Cdti abs fault interrupting engine?
 

Hi all,

We have a 2004 pre facelift 75 tourer cdti. Since we bought the car in October we have had an intermittent issue.

Issue:
Abs fault flashes up on the ipk screen along with the abs fault warning symbol. This lasts mere seconds but when it does flash up, at the same time the engine begins to lose power (as if the throttle has been disconnected). The Speedo can be seen losing speed.
As said, this lasts mere seconds and goes back to normal with no warning lights and abs works fine.
This can occur at complete random it would appear. It can happen at different speeds. Doesn't appear to be affected by bumps in the road or anything like that either, so may rule out a broken wire?

Because the Speedo was being affected, I figured the first thing to try was the driver's side abs sensor and bearing. Also changed the driver's side rear abs sensor and bearing.

This did not cure the fault unfortunately.

I have also recently been using the on board diagnostics and have been monitoring the voltage.

Normal resting voltage with the fuel pump etc priming is around 11.7v

When driving the voltage shown is between 14.2v to 14.5v

When the abs fault has flashed up, it doesn't appear to affect the voltage displayed (tried this to see if there was a sudden drop in voltage and was causing the abs to throw a wobbler but again, this appears not to be the case).

Has anyone come across this issue before?

I don't believe it is related to an abs sensor or wheel bearing as the abs works fine even after it has flashed up and no lights or issues are displayed afterwards..

I also fail to see how an abs fault can interfere either with the throttle or engine (unsure which) but as said above, when the fault flashes up for example at 50mph you feel the sudden loss of power as if you have taken your foot off the throttle and the Speedo begins to go down before springing back into life and the throttle comes back and all is normal like nothing happened.

On ignition the abs system does its self test and the abs light goes out, the abs works, Speedo works fine and the car brakes in a straight line etc.

Thanks
Rob

Is it a manual or auto Rob? And is the speedo dropping to reflect the drop in road speed, or is it just falling away completely?

Hi Mike,

It's a manual

The Speedo drops as if you have taken your foot off the throttle and the car loses momentum (even though your foot is still on the throttle). Because the blip is only a flash of a few seconds at most it never falls away to 0mph merely 5mph before everything goes back to normal.

A shot in the dark and I could be wrong here... But we might have come to realise the issue is apparent when it is raining or been recently raining. if anything was able to get wet, what could it be?
I've checked all the sensor wires in the arches for the usual chaffing but no damage found.

Thanks
Rob

Have you got traction control?

No traction control.

Are the plenum drains clear and ecu not swimming in a leafy bath?

Plenum is all clear and clean with no signs of water marks or anything. Drain under ECU clear. I need to check the others ideally.

This is a strange one. Only thing I can suggest is a T4 session - that might throw up some fault history clues.

Think you could be right there Mike. A t4 will hopefully throw some light on the situation rather than replacing random parts and hoping.

It's certainly an odd situation, the fact the Speedo was affected was the reason behind me changing the drivers sensor and bearing but to no avail.

Without traction control fitted, I'm stumped as to how the engine or throttle is being interrupted

I would go for a momentary electrical blip which could easily affect the engine and ABS at the same time. Intermittent faults are a nightmare, but the battery is on the low side, probably not long before it needs changing and with regard to that, I would check the charging circuit. Bearing in mind that you say it could be wet weather related, I would clean and check all connections, especially on the alternator as an intermittent s/c could explain your problem.