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Old 11-09-2002, 10:18 AM
Q Q is offline
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Join Date: Aug 2000
Posts: 839
Wow, $690!!

I had the same problem on my 380SE just recently. Per the manual's instructions and based on tips from people here, I pulled the relay and jumpered the pins that turn the pump on. Pump ran fine. Figured next step was to replace the relay. Before I ordered one, I took the cover off the "bad" one. Sure enough, there were three broken solder joints on the large (of the two) relay. 5 minutes with the $10 Radio Shack soldering iron was all it took. Popped it back in and it has run fine ever since. Best of all, it was free.
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Old 11-09-2002, 11:16 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Part of the problem with the dual pump system is that its very hard to pick one pump that is worse than the other. One would think that one pump should handle it but I have seen enough problems with the pumps that we always replace them as a pair. There might be testing that could allow otherwise but it would cost a significant share of the price of the one possible pump one could save.

Restricting flow is not going to cause excessive current as the pump self regulate (pressure relief) at some pressure.

Most causes of high current are electrical problems in the armature or connections or a mechnically locked pump (bearings siezing).
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 11-11-2002, 11:42 PM
Posts: n/a
thanks for all the help, but...

"Most causes of high current are electrical problems in the armature or connections or a mechnically locked pump (bearings siezing)."

Uh...could you explain that?

BTW, I have no reason to believe that Raul was trying to be dishonest. Reember, the car ran fine for three days before it stopped. If he was trying to jack me, he could've done day one.

Another thing. Prior to the repair, I had noticed that the exhaust seemed more noxious than normal. Didn't really think much of it since it is an old car. It does seems to be, so far, much less noxious after the replacement.

I noticed something different though. Previously, whenever I would get gas, when I took off the gas cap, I would hear a hiss. I understood that this was normal. Got gas today (first time since out of the shop) and nothing. Should I be alarmed by this?

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Old 11-12-2002, 08:14 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Refering to current used by fuel pumps. It was mentioned that a restriction could cause high current. MB pumps run at an average of 6-8 amps. If you deadhead them they will raise the current by maybe 10-20% before an internal bypass regulates the affair.

If one really wishes to look at the current it can be graphed over time with a sensitive inductive amprobe. The current flow through each segment of the armature can be analyzed and by knowing how many segments are in each armature the RPM of the motor can be calculated. Very easy with a good scope/graphing multimeter.

Each segment of the armature is a series of windings. The current flowing is determined by the accumulated resistance of the wiring, connections, brushes and most important the windings. Depending on design a winding will overheat if continuously powered. This will break down the insulation between windings and reduce the effective length of wire making the windings causing a short (or shorter circuit).

A few shorted windings will look like a small increase in total amp reading if using a gauge which is an average. Using the scope one can see the instantaneous current of individual segments.

If the bearings seize and the armature can't turn (on one pump say), the armature eventually will overheat and short out increasing current. Lots of possibilities for high current but a plugged filter/system is handled by the design of the pump.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 11-12-2002, 10:23 AM
Thomaspin's Avatar
Join Date: Jun 2001
Location: CA
Posts: 531
Relays are magic - like electricity

I have learned from the many postings on this board that fuel pump relays are not always responsive to logic.

I had similar problems to yours on my 1990 560SEL. I took the cover off the relay and searched hard with a magnifying glass for cracks and could find none. After the problem resurfaced I sprung for a new relay and it's been fine since. About $110.

As a back-up, I carry the old relay in the trunk and also a 4 inch piece of stout insulated wire. In a pinch, you remove the relay and insert the wire in terminals 7 and 8 on the relay socket. (The #s are on the socket, not on the relay). The pump will run if it is OK and the engine will start, though you will lose the rev limiter which, I understand, is part of the relay, so go easy on the gas. The pump will continue to run as long as your jumper wire is in place, so remove it once you have limped home.

If you place your ammeter between terminals 7 & 8 you will not only hear the pump(s) run, but will also see the current draw. Mine draws 6.8 amps, which appears within spec per Steve's email above.

BTW, you will see two relays at the firewall on the driver's side - the fuel pump relay is the one that is NOT marked 'Klima' - that one is for the climate control. Also, the positions of the two relays were switched (in 1988 or so - can't swear to the date, it's in the Electrical Troubleshooting Manual) so it's not enough to say 'it's the one on the left').
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