Ok, I know the subject of W126 fuel pumps, and fuel pump relays have been beat to death, and I have searched through all of the related threads I think. That said I'm still a bit confused about a few things.
What I'm trying to do is diagnose an intermittent "ghost" problem with the fuel and ignition system on an 88' 300SEL.
I'm starting to suspect the pump itself as the main culprit, but I want to be sure before I start throwing money at it.

Recently I've noticed that the fuel pump will get noticably louder from time to time which sometimes results in engine shutdown. The fuel pressure is good, so is the volume, and the accumulator holds. The problem is so intermittent though that I feel those tests are pretty much useless.
So I came up with the idea to connect a DVM to the X36 connector under the passenger kick panel. This way I could at least see what the voltage was going to the pump when the problem reared it's head.
Well, turns out the voltage is constant when the problem occurs (13.2-13.5V). This seems to rule out the relay. I would think that if the relay was a problem then I would see large changes in voltage.
Am I right in thinking that?
I guess a better test for the pump would be to monitor the actual fuel pressure, and flow. But I haven't come up with a practical means of doing this on the road. Any thoughts?

The wiring diagram shows two fuel pumps, but as best I can tell this car only has one fuel pump. Seems like I remember reading that certain years had two pumps?? If this car does have two pumps where is the other located?
The only one I've found is hanging directly under the fuel filter inside from the right rear wheel.

Thanks in advance for any help

I've seen this before. Was caused by severe corrosion at the electrical terminals on the pump itself. One pump, the terminal post was actually corroded right off, that pump was inop, the other only slightly better and cut out occasionally. Need to look under the rubber 90 degree boots over the terminals.
Gilly

__________________
Click here to see the items I have up for auction at EBay

Click here to see a photo album of my '62 Sprite Project Moneypit (Now Sold)

I've checked the terminals, and they are in great condition. I even checked and cleaned all my ground wire locations including the one for the fuel pump located in the trunk.

See my signature for model info - somewhat similar to yours.

It has one pump. From what I've seen in the MB repair doc, a lot of the 103 motors in the 300Es have 2 pumps and maybe the 420/560 V-8s - not sure about the latter.

My fuel pump, accumulator; fuel filter are all located next to one another. If you have 2 pumps, they will be next to one another. I'd bet a buck you have one.

I replaced my entire fuel delivery system at the rear of the car earlier this year. Here are a list of parts you may wish to consider obtaining beforehand should you end up replacing the fuel pump. Chances are they're either original or very old. This is not a tough job, but not one you want to do a second time.

Here's the parts list:

4 fuel hoses changed

Delivery Fuel Hose - 22mm attachment - 126 470 16 75
Return Fuel Hose - 19 mm attachment - 126 470 41 75
Fuel pump To Elbow Hose - 123 476 30 26
Accumuator To Elbow Hose - 126 470 40 75 - 5.5mm - only dealer has it

Fuel Tank Screen - 46mm or 1 13/16" socket will break this loose - part number - 201 470 05 06

Fuel Tank Screen Gasket - 110 997 01 45

Fuel Pump - Pierburg - 7.21659.52 - There's also a Bosch Pump - I Used Peirburg because that what the car came with.

Best to drive car 'til it's as close to empty as you can get it. Even after doing so, you're going to drain 2.5 - 3.0 gallons of gas when you pull the delivery line off the fuel pump. I first pulled grn. strap off battery and laid an old sheet on the ground in the work area to help prevent sparks that might occur should a hand tool get dropped accidentally on the concrete surface. If you create any spark while you're doing this, it will likely be you last DIY job. Easiest to remove the pump, accumulator; filter as a unit. You can do this by removing all required hoses first, then a couple of 8mm nuts that fasten this 3 part assembly to the car. Afterwards you can remove each part individually on a bench.

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

Thanks for taking the time to post all the details, Mike.
I didn't realize that the tank had a filter screen in it.
It sounds like the fuel system in yours is the same as mine. I was working on it this evening and noticed the accumulator mounted slightly above and forward of the filter. single pump (Bosch)

Thanks again

The larger delivery hose that connects to the fuel pump screws into the fuel tank screen which screws into the fuel tank. Once this hose is off, you can loosen the fuel tank screen with either a 46mm socket(baumtools.com) or a Sears 1 13/16" socket. The Baum tool is very pricey, but has a short profile which allows better access.

Others here have reported success using a Craftsman 1 13/16" socket.

I suspect you'll have no porblems changing out any of this if it is indeed the way you have to go.

Hope it works out for you.

By the way, my situation was as follows:

Get in car and drive approx. 15 mi. on highway. Car starts bucking intermittently, then dies. This ALWAYS happened only when it was hot outside(high 80s or better). I would have to let the car sit for 45 mins to an hr. B4 it would start. The first time this happened it was close to dark back in late March. It had been especially warm that day and was hot on the way home(high 80s). I had to walk home - 3 miles after the stall. By the time I got back to the car, the outside temp had cooled and I started and drove the 3 mi. to my house and just as I pulled in the driveway, it died.

A few days later, a cold front(tail end of winter) blew in and the car was fine during this cool spell. A few days later, the warm-hot weather was back and so was the bucking/dying out after about 15 mi. of driving.

One one of the many occasions this occurred, I jumped the fuel pump relay pins in my MAS unit(pins 1 & 2) and the car would not start, so I knew it was not the MAS. I had replaced that the previous year.

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

That's interesting. My situation is very similar in that hot weather makes the problem worse, it rarely dies in the winter time.
Mine has yet to leave me side of the road, as it will always restart although very difficult at times. What has been so frustrating for me is the intermittent nature of the problem. On the way home from work the other day it stalled probably 25 times on a 8 mile trip. Next day I drive 20 miles without it missing a beat.
Back in April I drove it to Savannah GA. (about 90 miles from here) and it ran perfectly all the way down there. As misfortune would have it, it started cutting out when I hit the downtown area in heavy traffic. NOT a fun trip. On the trip back it eventually got to running better, so when I get back home everything checks out good.

Lately the problem has become less and less intermittent so hopefully I'll be able to find the problem(s).