300CE24 722.5 auto box: 5th gear disappears with heat

[rayhennig]

Hello all,

And a challenge.

300CE24 5th gear disappeared again. What have I done to investigate this:

Replaced microswitches under gear lever. Confident that all's OK here.

Cleared 4 flash fault code: “Throttle valve switch (potentiometer) interrupted.”

Drove 8 km to refuel and 5th has gone again.

Checked microswitch again - seems OK. Now cannot clear fault code.

Several days without 5th
.
Colder this morning (clue?) drove off and we have 5th.

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

[Marcel107]

Hi Ray,

maybe the HGS Relay A008 545 66 32 (in my 1990 300CE-24) next to the Battery has a cold solder joint, or bad contacts


Best regardsrom Germany, Cologne

[rayhennig]

Thank you Cologne from grey Dordogne.

R

[rayhennig]

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

[rayhennig]

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 up a bit this afternoon (now done and cleaned up).

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