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Problems Converting From Positive To Negative Earth


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#1 1965 Classic Mini

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Posted 23 July 2014 - 07:44 AM

Tried unsuccessfully to convert my 1965 mini to neg earth - ultimately I'll switch to alternator:

Have changed battery terminals round (new ones fitted) "shorted" the 2 spade terminals on the dynamo and changed the coil wires round. Lights work OK etc etc.....however the engine just spins but will not fire up.....very frustrating .........Have reverted to positive earth and all works 100%

 

 

The fuel pump is electric -  does this need the 2 wires swapping round ?

I am having difficulty identifying the fuel pump type........pictures on web & info in Haynes manual  a bit vague (or is it me ?) There is a blue plastic cap at one end which suggests that it could be either an SU type or after market compatible . 

 

Help .....any suggestions ?

 

All advise gratefully received :gimme:

 



#2 Daviewonder

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Posted 23 July 2014 - 08:03 AM

I would say your fuel pump is sucking.



#3 1965 Classic Mini

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Posted 23 July 2014 - 08:09 AM

So if I swap the wires round will this solve it ?



#4 Daviewonder

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Posted 23 July 2014 - 08:11 AM

Yes. 



#5 grumpy dad

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Posted 23 July 2014 - 08:52 AM

Hi 

all SU pumps have a black cap

so its after market 

some of them have two wires 

and are polarity sentisf  so yes sap the wires round 



#6 1965 Classic Mini

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Posted 23 July 2014 - 09:44 AM

Thanks to all replies.....will be giving this a try :proud:



#7 dklawson

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Posted 23 July 2014 - 12:25 PM

On the fuel pump... SU pump or aftermarket?  (EDIT:  See footnote at the end... sorry)

 

On an aftermarket pump, wiring it backwards may let the white smoke out and you may create a non-operational pump.  On an SU pump, no... you don't reverse the wires on the pump because the body of the pump is an earth connection.  Yes, a separate earth wire may run to the pump... but the pump body is still an earth connection.  Do not reverse the wiring.  Early SU pumps did not have any components to protect the pump's points, later a capacitor was added.  On even later pumps the capacitor was replaced by a diode.  The capacitor was not polarity sensitive, the diode is.  So, what you DO have to do is remove the plastic cap on the SU pump and (unless you can tell the difference between the diode and capactor) reverse any points protection device that is located under the cap.   See the link below to Barney Gaylord's MGA web site and his pages on the SU electric pump.  It includes a discussion of the electric pump, the capacitor and diode.

http://mgaguru.com/m.../fuel/fp100.htm

 

The joining of the two wires at the dynamo to each other is not the end step.  To flash the dynamo those wires have to be tapped to the battery connection to change the residual magnetic field inside the dynamo.  Touching the dynamo terminals to each other does not do this... you have to make a battery connection also.  See the link below for steps on flashing the dynamo to convert its polarity.

http://www.minimania...trical_polarity

 

Note that most of your car's electrics will be fine.  HOWEVER, you do have to switch the coil low-tension terminal connections and you have to address any aftermarket electrics (such as a tachometer) that may have been fitted.

 

Someone is bound to post:  "Don't forget to change the polarity of your gauges or they will read backwards".  The gauges are not polarity sensitive so do not make any changes to their wiring (except for aftermarket tachometers as mentioned above).

 

An overview of the earth conversion is discussed in the MG link below.  The Mini is not differrent except that the MG came with an electronic tach.

http://www.mgexp.com...eg-convert.html

 

EDIT:  SU pump with a blue cap.  Those are typically the German aftermarket pump.  I am not sure how or if you convert their polarity.  It could be replacement is required for negative earth.  SU pumps typically have the black cap as mentioned by Grumpy.  For at least a while, SU/Burlen was using different colored tape near the black cap to distinguish between positive and negative earth versions.


Edited by dklawson, 23 July 2014 - 12:31 PM.


#8 Captain Mainwaring

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Posted 23 July 2014 - 01:58 PM

Can a diaphragm pump suck instead of blow? I'm not sure :-)

Possibly it may refuse to run though.



#9 Captain Mainwaring

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Posted 23 July 2014 - 02:01 PM

Wiper motors can be polarity sensitive - you'll tell by watching which way the move off after park - I can't remember if the Mini wiper motor fits in this group.



#10 dklawson

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Posted 23 July 2014 - 04:39 PM

Captain, the SU electric pump is only polarity sensitive if the diode is fitted across its points as I mentioned in my last post.  Regardless, it's a solenoid operated diaphragm pump.  It cannot pump the wrong direction due to the check valves installed in its inlet and discharge ports.

 

The Lucas wiper and starter motors used on the Mini are not polarity sensitive.  You do not change their wiring when switching polarity, they will not run backwards.



#11 Captain Mainwaring

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Posted 24 July 2014 - 12:06 AM

Captain, the SU electric pump is only polarity sensitive if the diode is fitted across its points as I mentioned in my last post.  Regardless, it's a solenoid operated diaphragm pump.  It cannot pump the wrong direction due to the check valves installed in its inlet and discharge ports.

 

The Lucas wiper and starter motors used on the Mini are not polarity sensitive.  You do not change their wiring when switching polarity, they will not run backwards.

 

 

Thus the smiley :-)

 

As a rule of thumb brushless DC motors will reverse on polarity change, but brush motors require their brushes swapping over.

 to run in reverse.

 

The diode in the fuel pump is just a clipper - it could be there for two reasons - one to prevent a spark across the points caused by reverse EMF as the field collapses or it could be there just to sharpen up the action of the armature.



#12 dklawson

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Posted 24 July 2014 - 01:07 AM

Since Burlen added a non-polar cap to the pump first, I expect the diode was added to quench the arc from reverse EMF as you said.






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