Ask the Expert: What is causing my Mercedes automatic to slip out of gear?

Most automatic gearboxes require oil changes every four years
Most automatic gearboxes require oil changes every four years - alamy

Dear Alex,

I have an elderly but lovely Mercedes-Benz E320 diesel estate with 140,000 miles. Recently, on two occasions, the automatic gearbox seemed to have slipped into neutral, not providing any drive. It only lasted a few moments and did not require breakdown recovery. Mechanics at my local garage seem baffled and suggested changing the gearbox oil; the cost could be £400-£500, which seems excessive if it does not solve the problem. Any ideas?


Dear GU

If your garage is quoting £400-500 for a gearbox oil change, I’d try somewhere else, because that does seem excessive; £200-300 is a more realistic figure. I suspect an independent Mercedes-Benz specialist would be happy to take on the job at that sort of price.

What’s more, a specialist will likely have the correct diagnostic equipment to plug into your car to see whether the gearbox has stored any fault codes.

If it has, that could help solve the problem without having to go through a process of laborious – and costly – trial and error. And even if there is no fault code stored, a marque specialist may well have experienced a similar fault and have a good idea how to fix it.

While your local garage’s prices are on the high side, they are right that with a gearbox fault such as this, changing the fluid is usually a sensible first step. That’s especially the case with old Mercs, given their owners usually adhere faithfully to the advice that the gearbox is “sealed for life” and that the fluid doesn’t need to be changed.

In reality, these gearboxes benefit from fluid changes as much as any other; many enthusiasts and specialists suggest doing so every four years or 40,000 miles. If your car has never had a gearbox oil change, the oil will be old and burnt out, so it’s worth doing regardless of whether it solves the problem or not.

If the issue persists, check the control unit connector plug. If this is wet, the bushing that isolates it from the transmission has failed and fluid is leaking onto the plug, possibly wicking up the wiring to the control unit.

Alternatively, it could be the conductor plate or speed sensor – these get clogged with swarf over time and can be cleaned, though it’s usually easier to replace them.

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