Electrovalve diodes - Myth or real?

xmexclusive · Post by **xmexclusive** » 14 May 2009, 11:25

Hi All

Have recently been trawling the site to try to add to my understanding and repair ability of XM suspension electronics. All my XM's use H4 or H5 ECU's and I have a few spares so up till now I have survived by swapping ECU's when needed to find a good worker. Now need to get a bit more professional about it so I have got in a supply of the VN05N solid state relays ready for repairing the duff ECU's. The spec sheets for these devices were very interesting as they show that the protective diodes for the electrovalve supply were built into the devices and have to fail before the VN05N starts to overheat and kill itself. Searcing the Citroen workshop manual it seems that the design of the electrovalve was changed about 1998 (for XM's) to incorporate a second protective diode encapsulated in the electrovalve body. Adding extra protective diodes makes no difference to the circuit function unless the original(s) have already failed so putting extra 1N4006's on the wiring or soldered to the ECU pcb seems well worth doing.

With a number of cars and extra doubtful bits I though it would be worth making up a test lead that I could move from car to car and also easily plug spare ECU's into. I have spare white and black ECU plugs and sockets so was going to make up short jumper leads with most wires straight through but the 1N4006 diodes added and a set of electrovalve function led's on wires long enough to reach the bottom of the windscreen. I was also thinking about putting in a switch so that I could swap the activate signals between the two electro valves (test on static car only) so as to be able to check to see if a failure is caused by the electrovalve or the VN05N driver. I assume that if it is a faulty driver the fault will move ends of the car while if a faulty electrovalve it will stay put. Was also wondering about adding switches, links or electronics to over-ride or simulate the various car sensors but will leave that for now.

I realise that perhaps a Lexia check might give me all the information but do not have access to one and there is a lot of options to check and note. Doing it this way seems to be the sort of simple electronics that I can understand, put together easily and not missinterpret. It would also be great to sort out which of the VN05N's do not need changing and I can add the diodes to the pcbs of those I open to repair.

Would appreciate anyones thoughts, ideas and words of caution.

John

Post by **CitroJim** » 14 May 2009, 14:00

Not myth at all John. The diodes are there not so much for protection but to suppress the back-emf that occurs on switching the electrovalves; a back-emf is generated by any coil when its magnetic field collapases, the principle of coil ignition. Any relay (and that is what an electrovalve is, electronically) when driven by a semiconductor driver will always have a reverse-connected diode across the coil to suppress the back-emf on switching to ensure a sharp clean change of magnetic state and to protect, to an extent, the driver transistor.

The diodes in the VN05s themselves are strictly for its protection only. Some Hydractive ECU's have the pads on the circuit board for the back-emf diodes but usually they are not fitted.

Strictly, the outboard diodes we fit to replace the ones embedded in the colis of the electrovalves should really be placed as near to the coil as possible to be fully effective.

I believe DickieG has successfully replaced VN05s but one thing you will certainly need is a Lexia to clear any faults stored in the ECU and if the VN05 pops, there will be recorded faults. The ECU will not work properly unless all recorded faults are clear.

The "breakout box" idea is a good one and that will then allow you to "look" at the electrovalve signals with an oscilloscope and this will be very telling.

The signal going to the valves is not a constant 12V as this would quickly burn out the coil. The valves are initially operated with a 500mS pulse of 12V and then a holding current is applied which is in effect a 1KHz squarewave which is why you can hear them humming when operated. The squarewave thus gives a mean voltage which is sufficient to hold the valve in it's operated condition but is insufficient to initially operate it.

If you inspect the waveform on a 'scope, the 1KHz waveform should look clean with sharply defined and very rapid rise/fall times with no overshoot. A waveform applied to a valve with a faulty back-emf diode will look fuzzy, the squarewaves will not be well defined and you'll see voltage overshoots on the tips of the squarewave where it switches. The fuzzyness and overshoots you see are a result of the back-emf generated by the collapsing magnetic filed of the coil interfering with the drive signal and distorting it.

Hope that helps John and apologies if I've made you suck a few electronic eggs there

xmexclusive · Post by **xmexclusive** » 14 May 2009, 15:01

Thanks Jim

You have no idea how valuable I am finding the responses to my questions. Any information is useful to me as it collects together somewhere that I can refer to it again. Age does make for forgetfulness so an easy reference is such a boon.

On the diodes issue some of us have sectioned a couple of early Mk2 XM electrovalves specifically looking for diodes and failed to find any. The spec sheet for the VN05N's shows that these devices were specifically designed to drive electrovalves. (Another one of those devices I think with a specific Citroen/Valeo number coding to make finding alternatives difficult). The spec also seems to indicate that the device diodes were intended to be capable driving an electrovalve without a diode on the coil. I seem to recall that I saw the reference to the modified electrovalve incorporating the diode is in the News Flash section of the XM workshop manual. The part number did not change as there was no restriction on use of the new item. You mentioned Pads on the ECU pcb's. Have you any links to pcb photos please. The H4 pcb I have open at present does not seem to have them just an extra set of unused pads to take a third VN05N. Not aware of any XM that uses 3 electrovalves though.

The checking and clearing with a Lexia. Does it have to be done on an ECU in the car or can it be done with the ECU on the bench?
In the longer term is there any reason why I should not locate the memory chip on the PCB, read its contents and refill it with zeros or the standard code for a clean ECU in much the same way that the experts do with coded radios or immobiliser codes in ECU's.

John

FrenchLeave · Post by **FrenchLeave** » 14 May 2009, 16:36

Hi John, another awesome posting from you.

If I understand you correctly the Mackay box of tricks that I fitted as a safety measure to my V6 is superfluous and would be better fitted to the '95 TCT?

Derek

DickieG · Post by **DickieG** » 14 May 2009, 17:38

As Jim states I did replace a VN05 in my estate, when it blew the ECU logged just about every possible fault code which could only be cleared by using a Lexia. As for clearing codes on a bench I suppose like most things in life its possible but you'd have to make leads up then find a way of stopping the Lexia re-checking for further faults so its not worth the hassle in my opinion. Simply send the ECU to someone who has a Lexia/Hydractive car for them to plug the ECU into their car etc.

One thing I discovered is that S2 Xantia's have a different Hydractive ECU to S1's and they are not compatible for swapping. You may have the same thing on XM's, look for the EOBD diagnostic socket for the later version.

From my understanding of things fitting extra diodes may prevent the VN05 being blown but once they have blown simply fitting new diodes is akin to shutting the barn door after the horse has bolted.

If you're a member of the CCC I did cover this very subject in my column last year so have a look through some back issues.

xmexclusive · Post by **xmexclusive** » 14 May 2009, 19:04

Hi Derek

Do not remove the kit from your V6. Get another one from Roy Makay for the Estate or fit the extra diodes yourself. At worst they will protect the VN05N's and electrovalves and may even return them to full working order. As I understand it extra diodes do not affect the circuit operation and heavy duty 1N4006's are most unlikely to burn out so are well worth the cost. Roys kit is less that the cost of a set of consumables to do an engine service.

John

xmexclusive · Post by **xmexclusive** » 14 May 2009, 19:33

Hi DickieG

Will look back through the CCC mags, thanks.
So far I have managed a bit of component identification. XM's seem to have had a fair number of different suspension ECU's over the years. I have seen H2 and H3's (metal case) fitted to Mk1's. Mk2's got H4's and H5's (Finned Alloy case). They look identical to Xantia ones but up to version H4 they were all factory coded using car type and model specific suspension tables. Individual part numbers on the case make it easy to separate out the different types. For the H5 version Citroen produced a universal unprogrammed ECU so they all have the same part number (Xantia included) but are not interchangable between cars. These can then be programmed once only with car specific suspension parameters. The instructions are that the car type is then to be marked on the outside of the case.
It seems that I will have to get access to a Lexia somehow as it is around a dozen or so ECU's that I want to sort through. I was hoping that someone knew which fault codes were permanent and needed Lexia resetting and which ones were transitory and self clearing as most of the TEVES ABS ones are.

John

DickieG · Post by **DickieG** » 14 May 2009, 19:52

As far as I'm aware none of the fault codes reset without a Lexia session, I have a Lexia and I'm willing to help you with this if I'm not too far away form you and you want to pop over. Alternatively will you be attending any of the CCC meets such as the DS/SM or national as we could do the lot there.

Post by **xantia_v6** » 14 May 2009, 20:23

Jim read the codes on my Mk1 Xantia a couple of weeks ago, and the only ineresting code was for an intermittant rear electovalve. Now how does the ECU know the electovalve is faulty?

The fault code reappeared after switching between hard and soft modes a few times.

I fitted a pair of diodes (28p for 2 from Maplins), and I think that the rear electrovalve is operating more reliably (but it is hard to be sure, because it was mostly OK before).

DickieG · Post by **DickieG** » 14 May 2009, 20:52

xantia_v6 wrote:Jim read the codes on my Mk1 Xantia a couple of weeks ago, and the only ineresting code was for an intermittant rear electovalve. Now how does the ECU know the electovalve is faulty?

I don't know how it works it out but it will state whether the fault is permanent/present or intermittent, in addition on S2 Xantia's the Hydractive ECU records the number of times it has been accessed.

On later cars the ECU/BSi records the date time it was accessed and what was done.

Post by **Peter.N.** » 14 May 2009, 21:13

Fitting the diodes will under most circumstances restore the ECU operation but the fault conditions when, presumably the diodes in the chip have failed are interesting. On opening a door you hear the solonoids 'clonk' and the whine as normal but no soft mode, when the electrovalves switch off again - no sound but on one occaision the suspension dropped and it depressurised the centre sphere! something I have been trying to do since I have been running them. I know because I changed it.

I think that what is happening is that the initial 12v pulse is opening the valve but the pulsed signal is not providing enough current to keep the valve open, so it is possibly sitting somewhere between the open and closed position. Fitting a couple of diodes on the board rectified the situation.

Your thoughts?

Peter

Post by **CitroJim** » 14 May 2009, 21:31

xantia_v6 wrote:Jim read the codes on my Mk1 Xantia a couple of weeks ago, and the only ineresting code was for an intermittant rear electovalve. Now how does the ECU know the electovalve is faulty?

I believe the ECU reads the current drawn by the electrovalve and if out of specification, logs a fault. Corroded or otherwise poor connections at the electrovalves can cause this.

Peter.N. wrote: think that what is happening is that the initial 12v pulse is opening the valve but the pulsed signal is not providing enough current to keep the valve open, so it is possibly sitting somewhere between the open and closed position.

I believe you're dead right Peter. Iff you look at the waveforms, an electrovalve with a duff doide will show a rough, ragged waveform and you can hear it as well as a sort of fuzzy, wavering and almost intermittent hum rather than a nice, smooth hum.

Interesting what you say about having dissected a duff coil looking for the diode. They'll be very small and my guess is it willr eside just under the connector pins.

This is turning out a most interesting thread

Post by **CitroJim** » 14 May 2009, 22:17

xmexclusive wrote:The H4 pcb I have open at present does not seem to have them just an extra set of unused pads to take a third VN05N. Not aware of any XM that uses 3 electrovalves though.

I expect there is only one board for all Hydractive ECUs and the unused set of pads are for the roll stiffness electrovalve when that board is used in an Activa; the boards will be common across the XM and Xantia.

Electronics manufacturers often use one standard board and in a "budget" model will leave large swathes of a given board unpopulated. My Freeview box is a classic example; it has the same PCB as the posher model with a smart viewing card for subscription channels but in my basic model, all that area of the board is a barren wasteland.

xmexclusive · Post by **xmexclusive** » 15 May 2009, 00:15

Hi Jim

I got that bit about a single spare set of VN05N lands on the H4 pcb wrong!!
There is a space and pad/tracks for an extra one in between the two that are installed at one end of the board but there is also another set pad/tracks for an identically packaged 5 pin device in the spare space at the other end of the board. I think the electrovalve sectioning was fairly comprehensive but am loath to waste another one checking for now. I did a search over at the unit this evening to see what suspension ECU's I could find. Have brought home four Mk1 XM ones and think there is at least 3 different types of those. I will start a listing of the part numbers and identification features before I start opening them up to see the internal differences and take some photos of the PCB's.

John

DickieG · Post by **DickieG** » 15 May 2009, 06:30

The spare slot for a VN05N on Xantia ECU's is used by Activa's for the additional sphere balancing block.