W220 (S320) Spare key wil open car but won't start engine

Gozaradio

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Hi Everyone,

First post to this forum here so I hope I can find some help.
I have a June 2000 S320, no major complaints aside from a couple of niggles which I will ask about in due course but the most important issue is that of the spare key not working (I'm paranoid about losing the main key and not being able to drive the car, especially as we are going on a long trip soon). This is the 3 button key (DAS3?)

Main key works absolutely fine, spare key will open car and will also perform global opening / closing when pointed at driver's door so I assume this to mean that the battery, radio and infra red are all in working order.

However, I can't turn the key in the ignition. I know from searching and reading here that the most likely cause is dodgy coil but I've opened up the key and it all looks in order.
I took pics in case I'm missing something:



Assuming that the coil appears to be in tact, what options remain for me before forking out for a new key from MB?

Thanks in advance for any help.
 

Chas

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Full marks for trying to fix it, however I suspect you will need a trip to MB. Its about £150 for a new one
 
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television

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OK you will need to check the through holes where they can go open circuit,, put in a strand of wire through each hole and solder up both sides, you only need do the ones visible in your first picture, that is the end for the ignition part.

You can bleep them but will not always find it
 

wardpaa

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If you have a multimeter (DVM) then measure the contacts on the right hand side of the coil as shown in the photo. The other side is a dummy connection.
You should see a reading of around 0.6 ohms - nearly a short circuit.
If you have not got a DVM then go to Maplins or B&Q and spend £20 on one (money well spent as your Merc will have plenty of demand for it later).
If the coil is reading OK then start checking the through plated holes as Television says. These can go open circuit.
 
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Gozaradio

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Thanks for the quick responses folks. I was just having a nother look at the key while I was reading them and I just noticed where the problem *might* be.

If you look at the second photo, the little cube next to the coil has a dry joint and is lifting off the PCB slightly. Is this part of the induction circuit? I'm thinking it's the actual point of contact for the ignition. I was concentrating so much on looking at the coil that I didn't notice it until now!

If so, should it just be a cae of carefully soldering it back in place? Mind you, I say 'just'; it looks like a tricky little soldering job, especially on the side next to the coil.
 

television

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That is an IR diode and it is off the board, and that will stop the key from working,,now a warning, that will melt,just like that if you touch the sides with a soldering iron, if you do not have a tiny tip on the iron, just ask any TV repair man to do it for you, it only takes one second to do it right
 

HelpThisDude

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Should that not also stop the windows from opening/closing with the key too?
 

television

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Should that not also stop the windows from opening/closing with the key too?

That part of the key uses the one on the other side of the board, immediately below it
 
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Gozaradio

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That part of the key uses the one on the other side of the board, immediately below it

It's all making sense and falling into place at last - I was begining to curse MB for creating cars with just *too* much reliance on electronics when mechanical solutions will do the job just as well
I'm thankful at least that it's a physical problem that I can deal with rather than a corrupt chip or summat.

However, that said, I'm a little disapponted in the build quality of the key considering that it's something which can (and often does) get dropped. I wonder how many people have forked out for a new key when they have had a similar problem? Has the design been improved upon for newer cars?
 
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Gozaradio

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That is an IR diode and it is off the board, and that will stop the key from working,,now a warning, that will melt,just like that if you touch the sides with a soldering iron, if you do not have a tiny tip on the iron, just ask any TV repair man to do it for you, it only takes one second to do it right

Luckily, I know just the man... my father. Only problem is that he's two hours away. Thankfully, he taught me all I know about soldering so I'll slip the techs a pint at work tomorrow to let me have a little go during lunch.

This time tomorrow, I'll be posting to ask which dealer does the best deal on a new key and whether anybody has a need for a melted 2nd hand key :p
 

television

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You are lucky,,you have the latest transponder coil,, the earlier ones fall off "just like that"
 

wardpaa

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Just looked at the photos - wish i had done before and it looks pretty obvious now. Luckily its the dummy side of the coil next to the joint that needs soldering.
 
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Gozaradio

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Well, the job is done - was a tricky soldering job but in the end I soldered it back in place. Meter seemed to indicate that i hadn't killed the diode so I was hopeful - went out to the car and.... It works!

A triumph over poor design! I now have two working keys which is a relief in general.

Thanks for the help, particularly to Malcolm; the definite Keymaster when it comes to MB!
 


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