722.5 trans 5th gear issue - PeachParts Mercedes-Benz Forum

Rital24 · #1 03-24-2021, 01:19 PM

Hi everybody!
Here is Johnny from Italy with his bad english!

I've this strange issue on my 1990 C124 300 24v supercharged with a 722.501 AT.
When I drive the car in "smooth mode" all the gears are working fine but, under partial or full load, the 5th gear takes about 4/5 seconds to engage even if I completely release the accelerator at, for example 180 Km/h.
This behavior is very annoying! The trans has completely rebuilt last year, tried to increase or decrease oil pressure, changed overload protection switch that I suppose has something to do with shift retard but nothing, the problem remains.
I think the switch near the lever is OK as well as the solenoid inasmuch if I don't accelerate to much the 5th gear engages properly.
Maybe the problem stays in the positive pressure in the manifold during boost that could create bad signal to some sensor or EZL unit?

I hope someone of You can please help me on this.
I really appreciate any suggestion!

rayhennig · #2 03-25-2021, 03:34 AM

Hello Italy, from France.

I have a very similar car without the turbo. For me, all changes are very good. BUT, if I am accelerating hard, 4th to 5th has a problem. It switches between 4th/5th/4th, until I release the accelerator and the 5th engages.

I call this "hunting". The gearbox hunts for 5th, it traps 5th but 5th escapes until it is trapped again.

Everything on mine has been changed - rebuilt gearbox, switches, HGS controller, electrical connections etc.

The only thing that has not need changed is the throttle position sensor (TPS) - the plug/switch and cable assembly you see in the attached image.

I have measured the resistance of this switch and the results are not a stable as those from another switch. So, one day I shall change the switch but it is not a high priority for me.

I have had this problem for many years, both before and after the gearbox rebuild and a major engine overhaul in 1997. Basically, it has done this since the late 1990s when I first noticed it.

Sorry I cannot be more helpful to you.

Good luck.

RayH