Johnny G Pipe

Does the 1990 C4 have lateral accelerometer? Checked search, where Adrian suggests that it does, but 'nothing to do with ABS'. I know Tips need 'em, but why does a C4 or 2 need one. There is no PSM/ESP variant on these cars. Why does it need lateral acceleration info.?


Reasons for asking as follows, if anyones got a couple of minutes to kill..

My ABS/differential lights are back, bless 'em. Same as before - ABS activated on stops under 24mph, then ABS removed, with lights and buzzer. Sometimes lights and buzzer with sharp turns, left or right.

Temporarily cured by a recent wet trackday, with four (4) offs, one of which being a spin. Then the lights and buzzer came back, on a sharp turn in town, and its back to every day. Black tape over the warn lights time..

A local auto-sparkie checked the wheel sensors as OK, and checked the OPC repair to the osf sensor earth, which initially caused the problem.
My theory? A stuck longitudinal accelerometer. Freed up at the track, but back to sticking. Explains why problem occurs some times on sharp turns.
zzzZZZ. Sorry, this is a little boring. Anyone got any better ideas?
BTW The car goes into an esteemed independent this week for MOT and service. He has a hammer.

Adrian

Dear Johnny,
I do not suggest that a C4 has a lateral accelerometer I clearly state that it does because there is one fitted.
The lateral accelerometer is used by the traction control system to determine how much the rear differential lock is to be locked up for how long.

The lateral accelerometer is used to provide transmission load information in the tiptronic.

The C4 is fitted with traction control or PSM but an early version of it. PSM is just traction control.

I would say you have damaged your lateral differential lock or the longitudinal lock. Spins in C4s can easily damage these components because they are supposed to stop you spinning and they are being overpowered. This is why you should disconnect the system when on the race track. Your symptoms suggest a failed lock because you were in a sharp turn the lock was required and it did not do the job and the system failed itself. The hammer will not show up a failed diff lock. There is a specific process for measuring and torque checking the lock and the plates.

Ciao,
Adrian
964C4

TMH

Adrian,
What is the process for performing this? Hopefully a simple disconnect of a plug, which is somewhat readily accessible? Do you feel that your above statement would also apply to auto-cross, or would be more appropriate for open race track driving only?

Thanks,
Tom

Adrian

Dear Tom,
You just pull the plug on the control unit in the luggage compartment. Tie it back to make sure it does not flop around.
I would disconnect the system which includes ABS when you do this whenever you are stressing the car and likely to have a spin if you pushg it too hard on a regular basis. One spin is not going to hurt anything but constant abuse is going to stress this stuff out.
Ciao,
Adrian
964C4

TMH

Adrian,

Thanks much for your reply, directions, suggestions. As always, well thought out and infinitely helpful!

Tom

Johnny G Pipe

Thanks Adrian, great advice about disconnecting this stuff at the track.

I never knew that the the C4 had active traction control like PSM, albeit an early version. Sounds like it varies the rear differential lock in sideways stress situations? (apologies for the alliteration) I never knew the car was so clever.
My spin was on mainly on very wet grass and mud, and so I hope there is no damage. The other offs were straight understeers into the gravel traps ( I kept panicking and putting 'both feet in' - but managed to get out of the habit, and get the drive back through the wheels)

Phoned my my mechanic today - he keeps getting a fault reading at OSF sensor, even though the sensor itself tests normal. He's replacing the harness, which was in a mess since the OPC's efforts.

Hope I don't have to delve into the diff locks..

Adrian

Dear John,
The C4 traction control system gives you a limited slip differential at the rear, traction transfer (torque transfer) front to rear and when the two diff locks work together it is an anti-spin device. Causes massive understeer. However it does not work on the grass.
The reason you disconnect for th track apart from the stress is that it will slow you down.
There is one little trick you can use (requires practice and a seat of the pants feel) to leave the system connected and take advantage of the limited slip rear diff at the rear but not allow traction transfer forwards or rearwards. If you feel the understeer coming on you tap the brake. This will release the traction control. As I said this takes practice and you have to learn to feel the diff locks start to engage. This is best done in the wet or on wet grass. On the grass is a lot of fun actually.
Ciao,
Adrian
964C4