73 450SLC Fuel System

[Pendragon]

Fuel Pump not operating.
No voltage to pump.
Traced wiring back to fuse block.
No voltage to fuse #14 (Master Relay/Fuel Feed Pump).

That is the basics of what I am looking at. I just aquired this car for my wife. Was in the process of getting all of the mechanical in shape before starting in on cosmetics.

Took the steering wheel off last night to replace some light bulbs in the instrument cluster. Put everything back on last night, but did not attempt to start.

This morning, went to start car and no fuel pump noise when key turned to run position. Started tracing problem at that point.

Ordered CD Rom manual from MB this morning. Will take seven to ten days for delivery. Was hoping someone could help me with this in the mean time.

Advice on troubleshooting from the ignition switch to the fuse block would indeed be helpful. A copy of the wiring diagram would be great.

Respectfully,

Bob Weaver
Avery, Texas

[Tomguy]

Not sure how it is on the 107 chassis, but the 108's have a smaller wire on the (+) terminal of the battery for the fuel pump relay. If it's supposed to be there on the 107s, make sure it's on the battery and tight.

[230/8]

Geeez, this is like Yogi Berra and the "de ja vu all over again" thing...

I just got my 107 back from the shop for this same issue. Ran fine for sevaral years and then would not start in the parts store parking lot. I replaced the fuel pump relay (always carry a spare), no dice...replaced its fuse, no dice...finally admitted defeat becasue it was 95+ degrees and I was out of ideas ana patience. Embarrasing to see your SL leave on a tow-truck hook.

My trusted indy began to sort through the system and found a possible problem in the fuel pump wiring...PO had spliced in a kill switch to prevent theft. Fixed that wire but it still died when warmed up. The fault seemed to be related to temperature somehow...died when hot, cooled down a bit and would restart (but would not restart that very hot day in the parking lot.)

My mechanic noticed the main power fuse was an old replacement fuse, aluminum type. He replaced it with a new fuse, copper conducter, and no problem since.

Maybe your problem is in the fuses. I bought a stock pile of good quality brass/copper 8, 16, and 25 amp fuses and will replace all of the bogus el-cheapo replacement fuses tonight in both my cars.

By all means check the battery connection. There is a main power lead from the + terminal on the 107s; my indy looked there first to confirm it was solid.

Good luck with solving your problem.

230/8

[Pendragon]

Cleaned the connections at the battery.
Seemed to have restored power to the #14 fuse.
Still have no power to the pump itself.

Where is the Fuel Pump Relay located?

Once again, thank you guys very much for this help.

BTW Tomguy, My dad purchased a 280SE new in 72. Burgandy with black interior if my memory serves me correct. It came with the Burl (if spelled correctly) dash and console. Ran for 310K, and then was traded in for a 300SD.
Good luck with the restoration on that machine.

Respectfully,

Bob Weaver
Avery, Texas

[230/8]

Bob:

Your fuel pump relay should be located in the ice cube stack that is above the fuse holder in the passenger-side footwell. Remove the fuse cover door and you can just barely see the relays located above the fuses. You ned to carefully remove the kick panel to get access to the relays. Relay number 1 is for the fuel pump. Relay number 1 is the top left relay in the stack. There should be about 8-10 relays in all, four or five in a vertical stack on the left and another bunch stacked on the right. They are numbered in the harness, but generally the left ones are 1, 3, 5, and 7, from top to bottom, and the right column of relays is 2,4,6, 8. You may encounter another one or two relays mounted on a bracket screwed to the bulkhead so that they are positioned atop the main bunch of relays. If so, you will need to carefully remove the bracket to access the fuel pump relay, unless you have really small and nimble fingers.

FYI, your power window relay is likely mounted on the bracket in position 9 and the sunroof relay may be next to it in position 10. This info can be found in the electrical troubleshooting section for the 74 model, so it may not be 100% true for your 73, but it has been accurate so far for my 72SL.

The wiring information for the 72-73 cars is, IMHO, a weak link in the information chain. I suppose good mechanics have gone to MB schools and learned the ins and outs, but us poor folks who depend on manuals can feel like red-headed step-children at times.

You might want to pick up a CD ROM manual for your car from Mercedes Benz. They are pretty cheap from this factory site:

http://www.classictechlit.mbusa.com/MB/

Good luck with your project.

230/8

[Pendragon]

230/8,

Thank you for the information.

I have ordered the manual from MB as noted in my first post, but would really like to get this running within the ten days they quoted for delivery of ROM.

I will have a look at the relays in the morning and let you know what the results are.

Once again, thank you all for your very informative information and help with this.

Respectfully,

Bob Weaver
Avery, Texas

[nhdoc]

If your engine is like the 4.5 in my '72 108, and I suspect it is, the fuel pump is only actuated for 1 second after the key is turned to run...are you checking for voltage right after turning the key?

After that power is off to the pump until the engine turns more than 100 RPM. This is to prevent flooding the engine if a fuel injector is faulty.

So, you may not have a problem other than the fuel pump itself...just a thought.

[Pendragon]

Marty,

Thanks for the information. I did not know that there was a timing issue regarding the pump.

I should have provided more information about my troubleshooting the problem.

When I first noticed that the pump did not turn on when turning the key to the run position, I disconnected the pump wires, then ran two leads to the passenger side of the car and attached my voltage meter to the leads. Then removed the key, waited a couple of seconds, inserted the key then turned to the run position. There was no voltage displayed. The next step was to check the same at the fuse location. Same result. No voltage.

I then took a second battery I keep for the Shovelhead and with the rear end lifted and blocked, checked to see if the pump was functioning. Not having a wiring diagram available, I looked at the rubber isolators on the pump/filter brackets and determined that the two wires had to be a hot and a ground since no indication of grounding to the frame was evident. Using the smaller terminal for negative, I connected the negative side with aligator clip and for a moment touched the positive lead to the larger terminal. The pump seemed to spin without a problem.

That is where I am now, looking at some problem related to the relay that 230/8 has directed me to. I am assuming that the one second time limiter for start up you indicate would be included in the relay. At least I hope so, or else there is something else I am going to have to track down.

Thanks for the help

Bob Weaver
Avery, Texas

[GaryF]

Hi Bob,

I tried attaching the PDF from the CD buts its too big, if you want to drop me a mail I'll send it to you if you havnt already received your CD.
The fuel pump relay is controlled by the ECU and this determines whether the engine is running or not and cuts power after a short time. From what I can make out it looks like pin 19 of the ECU connector drives this relay (pin 5) and checking this continuity might be a good place to start. Of course it could be the trigger points or other signals missing into the ECU but eliminate the easy stuff first right?

Gary

[nhdoc]

It sounds like you've pretty much eliminated the pump as the problem and have narrowed it to the relay, ECU or wiring...I would check the relay first. Sometimes just unplugging them and giving them a tap will get them to work again, at least temporarily. Some people have had look repairing them by removing the covers and looking for broken wires...they are pretty simple.

[Pendragon]

Problem identified and repaired.

I would like to express my thanks to all who assisted me in this adventure.

From the information that 230/8 provided me about the location of the relays, I was able to pull the relay, and looking at the diagram on top of the relay, able to test and verify that it was operating correctly.

Once that was eliminated as a problem, using the wiring diagram that GaryF provided me, I was able to trace the wires from the relay box to the fuel pump. While tracing the wires under the dash leading to the kick plate on the door jam, the meter started giving sporatic readings. Pulled the plate and found that one of the wires had a break. Repaired the break and the system has come alive.

Once again, I would like to thank all that helped with this problem, with a special thanks to 230/8 and GaryF.

Respectfully,

Bob Weaver
Avery, Texas