Ok…yet another stupid question as I continue to troubleshoot problems with the “Silver Shed”…
I’m in the process of diagnosing why the drivers side electric window doesn’t work - I’ve identified that both the switch in the cabin AND the motor/regulator in the door are goosed. Not much of a problem - I can source replacements from the “Bay of Flea” pretty easily/cheaply.
However, during the diagnosis process I identified that the car battery (at rest) was putting out just over 20V.
This surprised me - I’m used to measuring voltage on a car (at rest) of maybe 14 or 15V, which then dips to around 12V on cranking the car - I’ve never seen 20V (at rest) on a car.
Is this normal - or could I have identified a potential reason why the window switch and window motor/regulator failed in the first place…?
20 volt’s is abnormal, are you sure you set the meter on the correct scale, and read it correctly? I’m not questioning your knowledge, just that no 12v battery will push out 20 Volts!
A good battery should read 12.5 volts, and the alternator DC supply when the engine is running will be around 14.5 volts, give or take a few decimal places. If you got 20 Volt with the engine running then I would suspect the rectifier on the alternator output has failed - common factor, though unusual to go this way. Could also be the regulation is faulty, not sure on the Mk2, but I know on the Mk2.5 that’s part of the ECU (correct name PCM by the way) I hope not - PCM’s aint cheap.
If your car is a MK2 the alternator does not regulate the battery voltage with an onboard voltage regulator like most cars. The ECU does that on the Mk2".
The reading you have suggest water getting to the ecu via blocked cabin drain that are behind the seatbelt towers.
Lift the passenger carpet, that does not mean feel if the passenger carpet is wet.
Lift it and the ECU is where the passengers feet would be under a aluminium cover held on with anti tamper bolts.
There should be signs of water getting there, you should see rust marks where the wiring conecter is located and maybe dampness.
Thanks for the advice - I’ll take a look at the ECU location for evidence of water ingress, however (just for the avoidance of doubt) the 20V was measured across the battery terminals in the boot with the car ignition OFF. i.e. I wasn’t measuring the 20V at either the alternator or some other circuit, it was directly across the batter terminals.
My immediate reaction when I measured the battery voltage was that either:-
a) I’m doing something wrong here, or
b) My meter isn’t calibrated correctly
So - I took the meter and measured the voltage of some known “good” voltages (both AC and DC) to try and make sure my meter wasn’t overreading. My “calibration” efforts weren’t exactly scientific, but what I did reassured me that the meter I was using was accurate enough to tell the difference between 12V and 20V.
I was worried enough to disconnect the battery whilst I figured out that I am either doing something galactically stupid with my measurements or that there is something wrong with the car…
On a positive note, if it really is chucking out 20V, I could be inadvertently providing the BBQ for the forthcoming M25 Homes for Heroes trip. Make sure you bring your sausages everyone…
In a nutshell it was a faulty meter - although it was a subtle fault that wasn’t immediately obvious.
The fact that I could measure 20V across the battery terminals meant it HAD to be a meter fault (or a dangerously faulty battery!!)
Investigated further and it seems the meter had a fault where it would measure accurately any AC voltage, and any DC voltage BELOW 10V. However, as soon as you applied more than 10V DC to the meter it seemed to “scale up” the reading.
So…meter is now in the bin, new meter ordered and I’ve satisfied myself through repeated and differing tests that the battery is indeed only producing 12V (well, 12.01V to be precise!)
Thanks for the help and sorry for the false alarm!!
PS> It’s a shame really that it wasn’t really putting out 20V - I could have patented my car as the solution to the worlds energy problems!! Apply a 12V battery and get 20V out!!