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New Member
Jun 13, 2006
E320 CDi & E350CDi Estates
Having got a stuck bonnet open quicker than anticipated I:bannana: thought I would invest some time in a (LONG) maiden post to address this and some closely related topics.

This probably ought to be three separate posts but there are limits!:cool:

First off, I have been suffering a vicious circle of annoying "bonnet open" and middle of the night alarm incidents for some time from my 2005 W211 E320 CDI Estate:dk:.

If you have a W211 of this vintage and you haven't replaced your alarm unit and/or bonnet latch switch yet, then this post is for you :thumb:.

You might want to consider some preventative maintenance because these components are likely to be at or near their point of failure :eek:.

Then again, any DIYer fiddling with the bonnet latches or "latch open" switch (under the UK passenger side bonnet latch) is in grave jeopardy of locking themselves out (as I discovered :doh:).

It is all too easy for the grommet at the end of the sleeve of the latch cable to slip out of the slot in the latch :eek:.

If you have been working under the bonnet I suppose a useful protocol might be to close the latches manually and make sure that they still open by pulling the cockpit release under the steering wheel BEFORE finally shutting the bonnet :p.

If, however, the worst happens and your bonnet won't open...:wallbash:

In my experience, forget about poking coat hangers through the grille or trying to get at the passenger side latch (the one with the switch on it in a RHD vehicle). Even if you routinely paint your hall through your letterbox you're not going to get at the latch from under the car or by removing the passenger side wheel arch valance; fun as it may be to try :(.

No, kudos to this guy for showing the way: :thumb:.

However, don't be misled, he's using a wreck for purposes of illustration.

You WILL have to remove the driver side wheel (on a RHD model) and the front valance from the wheel arch but that still won't give you access to the latch cable; it is the other side of a steel bulkhead.

You will also have to reach forward and up toward the upper lamp assembly (the driver side latch is directly above it) in the engine bay and grope around for a stiff cable. Once you're sure you've got it (and NOT a wiring harness!) give it a yank, like for an old style train communication chord, and you should be rewarded with a satisfying pop as the latch on the RHD passenger side also opens. Hurray!:rock:

Since you are working on the RHD driver's side wheel arch, though, you might want to replace that irritating alarm, too. It's a self-contained unit and the trick is that the batteries corrode in due course and knacker the pcb - a process only accelerated by a faulty bonnet latch switch, which may be setting the alarm off in the first place; always assuming you have already killed the spiders, closed the air vents and/or disabled the cockpit and towing alarm sensors to rule out false alarms from these quarters :eek:.

It's going to cost you about £100 for the alarm unit from a dealer (they usually have them in stock because they are on numerous models and commonly go west after half a dozen years or more. I can't see much point in getting a secondhand one unless you know it's youthful :rolleyes:.

You just need to take off the rear valance from the same front offside (UK) wheel arch and either use a CR T30 torx bit to detach the screw or just wiggle the unit till it frees up and then work the screw out with pliers.

You will need Mercedes part number A 219 820 32 26 to replace it and an M10 nut (I think).

The replacement part is male and it comes without a nut:dk:; although if they have one in stock Mercedes will be happy to sell you it for £1.10!

Incidentally, to remove the wheel arch valances you will need 10mm and 8mm sockets/spanners for the half dozen nuts and screws on each valance and a flat screwdriver to ease four push pins from their plastic rivet fasteners - one at the front is recessed and it would certainly help to have the special, long-nosed, plier-like tool, but you can cope without it :D.

It takes about an hour to do this job - naturally, using the oblong centre jacking point (at the level of the front of the wheel rims) for your trolley jack and the side jack point for your jack stand, or vice versa if your jack has a puck-like rubber head that fits in the jack point without damaging it.

Now, at last, you can turn your thoughts to replacing the bonnet latch switch itself :eek:.

Like the alarm it has a limited life and when it malfunctions it will make your life a misery from false alarms and "bonnet open" warnings.

You can see it under the passenger side latch, as a shiny bit of metal perpendicular to the latch finger. Don't be misled, though, this is no electrical breaker circuit you're dealing with; the switch itself is under a pin at one end of the metal clip but the pin is plastic and spring-loaded, in a green plastic sleeve. The metal is just there for its flexibility, NOT its conductivity :rolleyes:; so you can shut the bonnet with the prescribed violence and the latch will just lightly bear on the switch, which should operate with an audible "click". If it doesn't you need a new one :(. On mine, not only had the switch lost its mojo but when I peeled off part of the cover the terminals were corroded, too :mad:.

Of course, if you are too violent in closing the hood you can actually knock the entire switch unit out of its plastic retainers to get a "bonnet open" warning. Then, however, you need only endeavour to click it back in place.

Either way, you are probably going to have to remove the two 10mm bolts retaining the latch.

It would be nice to pull the plug (with the two wires coming out) off the switch, beforehand but this is going to be tricky, even if you know where to find it, because the plastic retaining clip is under the plug and space is at a premium.

So, just wiggle the whole latch free, being careful of the switch wires and noting the position of the dreaded cable grommet and its retaining slot :devil: - you want to get those back correctly. You can use the other latch as a reference for this - it's a model of simplicity and ease of access, by contrast :crazy:.

You want Mercedes part number: A 203 821 92 51.

I tried to get this from Swindon Mercedes but they said it was going to cost £35 odd plus VAT and would take five days to obtain from Germany:wallbash:. What's more they said it was company policy not to give out part numbers (I can certainly see why ;)).

So, I called Bath Mercedes and got one for £10.08 that afternoon after a 90 minute drive (thanks to Steve from Parts doing 10 minutes overtime on his Saturday! :thumb:).

Strictly, you should probably remove the negative lead from the battery before changing the switch but, being as mine had evidently been beating out an intermittent SOS for some time anyway, I omitted this step :rolleyes:.

Replacement is the reverse of removal, as they say. It is easier to put the plug into the switch at the very last. Before you replace the screws, though, ensure that the grommet is fully seated in its slot or the latch may not open again :devil:.

All's well that ends well - but, of course, DO use a jack stand as well as a jack to work on the wheel arch AND do just check that the bonnet latches respond to the release BEFORE you finally drop the lid from the prescribed 12 inches...
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Excellent and helpful for me as future reference material.

Best part: "grope around for a stiff cable" :D

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