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The car is a Volvo S80, produced in 1998 for the European market.

Under some non-reproducible condition (it is in fact so non-reproducible that it appears random), emergency lights in the dashboard will suddenly indicate brake failure and the speedometer as well as the RPM gauge will immediately die (as in go to zero as if the engine is not turned on anymore). The backlights also go dark.

The kicker? The brakes still work and the engine is still running (and operates quite fine, at least from the driver's standpoint). It is possible to drive however long and far as needed, and the car does not feel or sound different in any way. However, if you turn off the ignition you won't be able to start the engine again. I assume it is the brake malfunction error that prevents the ignition.

Unplugging the battery for 30 seconds and replugging it removes all indicators of error or malfunction and the car starts normally. The error situation as described above may arise again later at some random interval - it may be as little as 2 days later or it may be several months later without any incident inbetween.

It's been in and out of the shop and no one is any wiser for it. The car has since this problem first arose had several services, so I assume all filters have been changed at least once. To my knowledge, no sensors have been replaced yet.

Any ideas on where to look or things to replace? Other troubleshooting tips?

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    A generic suggestion -- a loose earthing strap caused similar symptoms for me a few years ago after the garage forgot to fully tighten it.
    – Chris H
    Feb 16, 2016 at 15:32

1 Answer 1

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The S80 from this period had a high failure rate of ABS modules. Your symptoms seem to match this problem. A failing ABS module will show brake errors and corrupt instrument panel data (lights, speed, temperature).

What would help if you read the fault codes before disconnecting the battery, as doing so clears them before you can take a look.

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  • I think the error codes have been read at the shop, but I am not completely sure. I'll check on that. In the event that they haven't been read (properly), do you know if there is easily available equipment I can buy to read them myself?
    – Vegard
    Feb 16, 2016 at 7:05
  • They attempted to read the error codes at the shop, but were unsuccesful.
    – Vegard
    Feb 16, 2016 at 7:39
  • @Vegard The usual tool for reading error codes for Volvos is VIDA DICE. Original equipment will be expensive, as it is intended for mechanics. Counterfeit products can be found online, they are cheaper but troublesome. Usually people manage to get them to work, though.
    – I have no idea what I'm doing
    Feb 17, 2016 at 8:12
  • However I cannot guarantee that you will see the codes that you hope to see. As the ABS clearly didn't fail completely you might see a few errors that don't exactly pinpoint the problem.
    – I have no idea what I'm doing
    Feb 17, 2016 at 8:13

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