Re: Voltage drop


"Dave Plowman (News)" <dave@xxxxxxxxxxxxxxx> wrote in message
news:4f186f9970dave@xxxxxxxxxxxxxxxxxx
In article <faro61$i89$1$8300dec7@xxxxxxxxxxxxxxxx>,
moray <mtb_hyphen_rules@xxxxxxxxxxxxx> wrote:
I'm sure they vary but some use the full 12 volts to fire the
injectors etc. The regulated volts are kept for the logic side.

I'd expect all car electronics to be protected up to about 40 volts or
so - the rubbish you get on the supply lines is often this high.

Most ECUs will flag a fault once the voltage goes too high, and most
will shut down if it goes above a set limit (about 18V IIRC), in an
attempt to protect itself.

Yes. The trip computer on my car did that when the regulator went
intermittent causing the alternator to be turned hard on - I'd guess to
protect the LED display in the main as these ain't tolerant of overvoltage
- or rather too much current.

Sensors generally run of a regulated 5V source from the ECU, with
solenoids running of battery volatge and the ECU switching the earth.
Petrol injectors can vary from 5V to ignition voltage, with common rail
diesel injectors running at 70V+ generated by the ECU (common rail
diesels fail to run with a battery voltage below about 10.5V, as the
voltage multpilier circuit won't generate enough voltage below that).

ZF autos do much the same - they go into limp home mode if the volts to
them are low for any reason. I'd not considered they might use a voltage
multiplier - but they well might for the solenoids, etc.

Certainly with a semi decent battery fitted, even with a car alternator
that's jammed on full output (they'll only hit about 18volts with a
battery still connected), the electronics should be able to withstand
it for a short period of time.

Yes - I saw just over 17 volts. Of course if the battery had high
impedance for any reason - failed cell etc - it could go much higher.


We maintain vehicles at work that have a battery management system
installed that overrides the alternator regulator to provide a higher
charging rate, and they can fail in such away the alternator remains on
permanent full output. We've never had any cook any electronics yet, as
the batteries or alternator fail first.

My old Rover SD1 has an early EFI system - as well as other electronics.
One day while out it wouldn't turn over - seemed to be a failed or sticky
starter motor solenoid as the battery was fine. I called out the RAC and
was horrified to see him start the car by connecting his slave battery in
series with mine - so 24 volts. And nothing got damaged. I'd guess he'd
used this trick before. And the car - and my heart survived. ;-)


I always 24 volted the 12v starters when stock car racing, I never had a
starter fail in several years of racing.


.

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