How do you check whether an engine is positive or negative earth?

[Guest]

I have a 1970's Perkins 4.107 engine with a dynamo. There are no battery connections so does anyone know how to test if it is positive or negative earth?

 

[Guest]

It's the dynamo that needs to be polarised correctly to charge properly. It will almost certainly be negative earth. I would connect it negative earth anyway, if it dosn't charge the dynamo can easily be polarised for negative earth. Just connect the body to the negative terminal of a battery and flash the positive to the field winding. I personaly would replace the dynamo with an alternator, simpler wiring, easier and cheaper to replace if it goes wrong.

 

[Guest]

yup, ditch the dynamo - fit an alternator

 

[Guest]

Agreed, dynamo's are yesterdays techno. Fit an alternator, also ditch the dynamo control box, probably black with three relays

 

[Guest]

Being 70s it is likely to be +ve earth

Dhech markings on dynamo, fome times the field winding was marked f-, otherwiseit might be poss to deduce from inspection of the control box,otherwise speak to lucas/ perkins

david

 

[Guest]

why change to an alternator??

there are good reasons for this other than "its the upto date version"

An alternator will provide you with high charging currents immediately after starting your engine. This will restore the charge removed by the starter very quickly.

A dynamo has a very limited charging capability and therefore it takes a lot longer to recover the starting consumption.

kev www.yachtbits.co.uk

 

[Guest]

Is this a new install?

Mike,

At that vintage, the dynamo will almost certainly have originally been +ve earth. As suggested below, the dynamo can be either polarity and it is not difficult to reverse it if needed. When all cabled up, press the “cut-out” contacts together (if you remove the fan belt first, the dynamo may well run happily as a motor) this will set-up some residual magnetism in the field winding core and from then on it will be the correct polarity. This assumes the combined cut-out & regulator are Electro-mechanical (big coiled relays) and not semi-conductor versions. The starter is not battery polarity sensitive.

However, since it sounds as if you are starting from a new-ish install, an alternator from a scrap-yard will cost very little and can easily be fitted with almost no modifications other than a new fan belt required. This will give you over 35 amps charge, compared with 20 max for a dynamo, regulation will be better and the batteries will last longer.

Email me if you need more concise electrical directions for the Perkins as I have done this dynamo-for-alternator swap myself, including having to reverse the polarity of the entire boat.

Cheers, CRB

 

[Guest]

Re: Is this a new install?

Is not the Starter 'throw' and direction of rotation a function of polarity....!!!!

 

[Guest]

No, it’s all to do with “shunt” or “series” wound motors:-

Rotation depends on the magnetic flux induced by the DC voltage initially in the field and then the armature windings, which are fed via the commutator and brushes. In a shunt wound unit, reversing the polarity will reverse rotation, but series will always go the same way irrespective, hence your B&Decker drill works the same OK off AC or DC volts and actually requires quite complex switching to reverse the rotation. The starter motor is series wound and hence doesn’t care about polarity.

Or is it the other way round? – It’s been a long time!

 

[Guest]

Re: No, it’s all to do with “shunt” or “series” wound motors:-

Mmmmm me thinks it may have been a long time Colin.....