Hi Guys not sure if your 86 300e is simular in electronics as my e220 but i was haveing simular problems until i replaced the OVP relay behind the battery, problem solved . cheers baz

Special thanks to Dave Poole for offering his time to discuss this issue on the phone and to Mike Murrell for suggesting to check the ignition switch.

What I have found is I am not always getting a reading between pins 9 and 11 on the fuel pump relay. I turned the ignition switch on and left my meter to pins 9 and 11 and after about a minute voltage was detected. Wiggling the key in the switch or turning off and on doesn't have an effect but what is unusual is a reading will appear without doing anything but leaving it in the "on" position. One thing I tried was looking for voltage and when it appeared I removed my meter and replaced the fp relay but the car would still not start. At this time I am suspecting the ignition switch and will continue troubleshooting.

Thanks again to all who have contibuted their suggestions to this post.

Sorry but your problem got to me. Can you do one thing for me? Try hot wiring 11 to ground and see if it helps.

Not sure if this has anything to do with my issue but here it is. When I turn the ignition key to "on" there is a buzzing noise coming from behind the glove box, it is not very loud and can only be heard when the ignition is on. The noise is definitely not coming from under the hood.

OK...I'll take a shot at this one. Likely a "distant shot", but maybe worth thinking about.

Alarm.

You say behind the glove box. On a 126 and I'll bet on the 124 as well, the stock alarm system is beneath the passenger side foot well. That's "beneath" the glove box...sorta. Noises and their point of origin can be confusing.

Could this be the alarm controller yacking at you?

This suggestion may not be valid if the alarm system is the original factory unit. I want to say that the factory system would not stop the car from starting, but simply sounded an alarm in the event of dampering. Someone correct me if I'm wrong.

If your alarm is after-market, there's a much stronger possibility it could be involved here.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Well I decided to take a break from my electrical problem and find out what the mysterious noise coming from the glove box area. I removed the glove box and the vent duct to the passenger dash vent.
What I found was a little fan located directly under the passenger vent but not connected to the vent. I was able to see it with a small mirror and it runs when the key switch is on. There is a grill opening under the dash which allows air to/from the passenger footwell. So what does the little fan do? Circulate air under the dash?
By the way, my car is completely stock with the factory alarm system.

There have been many posts here that discuss this "little fan". Here's pointer to one.

Sheeesh it's hot!

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

I still vote for the grounding problem

After hours of troubleshooting and finding nothing wrong I had a friend tow my car to a local foreign car specialist. I couldn't get the car to start even with the fuel pump jumpered and it appeared it was getting good spark and fuel delivery.

The indy checked fuel flow and ran his diagnostics. I received a call today informing me the problem was distributor cap and rotor. This was a surprise to me as the cap and rotor were replaced about 10k miles ago. The car is still at the shop and I will provide an update when I get it back.

The car had a weak and intermittant spark. Since the cap and rotor were not that old and my timing light indicated a signal I ruled it out while troubleshooting. Now I know a weak spark will cause a no start situation, you live and learn.
Next is to troubleshoot the a/c and replace the water pump. The a/c has a good charge of R12 but the compressor has a mind of its own. My water pump is not leaking but the bearing in it is noisy and slinging grease.

Many thanks to all who contributed their advice as I learned much more about my car.