Red Dodge Diary

In my previous post I talked about the replacement of two electric fuel pumps on a GMC Jimmy I once owned, covering some of the steps I took to accomplish that job. In this post I would like to share with you the reason these two pumps failed, according to my investigation.

These pumps are quite typical of those found on most cars, and the pump I will show you in the photographs is not only used on the GMC Jimmy. It is a very simple device consisting of molded plastic parts, a small DC electric motor and the pump itself connected to the electric motor shaft. The components are fitted inside of a thin metal tube, the ends of which are roll crimped to complete a fuel pump assembly. Here is the assembled unit.

Fuel pump

As you will see in the accompanying photos, these little electric fuel pumps are not hearty, robust cast iron machines, but do seem “in general” to last a surprisingly long time.    There’s the little DC electric motor with permanent magnets, just like the ones from my radio control cars, or the one I built from a hobby kit in 1967. Pretty standard. There’s the actual pump section which is amazingly tiny, as you can see with a penny for size comparison.

Size comparison

This little unit is precision machined and corrosion resistant. There is also an impeller at the bottom to help pull fuel up and into the pump. It’s made of plastic and should last forever. The top end of the pump assembly contains the electrical connector, brushes and fuel outlet.

As I noted in my pump replacement post, I now had two dead pumps to play with, and my natural desire to know why they quit on me was in full swing.  I began by carefully peeling back the metal case starting at the pump suction end, opening it sort of like a sealed can of sardines. Here is a photo of the opened can with components laid out.

Major Sections

Here are both pumps opened up, showing how the little pump sits on the motor armature shaft, and how the plastic end cap/impeller sits on the same shaft right next to the pump segment.

Impeller view

Fuel Pump closeup

Armature Assembly

Let’s get the party started. OK, let’s get to it. The cause of failure. I’m not a chemist, or a physicist, nor do I hold a Doctorate in Basket Weaving, so when I identify “cause” of failure, that only means what I see that isn’t working. I’m not talking material yield strengths or hydraulic loads or anything like that. It’s just broke, see?   In the next photo you will see more detail of the armature, and where two plastic tabs used to be, and how they are now broken off.  Both pumps failed in the same way. These plastic tabs were part of the larger molded plastic end mounted on the armature, and their job was to drive the fuel pump.  Those tabs are very small, and I thought “TOO small”.   Perhaps four of them might be better.  So there you have it. I hope you have found this somewhat interesting, and I have to wonder if that third pump is still operating in the Jimmy. We sold it shortly after I changed that pump and bought something new.

Tabs broken off

Fuel Pump Drive Tabs