Post
by White Exec » Wed Apr 06, 2016 7:30 pm
HI Eddie,
Tip, about using test-meters:
Good at checking the presence of a voltage on a wire or pin, but NOT at telling you whether the connection is capable of delivering enough current (power) to make a device work.
Why? Because
- the meter draws very little current, in order to display the voltage (probably a just few milliamps)
- if the circuit contains a bit of resistance (eg dirty switch contacts, or damaged wiring), the few milliamps will probably pass through this resistance without too much of a problem, but
- a large current (enough to make a headlamp, or washer motor, etc) will not pass through the resistance, and the device (lamp, motor) will not operate.
How to test whether a circuit/wiring/switch will deliver decent current?
- Instead of connecting a test-meter, connect a 12v test lamp instead, between test point and a known good earth. This will draw meaningful current. If there is resistance in the circuit, the lamp will either not light, or be dim.
- What wattage of lamp to use? Ideally the lamp should draw roughly the same sort of current that the device you're testing would, if working properly: few examples...
Use a known good 5W sidelight bulb to test a sidelight circuit or interior light
Use a 21W indicator lamp for checking out motor and relay circuits
Use a good 55W headlamp bulb for checking out headlight and glowplug circuits
The worst you could do is blow a fuse by using too high a wattage lamp. If in doubt, use a 21W lamp - that'll do for most circuits, except delicate electronic ones.
So, to summarise:
- You can have voltage available at a point, but not necessarily anything much in the way of usable current.
- What stops the current "getting through" is resistance - like dirty contacts, or failing switches.
High current circuits are more prone to failure, as they put electrical strain on wiring, contacts, fuses and fuseholders, by generating heat, and contact arcing.
Hope helpful. Sorry for the physics lesson.
PS. Your 3.44v delivery voltage to that pump sounds very odd. Would have thought it should be more like battery voltage (12v). Possible that the measurement was taken between that connection and a less-than-good earth point? Try the test lamp.
Don't forget to check out the earth/ground connection of a device - most motors and lamps will have one of their terminals grounded. If necessary, run a temporary ground connection in, from a known good earth point.
Chris
1996 XM 2.5TD Exclusive RP7165 Polar White
1992 BX19D Millesime RP5800 Sable
1989 BX19RD Delage Red Deceased; 1998 ZX 1.9D Avantage auto Triton Green Company car 1998..2001; 2001 Xantia 1.8i auto Wicked Red Company car 2001..2003