Residual Code Storage

[addo]

At what point, did the PSA engine ECUs start storing codes in a manner that battery disconnection fails to fully erase?

Does it vary by ECU maker?

I'm curious, simply because one of the common things I see on other forums is people saying "Disconnect the battery to clear the codes". Sure it worked on a BX or 405, but I've long had the impression that stuff like Xantias held onto the data beyond a battery disconnect.

Thanks, Adam.

[xmexclusive]

Hi Addo

I think that it just depends on the design age of the ECU.
It revolves round adding to the circuit a serial static memory chip that retains data without power. Needs a software code revision as well.
I think that the first Citroen production use of these chips was around 1992 when they arrived to store security codes in radios and engine ECU based immobilisers. Chip cost was high to start with and storage capacity low.

John

[Thermonuclear]

The Sagem SL-96 Engine ECU in my 1.8 16V Mk2 Xantia has not got a live feed from the battery. When you turn off the ignition, the ECU keeps the double injection relay powered for about 20 secs or so, then remove the ground from the relay. Once this happens, there is no power available to the ECU, so disconnecting the battery has no effect on the Sagem ECU. I think some of the Bosch ECUs have a live feed at all times from the battery, so disconnecting them must have some effect

[xmexclusive]

Static memory chips retain the data indefinately even when power is no longer applied so the power supply situation to the ECU is irrelevant.
Battery disconection will make no difference to the data already stored.
Do not have any engine ECU's handy and open to check but I do have a couple of XM suspension ones. The 1994 H4 suspension ECU has a ST9405 serial static memory chip. The 1992 H3 suspension ECU has a similar device on the pcb. These chips have limited capacity and were only used to have fault data written to them. The ECU microcontroller would read and write the fault data as instructed by its main program. This maincontroller will be programmed to classify fault data according to importance and fault frequency. Some faults will be treated as critical others non critical. The main controller most likely has a power up routine that includes wiping non critical faults from the ECU when it is powered up. On the other hand it would take a Lexia or similar instruction to erase any stored critical faults.
Wiping the non critical faults can sometimes make a temporary correction to a problem but as it does not cure the faulty device running the ECU in the car will quickly clock up enough new fault reports to re-establish the ECU in its previous malfunctioning state.

John

[xantia_v6]

At what point, did the PSA engine ECUs start storing codes in a manner that battery disconnection fails to fully erase?

As a rough rule of thumb, any ECU that has a diagnostic port for OBD will have non-volatile storage of fault codes.
I know that the 1989 vintage XJ12 ECU on my V12 E-type (itself 1972 vintage) has neither.

[spider]

One possible concern about these and I was thinking about this a few years ago too...

With non-volatile memory which is going to hold essential ecu data, now some of these are getting on these nv chips are going to start to fail, I can see this causing problems in the future...