Noisy regulator drop test!
 

After cleaning all the grounds, new Bosch battery I come to check the Alternator I did the routine checks all were fine, I the did two quite definitive tests to see why I kept getting a P0340 code under hard acceleration on the alternater.
My CEL would light up, I'd do a test and DTC would read P0340 and P0340 pending code. 4 or 5 journeys later the CEL would go out. Rescan for codes and codes are still stored pending and other.

Checked all the wiring harness from the cam position sensor, did the test with multimeter, crank sensor also, all good.

As mentioned above I did a two tests on the alternater a "positive circuit drop test" @2000RPM and it read 0.54 and all under load (ACC's all on)

I then did a "Negative circuit drop" @2000rpm and it read 0.12 and all under load (ACC's on)

The positive battery lead all under load gets rather hot.

Now from what I have discovered is a failing voltage regulator can send signal noise and disrupt the wiring chassis it runs along this throwing a P0340 code.
I hope this may help others. I'll post back an update when I've reconned the alternater.

2003 Volvo S40 1.8 (Non GDI) B4184S2 123K MILES original alternator.


Positive or negative tests the accepted zones are 0.03 not above and 0.4 not below.