722.5 auto box: 5th gear disappears with heat

ray_hennig

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737
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MB 300CE-24 Sportline
Hello all,

And a challenge.

300CE24 5th gear disappeared again. What have I done to investigate this:
  1. Replaced microswitches under gear lever. Confident that all's OK here.
  2. Cleared 4 flash fault code: “Throttle valve switch (potentiometer) interrupted.”
  3. Drove 8 km to refuel and 5th has gone again.
  4. Checked microswitch again - seems OK. Now cannot clear fault code.
  5. Several days without 5th.
  6. Colder this morning (clue?) drove off and we have 5th.
  7. But not for more than 15 km. It’s gone again.
Is this temperature-related? The first time failure did seem to be in very hot summer weather.

I can say more, much more, on this but shall await initial feedback.

Best to all.

RayH
 
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ray_hennig

Active Member
Joined
Dec 1, 2008
Messages
737
Location
France
Car
MB 300CE-24 Sportline
Is this a clue to the illusive 5th gear?

Cold snap this morning so I left the engine running to clear the windows before leaving. Drove up the road for about 1 km and 5th engaged.

Switched to 4th with the gear lever - worked as expected.

Switched back to 5th and 5th reappeared. Just as it should.

10 seconds later, 5th disappeared again so I’m back to 4th.

And no more 5th for the other 10 km.

And still the 4 flash fault code: “Throttle valve switch (potentiometer) interrupted.”

Baffled.

Any suggestions anyone?

As always, thanks in advance.

RayH
 
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ray_hennig

Active Member
Joined
Dec 1, 2008
Messages
737
Location
France
Car
MB 300CE-24 Sportline
Well, 5th gear's back.

I took out the HGS module and started measuring. Good earths, ohms and volts all as per specification.

I installed a replacement, second hand HGS and it returned an error. And now I'm skipping 4th and going straight to 5th.

I cleared Code 9 and life has returned to normal. A couple of test runs and all's well.

This morning we did a 60 km round trip to get the oysters and all is well.

Cautiously optimistic so shall reinstall the interior trim when the weather warms (now done) up a bit this afternoon.

I'm not really sure what I did to fix this. I've no reason to suspect the original HGS so must assume that I may have not securely install the HGS. I've sometimes found it tricky to engage the MAS-style modules that have the twist knob on top to engage it. That's my only answer as all contacts have been well cleaned numerous times.

Time will doubtless tell.

RayH
 

alabbasi

MB Enthusiast
Joined
Feb 18, 2008
Messages
2,143
I believe that your 722.5 gearbox has an external overdrive unit on the back of the transmission. i've pretty much avoided these gearboxes and the cars that used them because of the wiring issues that came with them. But there should be plenty of information on www.benzworld.com regarding how to troubleshoot this box.

Good luck
 
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ray_hennig

Active Member
Joined
Dec 1, 2008
Messages
737
Location
France
Car
MB 300CE-24 Sportline
I believe that your 722.5 gearbox has an external overdrive unit on the back of the transmission. i've pretty much avoided these gearboxes and the cars that used them because of the wiring issues that came with them. But there should be plenty of information on www.benzworld.com regarding how to troubleshoot this box.

Good luck
Yes, it's an add-on to the 722.3 4 speed box. And it's pretty reliable too as my 450k Kms suggests. BUT, 4th/5th issues do crop up and it's usually electrical/electronic rather than mechanical. That's why I attempt to solve the problems, usually with some success - fingers crossed!

Anyway, progress this week has been encouraging with a nice drive in the sunshine and all 5 gears.

All the best.

RayH
 

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