[W126 Coupe] 380 died :-( Thank you

[Gerry Van Zandt]

WIth a bad alternator or voltage regulator, or a bad lower harness, or OVP, the dash will light up.  The fact that the dash did not light up, as well as your battery having an adequate voltage, tells me that it is not charging-related.  Rather it is something that is ignition or spark-related, caused by a single point of failure.

A bad fuel pump relay is definitely a single point of failure, and it is something that would cause a running engine to die (and not able to restart).  Often the relays heat up and then they work OK for a short time after they cool down (until they heat up again and conk out).  It is not a bad idea to listen for your fuel pump(s) engaging (you'll hear the whine behind and under the passenger side of the car) with the key in position 1 and if you don't, I'd tap them with a rock or a ball-peen hammer to see if you can start the pump.  Other than that, the relay could be a culprit. But I don't think that's the problem.

The fact that you're hearing clicking when you turn the key, but the starter is not firing, clearly indicates (to me) it's something electrical in between the key and the starter.  The single points of failure here really are the solenoid, the neutral safety switch and the ignition control unit.  A bad solenoid is not going to make the car conk out while it's running, becuase it's only used when the starter is activated.  

There is an inexpensive rotary switch that sits behind the key, but that is only activated when the key is turned (to start the car) not while the car is running -- it goes back to position 1 (running) after starting in position 2. Since you're hearing clicking when you try to start it, that tells me that a signal is being sent from position 2, but it's being interrupted so it can't activate the starter.

Have you considered taking a short wire and jumping the contacts on the solenoid (bypassing the ignition/key system), with the key in position 1 (running) to see if you can activate the starter and start the car?  That will tell you if your starter/solenoid is bad or not.  

In the end, I think you're going to find that your ingition control unit is bad or your fuel pump relay is bad, which are really the only two components that could both kill the car while it's running, as well as prevent starting subsequently.

Cheers,
Gerry

P.S. Your radio/antenna conking out I don't think is related, but it sounds like it could have a ground problem.  As a separate action, I'd check all of your ground points for proper contact and no corrosion.  The service manual shows all of the key ground points on the car, but the ones you'll want to check are under hood, a couple in the center console, and a big one behind the instrument cluster 


On Nov 6, 2011, at 11:15 AM, mbcoupes-request at mbcoupes.com wrote:


> In response to Gerry below, I have not checked all the fuses yet, but 

> will do that today.

> 

> The dash did NOT light up.  The only unusual behavior c. 7 minutes prior 

> to the car conking out was that the CD player cut out, then came on 

> again (several times), and the aerial retracted.  I turned the CD it off.

> 

> Re the ignition control box:  An auto electrician I met at the autofest 

> yesterday also suggested that as a possibility.  He also suggested that 

> I check the fuel pump relay.

> 

> I'll  let you know the results - it may be a couple of days.

> 

> BTW, I used to have a 72 VW beetle that had a habit of dying when 

> running. The problem was finally diagnosed as alternator.  The symptoms 

> are not quite the same as in the Merc - the VW's dash warning lights did 

> come on.

> 

> Once again, thanks to all.

> 

> Ingrid

> Cooma, Australia

> 380SEC


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