46 replies to this topic

[paulfoel]

 

Battery terminals are different sizes to prevent this from happening - did you hammer the smaller terminal on to the larger post ?

This is a "nice forum" friendly sort of thing, and I already got told off for growling at someone...which is fair enough....but if it wasn't...I'd say, well, let us just say that the OP was "confused".

Perhaps other people's earth leads are longer...but my battery wouldn't connect up if it was installed the wrong way around.

You could of course jump a flat battery and connect it the wrong way around...but let's be honest...it still wouldn't start.

 

 

Aware that I am a complete idiot of course....

 

But unfortinately leads reach easily and terminals fit. No good for idiots like me

[JonnyBMX]

whoops, i was once working on someones mini... asked em to connect the battery up.. then there was a bit of smoke so i told them to disconnect it.. went to investigate at the battery to see that they had connected it in reverse polarity.... luckily no damage to the car!

 

as allready said its only likely to damage polarity sensitive devices. i.e if an electrolitic capacitor is connected in this manner it will go bang ;P

[Captain Mainwaring]

A petrol engine with minimal ignition advance is perfectly capable of running backwards, just not very well

Exhaust valve normally open on piston upstroke.....reversed......open on the downstroke......which would mean a vacuum on the exhaust.....

Pre-engaged starter...so field wound...will run in reverse.

 

Perhaps he got the carb stuck on the exhaust when he got the battery on the wrong way around.

Yes a 4 stroke will run in reverse (notwithstanding that piston thrust sides and pin offsets will be wrong) - but it won't do so without changing the cam timing at the very least - if you did that and ignition was close to TDC then you might get away with it....otherwise you've got to glue the carb onto the exhaust manifold.

A two stroke will in principal run in reverse with the above caveats.

Edited by Captain Mainwaring, 09 May 2013 - 11:29 PM.

[Captain Mainwaring]

 

 

Battery terminals are different sizes to prevent this from happening - did you hammer the smaller terminal on to the larger post ?

This is a "nice forum" friendly sort of thing, and I already got told off for growling at someone...which is fair enough....but if it wasn't...I'd say, well, let us just say that the OP was "confused".

Perhaps other people's earth leads are longer...but my battery wouldn't connect up if it was installed the wrong way around.

You could of course jump a flat battery and connect it the wrong way around...but let's be honest...it still wouldn't start.

 

 

Aware that I am a complete idiot of course....

 

But unfortinately leads reach easily and terminals fit. No good for idiots like me

 

 

For what it's worth....there are two battery post standards SAE (which is normal) and JIS (smaller and found on Jap market cars). It is possible but very unlikely that your car has JIS spec posts. If it did, you'd never get a UK battery to fit.

Assuming it's got SAE posts...then you'd still have a struggle. If you had clamp type rather than cap type terminals then if you spread the -ve wide you might be able to hammer it on, but how you'd get the +ve to tighten up is a mystery to me at least.

 

 

That's the difference between JIS and SAE posts......

 

If you did succeed in connecting it up the wrong way, then you'd get smoke out of the alternator diodes the minute you connected it, if you were quick, you might save them. I would think that the MIMs would also go pair shaped. If you did cook it, I've got a jap spec one and for that matter a full SPi set up that I can sell you

 

 

Anybody ever done a -ve to +ve earth conversion? Do you remember what you had to change?

[Vipernoir]

I've done +ve to -ve on one of my Elfs.  From memory after 19-odd years all I had to do was change the terminals, and tab something across the dynamo to change its polarity.

 

Alternators are -ve earth only.

[Captain Mainwaring]

I've done +ve to -ve on one of my Elfs.  From memory after 19-odd years all I had to do was change the terminals, and tab something across the dynamo to change its polarity.

 

Alternators are -ve earth only.

 Also change the direction of the wiper motor - they will work but park funny and wear out quick if the wrong way.... Old permanent magnet starters always run the same way - you also need to change the coil polarity, if not the field won't build up as quickly as it should because of the way the HT is tapped.

[tiger99]

Not quite. Permanent magnet motors will reverse, motors with a wound field (including original starters and some wiper motors) will run the same way if the supply is reversed. Only a few Minis will have a permanent magnet starter, possibly one of the lightweight modern ones, so not many starters would be affected.

Absolutely right about the coil, but you should also fit a new set of points if you are intentionaly changing polarity, as they form a pip on one side and a crater on the other, which will try to reverse, leaving them both heavily eroded and making bad contact. They may last 1000 miles or so, maybe less, so best changed before the car fails to go.

[Tamworthbay]

A petrol engine with minimal ignition advance is perfectly capable of running backwards, just not very well

Exhaust valve normally open on piston upstroke.....reversed......open on the downstroke......which would mean a vacuum on the exhaust.....
Pre-engaged starter...so field wound...will run in reverse.
 
Perhaps he got the carb stuck on the exhaust when he got the battery on the wrong way around.
Yes a 4 stroke will run in reverse (notwithstanding that piston thrust sides and pin offsets will be wrong) - but it won't do so without changing the cam timing at the very least - if you did that and ignition was close to TDC then you might get away with it....otherwise you've got to glue the carb onto the exhaust manifold.
A two stroke will in principal run in reverse with the above caveats.

Most two strokes (with the exception of some rotary valve types) will run perfectly happy either way. Not sure which caveats you are on about but all you need is timing close to to TDC. No in principal about it, it happens - read my previous post.

[Captain Mainwaring]

 

 

A petrol engine with minimal ignition advance is perfectly capable of running backwards, just not very well

Exhaust valve normally open on piston upstroke.....reversed......open on the downstroke......which would mean a vacuum on the exhaust.....
Pre-engaged starter...so field wound...will run in reverse.
 
Perhaps he got the carb stuck on the exhaust when he got the battery on the wrong way around.
Yes a 4 stroke will run in reverse (notwithstanding that piston thrust sides and pin offsets will be wrong) - but it won't do so without changing the cam timing at the very least - if you did that and ignition was close to TDC then you might get away with it....otherwise you've got to glue the carb onto the exhaust manifold.
A two stroke will in principal run in reverse with the above caveats.

Most two strokes (with the exception of some rotary valve types) will run perfectly happy either way. Not sure which caveats you are on about but all you need is timing close to to TDC. No in principal about it, it happens - read my previous post.

 

The "in principal" refers to the fact that whilst they might well run, they won't run as intended - meaning things like gudgeon pin offset and piston thrust side will be on the "wrong" side.

It would be like saying that a 4 stroke mechanically injected diesel will run in reverse - yes it will in principal, until (assuming it has a rotary not plunger oil pump) the thing locks up because the oil pump is running backwards. 

[Tamworthbay]

A petrol engine with minimal ignition advance is perfectly capable of running backwards, just not very well

Exhaust valve normally open on piston upstroke.....reversed......open on the downstroke......which would mean a vacuum on the exhaust.....
Pre-engaged starter...so field wound...will run in reverse.
 
Perhaps he got the carb stuck on the exhaust when he got the battery on the wrong way around.
Yes a 4 stroke will run in reverse (notwithstanding that piston thrust sides and pin offsets will be wrong) - but it won't do so without changing the cam timing at the very least - if you did that and ignition was close to TDC then you might get away with it....otherwise you've got to glue the carb onto the exhaust manifold.
A two stroke will in principal run in reverse with the above caveats.

Most two strokes (with the exception of some rotary valve types) will run perfectly happy either way. Not sure which caveats you are on about but all you need is timing close to to TDC. No in principal about it, it happens - read my previous post.

The "in principal" refers to the fact that whilst they might well run, they won't run as intended - meaning things like gudgeon pin offset and piston thrust side will be on the "wrong" side.
It would be like saying that a 4 stroke mechanically injected diesel will run in reverse - yes it will in principal, until (assuming it has a rotary not plunger oil pump) the thing locks up because the oil pump is running backwards.

As a former midlands secretary of the British two Stroke Club I have a fair bit of experience of smokers. All this talk of gudgeon pin offset and piston side thrust is irrelevant if not just plain wrong. If you have worked on many smokers you would know how incredibly simple they are. The only issues are timing and rotary valves. The old villiers engines were famed for their flexibility, they were used in Invalid cars and mount either way. I used to run in a go kart series using these engines and the first job was always to check the timing to see which way they had been set up to run. There was absolutely no difference in the engines at all.

[Carlos W]

Back to the original topic!

To the OP, Chuck a new alternator on, check the wiring, turn the key and see what happens!

[Tanya]

my dick of a brother did this to mine once when I wasn't  looking and blew the alternator but everything else was ok.

 

Exchange the word 'brother' for 'husband' here and that's what happened to me!

[tiger99]

In case anyone is curious, the life expectancy of the alternator diodes with a reversed battery is less than 50 milliseconds, if the battery and earths etc are all in good order. But for the small resistance of the alternator windings, which constrains the current to only a couple of hundred amps, it would only be microseconds.

 

It is likely that the diodes failed open-circuit, however that is not guaranteed, and if they happened to fail short-circuit, the windings are also toast. If it was only the diodes, and probably the regulator, the parts could be replaced, if available, but in any case it is usually cheaper to ger a reconditioned alternator.

[Captain Mainwaring]

 

 

 

 

A petrol engine with minimal ignition advance is perfectly capable of running backwards, just not very well

Exhaust valve normally open on piston upstroke.....reversed......open on the downstroke......which would mean a vacuum on the exhaust.....
Pre-engaged starter...so field wound...will run in reverse.
 
Perhaps he got the carb stuck on the exhaust when he got the battery on the wrong way around.
Yes a 4 stroke will run in reverse (notwithstanding that piston thrust sides and pin offsets will be wrong) - but it won't do so without changing the cam timing at the very least - if you did that and ignition was close to TDC then you might get away with it....otherwise you've got to glue the carb onto the exhaust manifold.
A two stroke will in principal run in reverse with the above caveats.

Most two strokes (with the exception of some rotary valve types) will run perfectly happy either way. Not sure which caveats you are on about but all you need is timing close to to TDC. No in principal about it, it happens - read my previous post.

The "in principal" refers to the fact that whilst they might well run, they won't run as intended - meaning things like gudgeon pin offset and piston thrust side will be on the "wrong" side.
It would be like saying that a 4 stroke mechanically injected diesel will run in reverse - yes it will in principal, until (assuming it has a rotary not plunger oil pump) the thing locks up because the oil pump is running backwards.

As a former midlands secretary of the British two Stroke Club I have a fair bit of experience of smokers. All this talk of gudgeon pin offset and piston side thrust is irrelevant if not just plain wrong. If you have worked on many smokers you would know how incredibly simple they are. The only issues are timing and rotary valves. The old villiers engines were famed for their flexibility, they were used in Invalid cars and mount either way. I used to run in a go kart series using these engines and the first job was always to check the timing to see which way they had been set up to run. There was absolutely no difference in the engines at all.

 

Without flogging a dead horse here.....irrelevant it may be, but wrong it isn't. And I have worked on 2000hp marine two stroke diesels, and a few old stuart-turner two stroke petrols. 

As you say, my little Stihl brush cutter engine would run perfectly "backwards" except the cooling fan vane direction would be wrong.

 

Probably more important to this discussion (and it's not really important at all) is the fact there there is no way the OP's car started in reverse when he put the battery terminals on the wrong way around.

 

Regarding the MIM's, a little look at the block drawing I have for it shows it has crowbar protection, so the OP may have got away with popped fuses.

[Captain Mainwaring]

In case anyone is curious, the life expectancy of the alternator diodes with a reversed battery is less than 50 milliseconds, if the battery and earths etc are all in good order. But for the small resistance of the alternator windings, which constrains the current to only a couple of hundred amps, it would only be microseconds.

 

It is likely that the diodes failed open-circuit, however that is not guaranteed, and if they happened to fail short-circuit, the windings are also toast. If it was only the diodes, and probably the regulator, the parts could be replaced, if available, but in any case it is usually cheaper to ger a reconditioned alternator.

I saw one reverse polarity smoked - for what it was worth it was on an Astra diesel. It didn't die from it's injuries and yes, it did let the smoke out.