zero volts

[John Day <J.C.C.Day@xxxxxxxxxxxxx>]

Lawrence Hegarty wrote
> I'm trouble shooting my HVAC computer and the shop manual 
> (CarDisk) has many steps called "Ground check" that 
> requires that I test for zero volts at different points.  
> Does it mean I should:
 1. check for a good ground with my continuity tester.

 2. check that when I put the black probe of my volt meter 
> to ground and the red one a the point to be tested that I 
> do not have any voltage (ie. it reads about 0).
 
If you're testing with it switched on and current is 
flowing through the point then you will be able to measure 
a voltage at the supposed ground point if you have a poor 
connection. 
Obviously if you are checking with it switched off you will 
see zero volts if its connected to ground or not. 

I reckon it's always worth checking that the point has 
good continuity to ground. Ground is ideally the earth 
strap at the battery.  
If you measure the continuity be a bit careful of the 
continuity measurement on the meter.  Usually they go bleep 
if they see continuity but they may reckon that continuity 
is anything less than maybe 10 ohms.  Car electrics use 
relatively low voltage and high current so need good 
connections. e.g. a 6 amp current (eg headlight bulb) 
through even a 1 ohm resistance will drop 6 Volts. (Ohm's 
law V=IR). Best to measure the continuity using the Ohms 
resistance scale on the meter. But first put both probes 
onto the same point just to see how close to zero your 
meter really reads, there is always some offset and contact 
resistance. 

{All the electrical faults I've ever had on 33s have been 
bad earths ( sorry "grounds" in American) somewhere.}

Good luck 
John




(Opinions expressed are not those of University of Bristol)
- ----------------------
John Day
J.C.C.Day@domain.elided
Interface Analysis Centre
University of Bristol
121 St. Michael's Hill
Bristol BS2 8BS
UK
tel 44 (0)117 9255666
fax 44 (0)117 9255646

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