DMM and O2 sensor

[Scot Sealander]

Chief wrote:

> Ok, this may be a bonehead ? but here goes:
> 
> When measuring the voltage from an O2 sensor with a digital multimeter, 
> does it give the true reading or a slightly lower voltage.

It is slightly lower.  When first learning how ECM's worked, I tried to 
do this with a Fluke voltmeter and an open circuit O2 input to the ECM.  
The voltmeter was rock steady at some lower voltage.  The ECM was sending 
a varying voltage out the ALDL, along with O2 crosscounts!

The VOM was sampling the O2 input, and had a lower input impedance during 
sample time that lowered the measured voltage.  While not sampling, the 
VOM had a much higher input impedance that let the open circuit voltage 
rise.  This difference showed up in the ECM's measured O2 voltage, as it 
samples the O2 sensor much more often than the VOM does.


> The reason I'm asking is that the service manual says that the reading 
> should be .45 V but "may read as low as .32 V on a digital voltmeter"

It has been my experience that you will also see this.

Scot Sealander    Sealand at clarityconnect.com