P38 Speedo Head

[Simon_CSK]

I have an issue with my Speedo in that for some reason it doesn't work. It is electrical and hasn't worked for a number of years. It did however flicker into life for 0.6 miles today and got me thinking that perhaps I should get it functioning.

I have onother head which is from a (Very) high milage ar and whilst I accept the mileage should make no difference as I don't intend to sell it I don't really want that milage on my MOT history.

When I opened up the speedo I noticed that the odometer is on a separate card connected with a ribbon wire. Does anyone know if the odometer memory is in this card and can this be interchanged with my own card to keep my mileage correct?

[Puffernutter]

I could be wrong, but I suspect that mileage is stored in the BeCM anyway? That is where the pulses will be coming into and then the readings just transmitted to the speedo head? Happy to be corrected!

Cheers

Peter

[Simon_CSK]

Peter

Correct to some degree however I did change to another speedo head which had 125k on it, 7k higher than my current head. This one also had faults however when I changed back the speedo reading went back to my original the only difference now when I start up I get a message that states I have an odometer error. 

[elbekko]

3 hours ago, Puffernutter said:

I could be wrong, but I suspect that mileage is stored in the BeCM anyway? That is where the pulses will be coming into and then the readings just transmitted to the speedo head? Happy to be corrected!

Cheers

Peter

It's stored on both, and will sync both to the highest of the two.

[Escape]

As Ben said, it should sync both the the highest value. But I have encountered cars that kept giving the odometer error message, despite the value in the BECM being the same as on the dash.

As for the original question, it looks like the ribbon is for the entire message display, so unlikely the value is stored there.

[Simon_CSK]

50 minutes ago, Escape said:

As Ben said, it should sync both the the highest value. But I have encountered cars that kept giving the odometer error message, despite the value in the BECM being the same as on the dash.

As for the original question, it looks like the ribbon is for the entire message display, so unlikely the value is stored there.

Am I right in thinking the mileage must be stored on a chip? 

[Escape]

I guess so, it must be in one of the ICs on the board.

[Simon_CSK]

3 minutes ago, Escape said:

I guess so, it must be in one of the ICs on the board.

There are 3 chips on the separate odometer board. What are the chances?

[Escape]

Do you have an old one to try? If you disconnect plug C1280 from the BECM, the instruments should get power so you'll see a reading, but communication with the BECM is not possible. If I'm reading the diagrams correctly that is (Rave, electrical circuits 99, E1.3).

[Ed Poore]

Hmmm., if only there was someone with a spare BECM and instrument cluster 

[Simon_CSK]

6 hours ago, Escape said:

Do you have an old one to try?

Only have the one that is the donor and the broken one. Neither of which I really want to break any more that they are already.

[Ed Poore]

I was referring to the fact that I've got a completely broken for (other people's) spares P38 here. The BECM and instrument clusters are still here, happy to send them up at cost if you wanted?

[Simon_CSK]

Ed Have pm'd you

[Ed Poore]

I love remote debugging - to fill in a little bit of the PM between Simon and myself.

I came across an interesting post on LandyZone:

Quote

I considered long and hard before eventually deciding to post up on this thread. As many of you will be all to aware of the criticism i typically face on every post i seem to make and sadly this now means i am really now a quite infrequent visitor here and am certainly quite select on offering any further contribution to this forum.

However i certainly cannot stand by and watch any member potentially be ripped of for the want of bit a little bit of the knowledge i happen to posses.

The information i am about to provide may well have been of been of very high value to anyone of iffy tendencies 10 years ago, but the P38 is now quite an old vehicle and anyone buying one, will and should certainly consider its condition over and above whatever the odometer might show.

The Mileage of a P38 is indeed kept in both the BeCM and Instrument cluster and is often stored as the KM equivalent (Simply multiply the miles by .6214 to get the KM's) I really now forget which the P38 uses. If you change either up or down, the partner system will eventually auto update to whichever holds the larger of the two values, leaving those swapping either around with only all parts having the largest value in them.

This often happens instantly, otherwise, you may see a message in the Ipacks screen saying "ODO UPDATE" until it does. There is a diagnostic function "Odo re sync" to pre force this "to the highest value" synchronisation on demand.

However there is a very top secret trick that this ebay guy is exploiting with his so called Magic Instrument pack and i discovered over 10 years ago.
While the BeCM keeps it's value inside a secured microprocessor, that to date only we have been able to crack and provide direct access to, the Instrument Pack stores it's value in a totally industry standard parallel eeprom that can easily be read and edited by even the most basic of programmers. It really is not rocket science as the only thing this EEPROM chip contains is this value.

You have to de solder this chip to do this. But it is old through hole technology,
So that ain't any problem to anyone armed with little more than a regular solder sucker.

Normal protocol dictates that you disconnect the vehicles battery, remove the ipack, then its EEPROM, edit its contents then refit the EEPROM, the Ipack and reconnect the battery. But then as explained, the Ipack will just update to whatever the BeCM had in it. Putting you right back to square one.

However, the big secret, and the real trick to this Guys offering is that if you temporarily refit the Instrument pack without the EEPROM chip in at all, and power up the vehicle, the BeCM magically resets its count to zero. So the loan Magic Instrument pack is nothing more that one without an EEPROM chip in.

Having then edited the instrument packs EEPROM to whatever value you wish and then re fitting it, the BeCM will update itself from zero to whatever the Instrument pack has in it.

 

So from the photos above it looks like the CSI chip (CAT28C16AP I-20) is a parallel EEPROM so it looks like for Simon's instance, removing this chip, clearing the EEPROM and reinserting it will update the EEPROM from the BECM and job done. If you needed up to update the BECM then you can do the trick mentioned above.

I'll find the instrument cluster I have and remove it and see if I can read the EEPROM contents to at least verify some of the above process.

[Simon_CSK]

2 hours ago, Ed Poore said:

I love remote debugging - to fill in a little bit of the PM between Simon and myself.

I came across an interesting post on LandyZone:

So from the photos above it looks like the CSI chip (CAT28C16AP I-20) is a parallel EEPROM so it looks like for Simon's instance, removing this chip, clearing the EEPROM and reinserting it will update the EEPROM from the BECM and job done. If you needed up to update the BECM then you can do the trick mentioned above.

I'll find the instrument cluster I have and remove it and see if I can read the EEPROM contents to at least verify some of the above process.

So Ed to those who haven't a clue about electronics it is the big chip in the centre of my first photo which stores the Odometer and that can be removed to bu**er up the BCEM and reset the mileage to whatever.

That looks doable and should be able to take the chip out of the high mileage, reset the BECM to 0 insert my old clocks to update to existing mileage then reinsert the chip in the high mileage clocks which are now zero and the BECM should reset to the existing mileage.

[Ed Poore]

Not quite (if I've followed your procedure correctly).

Your BECM and "old" clocks are in sync, correct?

If you do what you propose, removing the high mileage chip will reset the BECM to zero. But when you put the high mileage chip back in it's contents have not been modified (an EEPROM persists its memory during power loss) so the BECM will pull the high mileage data from the instrument chip.

For what you are trying to achieve you need to reset the EEPROM on the high mileage instrument cluster. The good news, according to that website is that the only thing that chip stores is the mileage. I was going to see if we have a programmer in the office that can read this or if not I can make up something easily enough. I can then read the contents of my "sacrificial" lamb and see if I can see the mileage on it. If so then I can simply clear the EEPROM and send it to you. Provided it has a lower mileage than your BECM then when you plug it in the mileage should be updated to the BECM one because it is higher.

Make sense?

[Simon_CSK]

4 minutes ago, Ed Poore said:

Not quite (if I've followed your procedure correctly).

Your BECM and "old" clocks are in sync, correct?

If you do what you propose, removing the high mileage chip will reset the BECM to zero. But when you put the high mileage chip back in it's contents have not been modified (an EEPROM persists its memory during power loss) so the BECM will pull the high mileage data from the instrument chip.

For what you are trying to achieve you need to reset the EEPROM on the high mileage instrument cluster. The good news, according to that website is that the only thing that chip stores is the mileage. I was going to see if we have a programmer in the office that can read this or if not I can make up something easily enough. I can then read the contents of my "sacrificial" lamb and see if I can see the mileage on it. If so then I can simply clear the EEPROM and send it to you. Provided it has a lower mileage than your BECM then when you plug it in the mileage should be updated to the BECM one because it is higher.

Make sense?

Yes . Makes total sense.

I think it is the chip that stores the info. so simply transferring the chip from my old clocks to the high mileage clocks should be enough. Depending on my soldering skills. 

[Ed Poore]

That's one way to try it.

[XJC]

On 5/27/2020 at 11:53 PM, Simon_CSK said:

Yes . Makes total sense.

I think it is the chip that stores the info. so simply transferring the chip from my old clocks to the high mileage clocks should be enough. Depending on my soldering skills. 

Did you ever attempt this? Considering giving it a go myself.

[Simon_CSK]

Not yet but the BECM needs repaired after a trip to the valeters so has been parked up while I play with some of my other cars. 

[XJC]

Well based on this thread I'll be giving something similar a go over the next week. Will let you know how it goes!