420SEL Suddenly Stalls

[London380sl]

I have a 1988 420SEL with 130,000 miles on it. Recently the car has developed a nasty habit of suddenly stalling. You'll be driving along at a slow speed and suddenly, as if somebody flicked a switch, the engine will die. There is no stumbling or other warning. After about 5-10 seconds of cranking the car will start again and run with no problems. The engine is warm when this happens.

Apart from this issue the engine runs perfectly. Any ideas?

[London380sl]

Well ...reading from prior posts its sounds like it could be either a fuel pump relay or the overload protector relay - both are a bit pricey. Is there anyway of checking these parts other than "plug and pray" I've choosen the right one.

[davestlouis]

I tried all sorts of things when my 86 420SEL did the same thing...after spending a fortune at the dealer and throwing parts at it, I broke down and put new spark plugs on it, and lo and behold, found a corroded spark plug wire that was apparently intermittently cutting spark to one cylinder. At low RPM that was enough to make it stall. It runs like a top now, 1 year later.

[ABSCO]

Please where was the corrosion located? i have same problem and am unable to replicate stalling when thrashing those same plug wires at idle.
Thanks
Dave (AMCO)

[hey_allen]

I'm not sure of which pins off the top of my head, but you could do the fuel relay test with a piece of wire substituted for the relay, jumping the connections to the fuel pump. Make sure you remove the wire when you're done though.

If that is the faulty component, you can usually pop it open and see obvious connection failures on the back of the circuit board around the high current pins serving the pump. Just re-solder them and give it another try before paying for a new one.

I don't know if this is what Dave experienced, but on my SEC I had one spark plug wire not get seated fully on the plug and it developed a heavy layer of some white corrosion in the clip where it should have been contacting the plug. It measured at hundreds of times the resistance of the other wires when tested with a multimeter, as well.

[mak]

yes it seems classic symptoms of a failing fuel pump relay. in my car i initially mistook it for a ignition failure ,such was the abrupt cut off ,but engine always restarted.
Best to replace, as ageing has taken its toll, re-soldering works for sometime
mak

[Randy]

When my 1986 420 did this, it was the crankshaft position sensor. It's on the upper driver side of the bell housing, as I recall; a green wire goes form the ignition module on the driver's-side inner fender. It wasn't that bad to replace, but I recall having to do it mostly by feel. Anyway, replacing it cured my problem.
While you're at it: there is white silicone paste underneath the ignition module, which helps carry heat away from it to the steel inner fender which acts as a heat sink. You will probably find this to be dried out when you remove the module. Get some at Radio Shack and clean the old out and replace it. It will help the life of the module.

[david s poole]

that crankshaft position sender has green wire as original but it is black when replaced.[just so you can see if it's original].