Diodes

[davetherave]

In the eternal hunt for why the Xm rides so crap I am going back to the diodes.

I originally put in some internal diodes that were 1n4001, most people seem to use 1n4007s when doing the diode mod.
The difference seems to be the VRRM on the 1n4001 is 50 whereas it is 1000 on the 1n4007s.

So im wondering if the original diodes I put in are now truly rogered. Or would it make no difference?

The reason I used 1n4001s was because i had a load left over from fitting remote locking.

Dave

[xantia_v6]

No, it won't be the type of diode causing the problem. IN4001s and 4007s are very similar. from some manufacturers they come from the same batch, but after testing the breakdown voltage and leakage they put them into one bin or the other...

[davetherave]

Might take the ECU apart and check them anyways, there has to be something wrong with it!
I cant drive round in it like it is!

[xantia_v6]

Have you determined that the ECU is keeping it in hard mode?
On my Xantia the intermittently firm rear suspension was mostly due to low system pressure caused by a leaking rear electrovalve.

[davetherave]

No Its not definate it is very variable I will be riding along then boom hard mode untill i turn the ignition off and on.
I just dont have the time to take the electrovalves apart and figure the ECU is easy to check as getting to the electrovalves involves removing the exhaust.
How did you know the electrovalve was leaking?

[CitroJim]

a leaking rear electrovalve.

Internally and not externally...

Internal leakage in an electrovalve can be checked by disconnecting its leakage return and directing it into a jamjar. A good electrovale will 'spurt' when switching between hard and soft. At all other times leakage will be minimal. One that flows constantly in soft or hard is suffering internal leakage.

Diodes are either good or bad in this context. If they test OK on an AVO or similar then they're OK for the job they are being called upon to do.

Almost any silicon diode intended for rectifier duty will do.

[addo]

Hard - spurt - soft... Where's Malcolm when you need him?

[CitroJim]

Hard - spurt - soft... Where's Malcolm when you need him?

[davetherave]

Right well ive tested the leakage from both electrovalves and it is very very minimal so its not those

[CitroJim]

Have you checked your main-line pressure Dave?

[davetherave]

Sadly do not yet have the equipment to do that, thats next on the list! Ive bought a pressure tester today and will await its arrival.
When it stops raining ill try and lexiate again to see what faults it threw up in france. When I did it just before it was steering wheel angle sensor
and speed calculation intermittent.
Dave

[xmexclusive]

Hi Dave

I can probably find and test good a spare suspension ECU to lend you to try.

John

[davetherave]

Very kind John! but what would cause the problem inside the ECU? I have replaced a vn05n chip in the ECU and put two diodes in which made me think perhaps they were not up to the job being less than original spec.
I may just take my ECU apart just to check whats going on.

The question as ever is is it hydraulic or electrical in nature, I beleive because at times it does work fine that it is more likely to be electrical but I suppose it could be main pressure.

[xmexclusive]

Hi Dave

The diodes you used are actually way over original Citroen spec.
The reference is the original diode encapsulated in the electrovalve.
This is just one of the two diodes fitted to the electrovalve drive wires.
The other diode is encapsulated in the VN05N on the output wires.
Both are very low spec items and damage easily in service.
The extra diodes put on the wiring provide continued protection if the originals fail.
They can do nothing to rectify a already faulty VN05N.
A diode prevents the VN05N getting killed by voltage spikes burning it out.
VN05N's can still partially work as they kill themselves.
Failure to check the output waveform as equal spaced using a scope can pass as good a failing VN05N.
It matters not how many diodes are fitted as long as one is present and working.
There is one diode failure mode that can cause problems.
It is unusual but not unknown for a diode to go short circuit.
With a few identical cars and spares my normal fault finding method is part swapping and watch what the fault does.
Hence the ECU offer.

John

[davetherave]

Thanks John, I think I will take you up on the ECU offer, unless anyone has 5l of petrol and a lighter?

I took the ECU apart this afternoon and all looks fine since the Vn05n replacement and diode mod.
Replacement chip is on the right, I left the other one as it looked fine


Here is my internal diode mod from a while back.

All looks ok

I also took apart the ECU of the estate out of curiousity


You can see that exactly the same thing has happened the chip on the right looks to have been getting very hot, this is more apparent when looking at the other side of the board where it has sort of delaminated it a bit. So although this doesnt look completly blown I Think I may replace it anyway, and do the diode mod to this one as well.