transmission. Make sure it's connected. If it is, you can adjust the vacuum modulator to dialing the key clockwise to firm or counter clockwise to soften the shifts. Someone may have gone in before you got the car and adjusted the modulator to compensate for the leak.

Thanks

 

[Gear1275]

Your engine is under load-before engaging drive your torque convertor was freewheeling now its trying to drive the gearbox- the gearbox is effectively locked in a gear but not moving so slippage/drag has to occur- like your clutch analogy.
the clunks are more likely to be coming from the engine/gearbox/propshaftcentre bearing mounts being soft or worn- or possibly the flexible prop shaft joints moving on drive takeup as the engine torque is applied? Most of these early boxes are vacuum controlled so make sure there are no vacuum leaks [ the rubber joins/branches are prone to perish and crack. Here's a link to fault finding the 722 box- diffferent car but same design of box
http://www.w124performance.com/docs/mb/other/tranny_722_Mitchell.pdf

Thanks for the in-depth reply!
I will have a read through that and see if I can get to the bottom of it. I’ve never had auto box issues In my years of driving so not had the need to research that area so it’s a good excuse to learn...

 

[Gear1275]

It sounds like your vacuum modulator is not connected or out of adjustment. There's a line that goes from the back of the engine to the side of the transmission. Make sure it's connected. If it is, you can adjust the vacuum modulator to dialing the key clockwise to firm or counter clockwise to soften the shifts. Someone may have gone in before you got the car and adjusted the modulator to compensate for the leak.

Thanks

Ok thanks.
Would adjusting the modulator affect how hard the gearbox engages drive?

 

[alabbasi]

Ok thanks.
Would adjusting the modulator affect how hard the gearbox engages drive?

Yes, but trace the vacuum line first because the character you describe would indicate no vacuum is getting applied

 

[219]

To a degree that is normal , there is a load on the engine when drive is applied , but usually it is compensated for and the idle stays the same , but you can feel the load on the engine ; most of my cars have done this , including my 500SEL which is the same engine and box

 

[spike60]

I have an 1985 500 SEC and it idles at about 600 rpm hot falling to about 450 rpm in drive. I don't consider it a problem as it's smooth and never stalls.