1979 Mercedes 200D W123 - mojo risin'

Sadly the 200D is right at number 1 on my list of cars to have,check out "Mobile.De"here tinyurl.com/ca66dyy
It's the german autotrader click the union jack for english and save searches and individual cars.Always loads of w123 models of all types and budgets.I didn't realise the 200D actually sold here, I have an english brochure and price list from '81 and the 200d isn't listed,good luck .

Heater plugs must have been working last week as these engines absolutely will not start without heat no matter how healthy they are

what kind of mpg have you been getting? good luck fixing the lastest issue

Clubbymatt - I haven't done any calculations but I would say about 40mpg for mixed driving.
ChasR - yeah hopefully it'll be an easy fix... it shouldn't be too complicated since there's nothing complicated about it!

After a bit of research, and thinking about it logically, I discovered that the garage have made a right balls-up of the wiring. For loop-type plugs to work, the live should go into one terminal, down the plug, through the loop (consequently heating it up), back out the plug and into the other terminal (which is wired to the next plug, and so on). With the wiring as it stood, in parallel, the current would take the path of least resistance and not go down the plugs at all!

The glowing wiring is apparently normal for these engines - the fact that the first and middle wires were glowing must've been merely because of a poor connection, forcing the current into the plugs and heating them. So it was only ever starting on the first two plugs, hence the poor starting when cold (until the weather warmed up).

So, I re-wired it in series as it should've been, but still they didn't come on. I checked the input for the first plug and it showed 26 volts, but the output showed nothing. I suppose this means the plug has fried itself, hence it not starting. I'll replace the plugs tomorrow and re-wire it properly, and it should be fine.

I'm also unsure as to how to approach the garage about this. They've clearly made a mistake but I'm unwilling to let them work on it again (I spoke to them about the earlier poor cold-start issue after their work, but they fobbed me off and then the weather picked up so it started fine).

Wondering how I held the starter in the glow position whilst simultaneously going round the plugs with a multimeter? No, not a glamorous assistant, but a rather un-glamorous pair of old fanbelts:

Like a boss.

Yep, I only did it because I was short on time - I had to get the car driveable before I moved house! In hindsight, I'd rather have paid a transporter to move it and worked on it myself. Finding a mechanic that takes old cars seriously seems to be difficult.

Yup. I measured voltage on the input and output of the first plug - input had voltage (voltmeter showing 26 volts from a 12v battery, not sure how...), output didn't show anything - I guess this plug is goosed! So I bypassed it and wired the remaining three plugs in series...

Pulled the starter and the glowplug light came on, for a good 20 seconds, rather than 2-3 seconds like it did before. Checked voltage at the three good plugs - good... then tentatively pulled the button to the next notch and it started up straight away!

I've ordered a new set of proper Bosch glowplugs and I'll be visiting the garage tomorrow and asking for a refund on the labour and a replacement first glow plug. Pretty disappointed, especially as I've already been back there once to ask why it was taking 10 seconds of cranking to start.

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No need to bump the thread, but I took it back to the garage this morning. The mechanic did initially try to fob me off and blame the poor starting on something else, but as soon as I opened my mouth I think he realised that I knew more about the wiring than he did. Refund on the labour + replacement for the blown plug = happy with that.