"Ed" <jag_manR__EM*-0_V_E653@xxxxxxxxxxx> wrote in message
> "Ed" <jag_manR__EM*-0_V_E653@xxxxxxxxxxx> wrote in message
> > I have two digital multimeters of the type typically used by auto
> > techs. One has
> > a 0 resistance reading of 0.6 Ohms, the other 0.3 Ohms. What
> > accuracy can be expected
> > if I measure a resistance in the 0.5-1.0 range and subtract the
> > meter 0 value?
> > TIA
> > Ed
> Thanks to all for their responses.
> I guess I should have given the specific context of my question. The
> resistance in question
> is that of the primary winding of an ignition coil, more specifically
> a 2-coil pack as used on
> the GM 3.8L V6 engine. The spec is 0.5- 0.8 Ohms. By subtracting the
> short resistance of
> the meter(s) I get about 0.4 for each of the 3 coil packs. The dealer
> replaced them because
> they were "out of spec," although the tech didn't record the actual
> readings he got. The
> charge was close to $300, parts and labor. Unfortunately, it didn't
> fix the problem, which
> is an intermittent engine miss. Right now I'm trying to get my facts
> together to approach the
> dealer for possible refund or credit towards a second try at fixing
> the car. At the same time
> I'm under pressure at home to get on over to the Toyota dealer.
It is very difficult to tell if coils are bad just by going by the DC
resistance unless they are open. A few shorted turns will not show up.
Also there is a differance in the hot and cold resistance. The shop was
just doing what many do, relpacing parts and hopeing to hit the correct one.
It takes more than the usual VOM to check the low resistances. I have a
Fluke digital meter that costs several hundred dollars. I don't trust it
for the very low ohm readings. Just the resistance of the leads, the
pressure I put on the probes and a few other things will throw off the
readings a small ammount.