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Old 11-12-2002, 08:14 AM
stevebfl stevebfl is offline
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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