Monday, January 5, 2015

Starter Motor Overhaul

<This post is part of the 'Winter Project 2014/15 - New Engine Rebuild' project>

While I was in Stafford buying my latest engine, I took the opportunity to pick up a few other items I needed. One of which was a pre-engaged starter motor. While it did look rather dirty and there was no guarantee it would work, I thought it was worth a punt for a tenner. When I got it home and checked it, I found although the motor ran as expected, the solenoid was not activating. I could either try to fix the solenoid, or buy another.  To be honest, as it was knackered anyway, I had nothing to lose by taking it to bits...


As I have no real idea what is inside either the solenoid or the motor itself, this was something of a 'feel your way in the dark' strip down. I started by taking out the screws that hold the end cap on the solenoid. Trying to tease it away, I found that the internal wires were soldered into terminals in the cap. So using a soldering iron and a solder sucker, I desoldered the wires allowing the cap to come free.



Taking the innards out, there was nothing obvious that could be causing the problem, but what I did see was how the mechanism is supposed to work and it's as brilliant as it is simple. When you turn the key the solenoid is energised creating a very strong magnetic field, pulling a metal plunger inside the solenoid. As the plunger is pulled, it engages the pinion into the flywheel and also closes the circuit that starts the motor.



This is clever for the simple reason that unlike the earlier inertia-type starters, the pinion is not spinning at the point it engages the flywheel. This makes for a smoother meshing of the gears giving much less wear and tear at the point of impact. Anyway, as lovely as all this discovery is, it doesn't solve my problem.

Thinking about it logically, it's either an electrical problem with the solenoid windings, in which case it's scrap as it can't be opened, or it's mechanical. If it turns out to be mechanical, I might just have a chance of fixing it. Testing the continuity of the solenoid windings, it seemed to be fine, so connecting up the terminals to a battery, I introduced the plunger, and hey presto, it was whipped out of my hand and pulled inside the solenoid. Somehow, taking it apart seems to have fixed the problem.

At this point, I could have put it back together and the motor would have been fine, but if you've read any of my blog before, you'll know that I don't like putting dirty parts back on the mini, so... I decided to take the whole thing to bits! There is a little method to the madness though, apart from making it look nice, it will also give me an opportunity to check the condition of the brushes and commutator.

After a little Google guidance, I started by removing the end cap, 'C' washer and spacer...



Then undoing the two screws, the back part of the cover was removed...



Inside, there was a plastic housing which sat around the commutator, within it were four clips that held the brushes in place...



Using a small screw driver, it was fairly easy to pop the clips off, but unbeknownst to me, there was also a spring under the clip which came catapulting out, launching the clip across the room. Luckily I found it again.



Once the brushes were out, the wires could be un-threaded and the plastic housing removed to reveal the commutator.



Looking at the brushes, they were made from copper and were well over the 3.5mm recommended in the Haynes manual. The commutator though, did look to be quite worn down, I still think it will offer many years of use though before it's completely knackered.



Turning the motor over, there were two more screws in the pinion housing. Taking these out, it was possible to remove the middle part of the outer housing.



The armature itself soon followed.



With all the components now apart, I could start the job of cleaning everything. There was a nice layer of crud inside the pinion housing, but nothing a little red diesel couldn't shift.



Once every part had been thoroughly cleaned, I masked off the parts that needed painting. After a base coat of acid etch, they received three coats of black gloss and a top clear coat.



Once that was done, everything could go back together. As the Haynes manual would say, it was just the reverse of taking it to pieces.

Armature in
Housing on.
Brushes re-seated.
Internal wires tucked back in place.
Final part of the housing back on.
End cap in place.

Now as for the solenoid, the only slight complication was re-soldering the cap back on.



Once it was done though, the solenoid could be re-mounted. I found it easier to put the plunger on the lever first, then slide the solenoid over it.




Once the solenoid was bolted into place, a quick check with the battery confirmed that everything still worked and finally... the 'Ta-Dar' moment.



This was a great little project that could be conducted in the spare room on some newspaper away from the deathly winter chills of the shed. I had no idea how it would go or how to do it, so just got stuck in. For a tenner and a bit of hard work, I am very pleased with the outcome and also the opportunity it gave to learn how it worked and why it's called a pre-engaged starter.

Now the started motor is finished, it can be wrapped in a plastic bag and put into storage ready for the day it will be attached to the new engine.



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