Fuel Distributor Pump Overhaul

[n/a]

Hi,

I'm looking for advice from anyone who has worked on the LATER, smaller fuel distributor pump (ie the pump at the rear adjacent to the fuel tank). I have a 1967 250SL RHD.

My pump was leaking at the point of the electrical contacts! I drained the tank, removed the protective cage and pump from the car. And I have replaced the perished seals with a pair from a local seal supplier (it appears that the full set of seals(6) are not available from Bosch, MB, SLS, etc. so hence I tried to get the closest match.). Hardest part above was finding seals that were a close match.

All sounds OK so far...

But when I did a bench test before fitting it to the car I found that the pump impeller is either not spinning or is doing so erratically. And I note the pump was running fine before the leak!

Testing without the impeller in place (ie part disassembled) yields mixed results - Yes the shaft from the motor always spins when receiving current, so presumably the electics are OK, BUT the speed of the shaft (the RPMs) is not consistent sometimes it is fast, other times about half the speed of revolution.

Wondering what may be my source of difficulty? Anyone have any ideas???

I note all the above tests were with the pump dry - is my problem simply that the pump needs to run wet?

Also since it appears that the impeller disk is meeting resistance I wanted to confirm that that there are indeed no washers, etc. in the area where the impeller wheel is attached to the shaft at the bottom of the pump? I'm pretty certain that I encountered none when I opened it up, but oh when doubts creep in.....

Appreciate any guidance anyone can offer.

Martin

1967 250SL RHD Auto

[George Davis]

Martin,

it's been a while since I messed around with my fuel pump, but here are a couple of thoughts:

I don't recall if there are any washers/shims/etc. above the impeller, but I suspect not since the impeller just "floats" on the shaft.  I believe there were some washers/shims between the armature and the lower armature bearing plate.

I don't really know what might be causing the speed fluctuations when you test it.  My first guess would be that the brushes aren't making good contact.  I inadvertently ran my pump dry for many seconds (before I figured out it needed to be bled), and it seemed to run fine at steady speed while dry.

The pump needs to have the air bled out of the impeller housing before it will pump fuel.  If your car has the third small bypass hose, it should self-bleed.  If it doesn't have the bypass hose, there will be a small screw in the end of the third (unused) fitting.  After installing, loosen the screw and let the air bleed out until you get fuel from it, then retighten and go.  For bench testing, you may need some sort of gravity feed arrangement to simulate the setup in the car.

Last, if it still leaks, Permatex No. 2 works well to seal it.

Good luck,

George Davis
'69 280 SL Euro manual

[Benz Dr.]

Strange. I've never had to bleed a pump. I always thought they would self bleed or just push the air out. This goes for all new pumps I've intalled and ones we put back in a car during a restoration.

If the pump motor runs at fifferent speed it's basically had it or the brushes are worn. I simpliy refuse to play around with stuff like that. New pumps are available and it's what I'd install. Most of the time they continue to leak or start to leak after you play with them.

Since a customer expects his/her car to run properly after I work on it I can't waste time on electrical componets that might fail in a few months. A new pump will last over 100,000 miles or 25 to 30 years of service - maybe more. This gives me peace of mind because fuel volume is VERY critical to how well the engine runs.
It may cost more and some people have been able to fix their own pumps but it's not for me. My call back rates are very low because I try and do everything right the first time. We rebuild and fix a lot of other things - not this part though.

Daniel G Caron

[gugel]

quote:Originally posted by Benz Dr.

I simpliy refuse to play around with stuff like that. New pumps are available and it's what I'd install. Most of the time they continue to leak or start to leak after you play with them.

I bought a new Mercedes pump for my '65 230SL for just this reason and it works fine.  However, the old protective housing doesn't fit over the new pump -- the outlets are in different places, and the bottom of the new pump is a little lower than the old.  I tried a new housing, also from Mercedes, but that didn't fit either.  Is there a new housing available that does fit over the new pump?  Or an easy way to modify the old one to fit?

Chris Earnest

[George Des]

Martin,

I've redone several of these pumps and there are some things you need to watch out for. First, it is possible to mount the can incorrectly which will cause the pump to run backwards. Second, it is quite possible to mount the can so it is cocked in position which will put pressure on the armature which will in turn cause it to revolve erratically. This is easy enough to check by trying to manually rotate the shaft before you mount it on to the bottom portion of the pump. It should rotate fairly easily. As a further test you can connect it to a battery charger or other 12 volt source to see how it spins. If the can is cocked try disassembling it and when you tighten up the two nuts that hold the can in place, try alternating between them so they tighten up evenly and draw the can and the rubber seal in straight. There are several very thin shims between the shaft e-clip and the bearing surface on both the brush side of the armature and the impeller side, but these are in board of the e-clip only. You may have to play around with these to get the right combination that will not bind up the shaft. Unlike the older large style pump, these are "wet" pumps and are designed to have the can filled with fuel while running i.e. the armature runs surrounded with fuel. This cools, lubricates and cleans the internals of the pump. Unlike the older pump, running this style pump dry won't burn up any shaft seals but it may cause your impeller to gouge the flat bottom plate to the point where it will compromise the pressure output. An incorrectly sized bottom seal i.e. too thin may also cause the impeller to bind. These pumps are much easier to rebuild than the older pump--no ball bearings, no main bellows seal and considerably less screws to break off because they become "welded" in place after years of use. Personally, though I think the older pump is a tribute to true German and MB engineering and find rebuilding them a real challenge. Hope the info helps.

George Des

[George Des]

Dan,
I have to agree with you, if I were in this for a business, I'd just go with installing a new part. The chance of installing a used or rebuild part that later fails is just not worth the risk and aggravation and is a sure way to drive customers away. I do these rebuilds strictly as a hobbyist.

George Des

[Benz Dr.]

Oh, you can rebuild them. I might replace one a year so it probably wouldn't pay me to start doing them too. Distributors are hard enough to do but they can't be replaced with similar new units in most cases so rebuilding is a viable option.
Kind of like water pumps - you can rebuild those too but why bother, they're cheap. Now if you have a 6.3 then maybe a rebuild is a good idea.

 Nice thig about these cars is that almost everything can be rebuilt.

Daniel G Caron

[n/a]

Thanks for the comments guys.

Thanks George for confirming that there are no shims near the impeller - good to get confirmation on that.

I'm confident that the magnetised can is mounted correctly because I marked it before removing it (there is always a bottle of TippEx nearby when I'm stripping things!).

I hadn't replaced the brushes - because the pump was running correctly - but I will check to confirm that they are still correctly seated after having had them off.

Thanks for the advice re. air locks, I'm not at that stage yet but I'm sure that advice will come in handy. My tests have been with the pump DRY - which may be a source of the problem.

I agree with Dr Benz that replacement of this part would be a simple and reliable option but this comes at a fairly high cost.
The correct replacement pump is available from various sources (inc. Bosch themselves) but at a cost of c. 400 Euros.
So far the seals have cost me 4 Euro, but of course I am not yet running so too early to tell.

Of course replacing the pump with a later model is also an option, and indeed if I fail to repair my current pump that is the most likely option I will take. Guessing that can be done for 100-150 Euro but this is not without the complications of finding the right spec. of pump and modifying the cage and protective housing at the rear of the car as Chris is commenting on.
Chris take a look at this posting which may help:
http://index.php?topic=1401,pump[/url]

Martin

1967 250SL RHD Auto