W124 E320 Cabriolet hydraulic top - operates once then dies - is it the relay or the pump

Started by Hugo_Darlinghurst, 11-04-2022 11:48 AM. 13 replies. 2,031 views.

11-04-2022 11:48 AM

Hugo_Darlinghurst Senior Member

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Join Date: Feb 2010
Location: Darlinghurst NSW
Posts: 524
Club: Mercedes Classic Drivers Australia (MCDA) - chapter committee
Car: 1968 W108 280SE 5.0 Coupe

i know this came up before.

1996 E320 Cabriolet (A124). The hydraulic top operates once from closed to open, then will not operate again until the car has sat for about 4 hours. Then operates once more and dies again.

A124 convertible top control unit reference

A124 convertible top hydraulic control schematic

sounds like the pump relay getting hot and cutting out, not the pump itself. WIS calls it K9 in the auxiliary fuse box.

Question: anyone confirmed this specific failure on an A124, and is the K9 relay still available in 2017?

I think these are the parts i need? Found this is an old manual

11-04-2022 01:18 PM

Vasilios_Billy

Lifetime Benzer

Join Date: Mar 2008
Location: Campbelltown NSW
Posts: 1,864
Club: M.A.L.A.K.A.S.
Car: 2020 AMG GT C

Hugo.

Yes! This is a textbook K9 thermal-cut-out failure on the A124. The relay overheats during the first top operation, opens, and requires about 3-4 hours to cool to closed state. Once cool, it operates once more and overheats again.

The K9 relay is Mercedes-Benz part A 124 821 02 51. Still available from Mercedes-Benz Classic Centre Stuttgart in 2017. Approximately $165 retail. The aftermarket equivalents (Hella, Bosch) are listed but the internal coil specification is different and they do not reliably handle the inrush current of the top pump motor. Genuine only.

Replace in conjunction with inspection of the top pump motor brushes. If the relay has been heating, the pump motor has likely been drawing more current than normal which suggests brush wear is contributing. Brushes are user-serviceable on the A124 top pump.

11-04-2022 04:18 PM

Hugo_Darlinghurst Senior Member

★ Gold

Join Date: Feb 2010
Location: Darlinghurst NSW
Posts: 524
Club: Mercedes Classic Drivers Australia (MCDA) - chapter committee
Car: 1968 W108 280SE 5.0 Coupe

Vasilios.. excellent. Ordering the K9 from MB Sydney Service today. Will inspect pump brushes while waiting for delivery..

19-04-2022 09:48 AM

Hugo_Darlinghurst Senior Member

★ Gold

Join Date: Feb 2010
Location: Darlinghurst NSW
Posts: 524
Club: Mercedes Classic Drivers Australia (MCDA) - chapter committee
Car: 1968 W108 280SE 5.0 Coupe

fyi - K9 replaced. pump brushes were still in limit, 4mm left from original 8mm. top now runs thru cycles properly. $165 and half hour. Vasilios was right again..

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