[kwlug-disc] OBD-II/OBD2 Diagnostic Scanner

[John Johnson]

On 2014-03-07 11:32, Chris Irwin wrote:
> It's amazing how long this technology has been sitting around in cars, 
> though. I just bought a bluetooth adapter for my 1987 Fiero, (it 
> pre-dates ODB-II, and doesn't work with my existing adapter). I 
> haven't used it yet, though.

As a kid in the 1960s I got hold of a wiring diagram for a car, maybe my 
father's. Even without an ECM the car had built in diagnostics. If one 
is familiar with the cars of the day, when starting, the dashboard 
'idiot' lamps should light up in a particular patter, if all is well. 
IIRC With the ignition on but before cranking the starter the oil lamp 
would come on. The Oil lamp would go out while cranking but the Alt 
light would come on. When the car started both the Oil lamp and the Alt 
lamp would go out. The patten that these lamps (and others) would 
follow, during starting and at other times, would give an indication of 
the health of other systems not just the area associated with their labels.

A 1970s Volvo had an 'idiot' light that would give an indication if a 
headlight or taillight was out. It would do this with a circuit that 
would compare the current in the light circuit on the right side of the 
vehicle with that on the left. And if there was an imbalance, light the 
'idiot' light. The circuit was comprised of a reed switch inside two 
coils. The right side current would flow through one coil. The left side 
current through the other. The coils were arranged such that the 
generated magnetic fields would cancel each other out if all was well. 
If all was not well, the resulting magnetic field would activate the 
reed switch lighting the 'idiot' lamp. (Aside: A similar differential 
measurement is used in electrical ground fault monitors.)

Some early ECM / OBD systems (circa 1980s 1990s) had a mechanism where a 
trigger action, like connecting a wire on the diagnostic plug, would 
cause one of the dashboard lamps to flash in pattern that indicated the 
codes. One would need to have a good eye to count the flashes to 
determine the code. At the same time, codes would be signaled on another 
connector in the diagnostic plug where one could connect a voltmeter. 
And one could count the swings from 0V to 12V. Again, one would need to 
have a good eye to count the voltmeter swings to determine the code.

And while I am writing this, I will toss in a personal trick, learned 
while trying to maintain old cars. To distinguish between electrical 
problems cause by a possible failing battery and a possible failing 
alternator I would, sometimes use the radio. I would set the radio to 
the AM band and tune it off station. If the audio sounded like a 
'motorboat' them there could be a failing diode in the regulator circuit 
in the alternator. (The term 'motorboat' is commonly used in these 
situations.) With a degree of certainty, I would pull the alternator and 
take it to the Parts Source (shill intended) and use their free 
diagnostic system to verify my findings.

Attn: Kwartzlab members: The KWLUG list has a good thread on OBD-II/OBD2 
diagnostic tools.
And in the thread a DIY solution has been mentioned.

Regards
John Johnson