thesmithsUK

Hi all.

I'm replacing the failing Bosch starter motor in my 4.5L 928 with a new, high-torque unit from Powerlite.

The new unit has two terminals: the larger one for the positive battery connection, and a smaller one for the ignition connection.

Simple, right?

Except that the old unit has three connections!

See new and old pics below.

My question is: how do I make three into two? What gives?!

FredR

The connection with the two bigger lines on is the live feed from the battery and cross connects to the alternator. When the 4mm yellow cable is energised from bus 50 by the cranking position on the ignition switch the solenoid in the starter motor is energised, the power supply then connects to the motor that duly cranks over.

On the early models like yours [model year?] it seems that smaller cable [1.5mm] is also energised and that appears to take power to the ignition system- why it does this I have no clue. Later models like mine have a similar setup on the large terminal and the feed to the solenoid but do not have that additional connection as you have now discovered. As to how you work around this needs a more informed opinion but doubtless you will get one when the US wakes up.

Needless to say the smaller cables look buggered- the big one form the battery may still be OK.

thesmithsUK

OK, so I've established that the yellow wire is the trigger wire.

And that the (badly frayed) brownish wire above it goes to the ballast resistors - to bypass them when the starter is engaged (to give a bigger spark).

I don't think this is necessary with new, High Torque starter motors but do I:

a) just disconnect it, or
b) add it to the same terminal as the trigger wire?

ZEUS+

That wire bypasses the.4 ohm resistor to the ignition coil for better starting, especially when cold. If you connect it to the trigger wire, current will flow to the starter with key on

Jason89s4

Yes, that third wire is to the ballast resistor to direct more power to the starter during startup. But if your new starter doesn’t even have a place for it….I guess just tape it up and keep your battery charged, let the yellow wire do it’s thing.

thesmithsUK

Many thanks, all. Great advice.

Works a treat with that ballast resistor wire disconnected.

And what a difference these modern high-torque motors make!

Alan

The ballast bypass wire is for enhancing the spark during starting - while the faster turn over is good and probably gets you faster starts normally - it's not really a substitute for a better spark in adverse (cold) starting conditions. If you wanted to connect it and have it function as before you'd need to add an extra relay controlled by the (50) starter signal (Yellow wire) from the ignition switch. If you'd never drive it in such conditions it wouldn't really matter anyway.

Alan

Zirconocene

@thesmithsUK : Which starter motor did you get from Powerlite? How is the fit on the car? Any strange noises when starting as compared to stock?

I don't know that it's feasible to get one here in the US, but their prices look very reasonable for new units.

ETA: RAC509; I found it, there was just a lot of scrolling. Thanks for posting about this, it's nice to know what other options exist for our cars.

Cheers