Starter Motor not Engaging - Advice Please

[John P]

Yes,   I have just been trying it on the floor powered by some jump leads.

Before that I did strip it down again and checked for continuity of the field coils and that they weren't down to earth.   Obviously it's only checking at meter voltage but it all seems OK.

I also tried a continuity check between the various commutator segments and earth and again this seemed OK in that there was no continuity.   I did also measure the resistance between various segments but whichever pair I chose they always showed continuity between each other and a very low resistance figure (probably meaningless with my cheap multi-meter) but I didn't really know what to expect.

Running on the jump leads it runs quite fast but there's still not much of a jolt when it starts up and the pinion doesn't even try to release itself from the clip.   If you wind the pinion out of the clip before starting then it does move forward but not with any great urgency.

I am just cleaning up the Bendix helix again but it was already pretty clean so I'm not too hopeful.

I did wonder if running the motor for a few minutes would help bed in the new brushes but other than that I think I'll try it back on the car and then admit defeat - at least for now.

John.

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Nick,

It's a brand new battery which shows around 6.3 volts at the terminals and 6.2 at the starter.

I haven't charged it myself but it was supposedly fully charged when I got it just under a week ago.

John.

[Nick Turley]

Whilst I hear that, I still say, stress test or try another battery just to be sure.
I have myself picked up a new battery only last year which seemed poor, when I stress tested it it failed.
If you do this it then does rule out the battery.
I have just been to my chummy and checked the voltage on the battery 6.4 then had the starter turn the engine over for three two second bursts. When it was cranking the voltage dropped at the battery to 5.9 volts at completion of this test it went back up to 6.4 Volts.
I must agree it is a bit of a mystery. What about trying another starter ? would someone in your club loan you one ?

[Colin Wilks]

Woodrow does recommend cooking the armature for a couple of hours at gas mk ¼. Perhaps now is the time to sit in the kitchen with a glass of malt whilst the oven does its stuff and you can consider your plan for the morning?

[Zetomagneto]

Really? Can't image cooking would help.

[Ivangardiner]

The starter on my Bantam had been lying outside for years. I tried the usual new brushes, clean up, new bearings etc but it was really sluggish. I took it to a local motor rewinding place, and they said they would give the armature "a bit of a bake".  They may have done other things but it came back working like a new one, so baking does do some good.  It's an Autolite starter but very similar to the Lucas one.  I wouldn't try this baking thing myself though.  Best of luck.

[Ivor Hawkins]

John, I don't know whether this helps, but following on from bedding in the brushes, as I described earlier and spinning up the stater with a windy gun to loosen things up, I connected the starter up to a fully charged 6 volt battery this morning and videos the result for you to check, to see if we both have the same problem.

The starter doesn't have enough grunt to throw the starter dog out of the retaining clip. When the dog is unclipped, it whizzes forward on the bendix quite nicely.

I have also connected the starter to the 100 amp 6 volt charger/starter and the result was the same...a little try at 12 volts gave the sam result only that the motor ran faster!

https://youtu.be/gNLnop_IQm8

[John P]

Ivor,

That's almost exactly the issue I have and, interestingly, your starter seems to be running at a very similar speed to mine so at least I'm hopeful that electrically it may be OK.    (Or neither yours nor mine are OK I guess).

Although my battery is new I had been thinking of putting it on charge as the voltage has now slightly dropped to around 6.27 volts but in light of your other tests that doesn't seem a likely solution.

I know you can fairly easily bend the pinion clip to reduce it's holding force but even this hasn't made enough difference for mine to work.

Actually I'm amazed that the pinion has enough inertia to cause it to move out of the clip so I suspect a lack of initial acceleration is the problem but what's causing it I've no real idea.

John.

[Ivor Hawkins]

I’ve come to the same conclusion John, I’m now going to run an Avometer over everything and try the armature in the oven at 100 degrees for an hour and see if that helps.

I will also take the dog out of the clip, mount the starter and see if it has enough grunt to turn the engine.

I’m doing this in between bead blasting and painting my wheels, so results depend on drying times!

[John Cornforth]

Hi John P

When a starter motor is first turned on, the armature must speed up rapidly. This high acceleration needs high motor torque. Because starters are series wound, their torque is proportional to the square of the current. If things are working properly, when the battery is connected to the starter there will be an initial very high current surge of hundreds of amps giving a burst of high torque. Only a tiny part of this is needed to overcome friction, and there is not yet any load from the engine, so nearly 100% of the torque goes into acceleration of the armature. The pinion lags behind and is thus wound up its thread. If the starter is off the car, the torque reaction manifests itself as a "kick" if the starter casing isn't firmly held down.

Once the armature has sped up, it generates a back EMF in opposition to the battery voltage, so the current diminishes to a lower steady running value. At this point the motor torque is balanced by friction plus the load from cranking the engine. If the starter is off the car the torque is balanced by friction alone and the steady running current won't be very much. The speed when off the car will be more or less proportional to the battery voltage.

The upshot of all the above is that if there is any unwanted extra electrical resistance somewhere in the starter, its wiring or the battery it will have a MUCH more significant effect during the initial acceleration than it will during the steady speed running state.

To put some figures on it, say you have a total resistance of battery, starter and wiring of 20 milliohms. With 6 Volts the initial current will be 300 Amps. If you were to then add 10 milliohms of extra resistance the total would rise to 30 milliohms and the initial current would drop to 200 Amps. The torque would be only 2/3 x 2/3 = 4/9 of the previous value.

I don't know if new brushes are contoured to fit the armature. If they are not, then you should be able to get a closer fit by the use of thin abrasive paper. The increased effective brush area will reduce the resistance.

[John P]

John,

Thanks for the theory.   One of the frustrating things is not understanding what to look for.

The battery, battery leads and isolator are all new and the starter button re-furbished so I'm fairly sure any high resistance must be in the motor itself.   This is supported by the fact that I had the same problem on the bench when the motor was connected via jump leads direct to the battery.

The brushes are new and come contoured to the commutator but, in any case, the old set had obviously worn in but gave exactly the same problem.

Within the motor I'm guessing there's a lead from the external connecting stud to the field coils and then another lead from the field coils to one of the brush holders.   (I take it this is the lead you can see when the motor end is removed to expose the brush holders and is wrapped in a spiral of insulation).

The only other connection is from the second brush holder to earth.

In terms of hunting down any high resistance the only parts of this I can physically check are the ring tags which connect the leads to the brush holders but visibly these look fine.   The brush which is connected to earth uses a small copper braid riveted to the starter motor end plate so I guess any corrosion on the mating faces between the starter body and the end plate could have an effect but it looks OK.

I've only got a low cost digital multi-meter but could I get any meaningful results using this.

John.