Engine: D5244: Voltage drop

[john f]

Hi all
A baffler for you all (and me)
I have noticed on my 2007 phase 2 v70 185 D5 that when pulling away from standstill the headlamps dim very slightly for a second and then all is ok
Living in a rural area and driving country lanes highlights this
I have plugged into my cigarette socket a cheap volt tester which on idle delivers in excess of 14v and driving normally runs to 14.3 volts at times , How accurate this is I do not know
This morning while car was stationary delivering 14v+ I pressed every button and switch with no results until I pushed in the clutch pedal and released it
on release the volts dropped to 13v and held there for a few seconds and then back to 14v I tried again with same result on clutch pedal release
I then drive the car and while moving dipped the clutch and released but the voltage stayed at 14v
The volts drop when the car is at standstill or pulling away from standstill and clutch is released in gear or not the only electrical item I think is connected to the clutch pedal is the cruise control switch but why is there no fluctuation when rolling has got me

John F

[cheshired5]

It's certainly an odd one.
I would begin by disconnecting the clutch position sensor and see if that makes any difference.

[john f]

Seems like the only possible thing it could be
Why it doesnt do it while the wheels are rolling though is most confusing
Speed monitor related ??
No cruise control at stationary or below a certain MPH ??

John F

[Brendan W]

Could the brake lights be blinking as the clutch is going in and out ?
The brake light switch on the earlier cars sits on the same bracketry and if on the edge will flash the lights at gear changes.

[john f]

Hi
Just had somebody look at the brake lights as I repeatedly hit the clutch
No flicker I,m afraid
Voltage drop only happens when stationary when releasing clutch or just pulling off from staionary releasing clutch

John F

[john f]

Disconnected clutch position sensor (electric plug)
No voltage drop
Still baffled !!


John F

[Dancake]

A failed (internally short circuited) clutch position sensor can cause voltage spikes/discrepancies if it is of the potentiometer variety. My guess is that the sensor is the culprit.

[cheshired5]

The clutch position sensor is a variable resistor.
Approx 3300ohms extended down to 200ohms contracted.
I'm guessing that something like a 5V signal is fed into the sensor and dependant on the return voltage, the ecu knows the clutch position.

If it's no longer variably resisting and heading towards short circuit, it has the potential to cause voltage issues.
That's my take on it anyway.

Unclip it and test it....... more out of curiosity really because it's likely junk.

[john f]

Just order a new part I will fit it next week and let you know

Thanks

John F

[john f]

New Switch Fitted

Voltage Drop as before


John F