little diode thing

[Barry S1]

Any ideas about the best place
to find one please

[glenrobbo]

Morning Barry,

RS Components is your best bet, but you need the spec.
Does anyone on here know?

http://uk.rs-online.com/web/c/?sra=oss&r=t&amp...

Edited by glenrobbo on Sunday 19th June 07:24

[phillpot]

Barry S1 said:

Any ideas about the best place
to find one please

My shed (and no specs needed)






Drop me your address Barry and will be in the post by the end of the day

Edited by phillpot on Sunday 19th June 07:56

[Barry S1]

Thanks Mike
Will do.
Can I pay you next time we meet if that's
ok with you

[phillpot]

Barry S1 said:

Can I pay you next time we meet if that's ok with you

No problem , give you time to save up




Got your email with address

[driverrob]

I hope the OP remembers which way it was connected or the replacement may not last long.

[magpies]

driverrob said:

I hope the OP remembers which way it was connected or the replacement may not last long.

that's why links like these should always have a male connector one end and female the other - so no doubt as to which way round it should be connected

[phillpot]

driverrob said:

I hope the OP remembers which way it was connected or the replacement may not last long.

It's a diode, it prevents current flow in wrong direction as I understand it so would it not tolerate being connected wrong way round and just "do what diodes do" or is there more to it than that?


New one is marked up red and black, find which terminal is live with a meter and that's the red one?

Looking at Barry's photo the band on his one is to the right so that's +ve?

[Buzzlt]

Yep the band is positive :-)

[mentall]

phillpot said:

It's a diode, it prevents current flow in wrong direction as I understand it so would it not tolerate being connected wrong way round and just "do what diodes do" or is there more to it than that?

If you put it the 'right' way round (band +ve) it will be open-circuit to the relay coil voltage, and (briefly) short-circuit to the back-emf from the collapsing field (which is what it's meant to do).

'Wrong way' and it will present a short to the coil operating voltage, and blow either the supply fuse or the diode junction or both.

[driverrob]

magpies said:

driverrob said:

I hope the OP remembers which way it was connected or the replacement may not last long.

that's why links like these should always have a male connector one end and female the other - so no doubt as to which way round it should be connected

Sorry. It looked like 2 male connectors in the first photo.

[phillpot]

mentall said:

If you put it the 'right' way round (band +ve) it will be open-circuit to the relay coil voltage, and (briefly) short-circuit to the back-emf from the collapsing field (which is what it's meant to do).

'Wrong way' and it will present a short to the coil operating voltage, and blow either the supply fuse or the diode junction or both.

So, as I suspected, it's not as simple as an "electrical one way valve"?

I thought there was a diode in the fuel pump relay that did the back EMF thing, to protect the ECU?

I also thought (possibly quite incorrectly) that this little chap was something to do stopping the fuel pump running if the engine stops in a crash?

driverrob said:

Sorry. It looked like 2 male connectors in the first photo.

Nowt wrong with your eyes, it does have two male connectors.

[mentall]

phillpot said:

I thought there was a diode in the fuel pump relay that did the back EMF thing, to protect the ECU?

Sorry: could be me getting confused. Where is this diode if not across the relay coil (not all relays have a diode included)? I haven't seen one on my car.

[phillpot]

That's it in the the fuel injection relay feed................ wiring diagram

The diagram in Steve Heaths book shows the relay internals, on injection and pump relay the left hand pair of wires operate the coil, right hand two are switched.


I thought (lot of thinking going on) the fuel pump relay had a diode built in and that's why the terminal layout is different to a "standard" relay to prevent wrong one being used?

Edited by phillpot on Sunday 19th June 21:52

[tdiquattro]

Some have the diode built in some dont, should be shown on the casing... (pointy thing! lol)..

http://www.halfords.com/workshop-tools/garage-equi...

[GreenV8S]

phillpot said:

I also thought (possibly quite incorrectly) that this little chap was something to do stopping the fuel pump running if the engine stops in a crash?

Mine had an impact sensor on the front of the passenger footwell. Which had reduced itself to a pile of rust and became one of many sources of reliability issues a few years ago and has now been deleted. I believe the diode connected in reverse parallel with the fuel pump relay coil is just there to protect the driver circuit in the ECU from the back EMF when the relay is switched off.