MK3 wiring - start realy problem

Jim

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Apr 30, 2005
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Just finished modifying the electric starter on my MK3. Four field coils & brushes plus ball bearings on the armature. Installed it along with a new starter relay & heavy wires for the starter. Seems to work very good then something really strange happened. The engine would not shut off. Switched it off at the handlebar and even turned off the electrical system with the key. It still continued to run. So I pulled the sparkplug leads and it stopped but then I realized that the starter was still turning the engine over. I had to disconnect the lead on the starter to get it to stop cranking the engine. My guess is that the new relay or the start switch hung up. I was wondering if any of you folks had ever experienced such a thing? My electrical diagram is in bad shape and not real clear so I was wonder in if any of you folks could look at a MK3 wiring diagram and see if voltage is fed to the ignition system when the start relay is engaged………even with all other switches in the off position?
 

L.A.B.

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Jim said:
My electrical diagram is in bad shape and not real clear so I was wonder in if any of you folks could look at a MK3 wiring diagram and see if voltage is fed to the ignition system when the start relay is engaged………even with all other switches in the off position?


Yes there is.

There would normally be a white/purple wire to the coils from the starter relay (or starter solenoid).

The white/ purple wire bypasses the ballast resistor fitted in the ignition circuit thereby giving the coils a voltage boost while the starter motor operates, so runs directly from the battery through the relay.

This wire is no longer used if an electronic ignition system (like Boyer Bransden) is fitted.

The ignition switch should connect power to the handlebar starter switch (white wire) so if that stuck on then with the ignition switched off there should be no power to operate the starter relay.

So I would guess the fault probably lies with the relay itself?

What type of relay have you fitted, is it an original Prestolite type?
I believe if there is insufficient battery power available (possibly due to the increased power drain from the uprated starter?) then the relay contacts can 'chatter' the heat produced by arcing between the contacts can cause them to weld themselves together if allowed to continue for long enough.
 
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