negative earth article

Post by **scarrington** » Mon May 18, 2009 8:28 pm

I've just read the change over procedure in the new issue of minor matters, actually I changed my van over 3weeks ago after down loading same article from technical tips. However there is no mention of the fuel pump which I had to rewire round the other way because it wouldn't work until I swapped the wires over.

Post by **PSL184** » Mon May 18, 2009 8:48 pm

You shouldn't have to (I've certainly never had to on many that I have done). Only thing I have had to do was fit new pump points because they try to pip and dip on the opposite contacts to before. Same goes for the dizzy points too......

Post by **Leo** » Mon May 18, 2009 9:02 pm

beware of electronic Fuel Pumps, the SU type look exactly the same as the points type apart from a small embossed tag.
They are polarity sensitive and will fail with out warning if connected the wrong way round

Post by **bmcecosse** » Mon May 18, 2009 10:14 pm

Standard pump will work either way - if you had to swop the wiring - does that now mean that the metal pump body is not the same polarity as the metal body parts round about it ? If so - beware shorts - and as it's not fused you could easily start a fire! I suggest you at least fit a fuse - and if possible get a pump that WILL work on negative earth. I haven't seen the 'article' - but hopefully it advises you to fit a new condenser, to swop the coil wires over, and to fit new points - or at least clean the old points in both dizzy and fuel pump!

Post by **PSL184** » Mon May 18, 2009 11:53 pm

There was no mention in the article about replacing and / or cleaning points (either) or condensor. True enough if you have an electronic pump then it will be polarity sensitive but then you would originally had a pos earth pump and it wouldn't work either wired the other way round....

Post by **bmcecosse** » Tue May 19, 2009 6:53 pm

So - MMOC official 'article' doesn't tell all the important points about changing polarity! Dear oh dear. Good thing we have this website to sort out the problems!

Post by **downsey** » Tue May 19, 2009 10:10 pm

You can just use an mgb fuel pump, its physically the same as the morris pump but neg. earth.

Post by **bmcecosse** » Tue May 19, 2009 10:25 pm

Any MGB pump I have seen is a good bit larger! Also - it's a 'pusher' pump, not a 'sucker' as required for standard Minor installation. The normal SU electric pump works with either voltage ground - it's only the 'electronic' pumps that have this polarity problem.

Post by **autolycus** » Wed May 20, 2009 8:35 am

Fully agree about Minors needing suck pumps not blow pumps (or LP as against HP in SU-talk), but there are some non-electronic SU pumps which are polarity-conscious, i.e. those fitted with internal diodes. Those with capacitors across the points are OK.

Seehttp://www.sucarb.co.uk/TechnicalDetail.aspx?id=49 for details

Kevin

Post by **bmcecosse** » Wed May 20, 2009 11:06 am

Ahh - diodes hadn't been invented when my Minor was built!

Post by **d_harris** » Wed May 20, 2009 12:18 pm

You must have a VERY early minor then.....

Crystal Diodes were invented in 1899

Post by **ASL642** » Wed May 20, 2009 1:06 pm

Post by **autolycus** » Wed May 20, 2009 1:10 pm

bmcecosse wrote:Ahh - diodes hadn't been invented when my Minor was built!

That's odd - I'm sure the Haynes manual shows it with valves. Perhaps they're triodes.

Kevin

Post by **Leo** » Wed May 20, 2009 2:38 pm

Very early pumps were fitted with a Cat's Whisker

Post by **bmcecosse** » Wed May 20, 2009 6:02 pm

Doubt there would be a 'crystal' diode in a fuel pump!
Why would fuel pumps be fitted with diodes anyway ? And if they are - and it prevents operation when earthing polarity is changed - then presumably it's just a matter of turning the diode round to make it work again?

Post by **downsey** » Wed May 20, 2009 8:21 pm

bmcecosse wrote:Any MGB pump I have seen is a good bit larger! Also - it's a 'pusher' pump, not a 'sucker' as required for standard Minor installation. The normal SU electric pump works with either voltage ground - it's only the 'electronic' pumps that have this polarity problem.

I used an mgb pump with my 1275. Worked fine.

Post by **autolycus** » Wed May 20, 2009 10:15 pm

bmcecosse wrote: Why would fuel pumps be fitted with diodes anyway ?

I believe they're known as "quenching" diodes - they are to reduce sparking when the contacts open.

bmcecosse wrote: And if they are - and it prevents operation when earthing polarity is changed - then presumably it's just a matter of turning the diode round to make it work again?

Yup. It may need un- and re-soldering. My main point was to warn that it's not quite as simple as "mechanical - easy, electronic - expensive". I'm just contemplating swapping diodes round on a twin-bodied SU pump mounted under a car of moggy-era. Clearly not the original pump.

Kevin

Post by **bmcecosse** » Wed May 20, 2009 11:40 pm

Can't see how a diode would 'quench' anything. Unless there is a reverse emf in the coil when the points open - and the diode shorts that to ground ?

Post by **autolycus** » Thu May 21, 2009 8:17 am

bmcecosse wrote:Can't see how a diode would 'quench' anything. Unless there is a reverse emf in the coil when the points open - and the diode shorts that to ground ?

Yup. Collapsing magnetic fields, and all that. The diode provides a path to the battery, which you can think of as behaving like a big capacitor.

See
http://www.bcae1.com/relays.htm (quite a long way down the article, but some useful stuff about relays.

Kevin
who has just realised this is all in the "Mechanical" section.