Date: Sun, 17 Jul 2011 14:05:46 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: ground wire ohms
In-Reply-To: <1310917469.49517.YahooMailClassic@web180512.mail.gq1.yahoo .com>
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At 11:44 AM 7/17/2011, Gus Cairo wrote:
>On an 1982 aircooled Calif Specs checking the ohms on the ground
>wire from the block through the harness to the fuel pump double
>relay, I get a reading of 9.67 ohms is that correct or is there
>problems with that wire Thanks Gus
Gus, none of the wires or connections in the van have enough
resistance to make it practical to check them with an ordinary
ohmmeter. Ten feet of 16-ga wire has a resistance of about fifty
milliohms. It's also necessary with an ohmmeter to disconnect the
device being tested from the circuit it's in and test it in
isolation. So the short answer is no, that's not normal. Even one
ohm would indicate a serious problem, and the higher the current in
the circuit, the more serious.
However there's another problem, which is that the ground wire (85
terminal) from that relay goes into the ECU (97.13 track 21) and not
directly to ground. So what exactly are you measuring? It's
possible that there's an active circuit inside the ECU that is
interfering with your ohmmeter reading.
The practical method is to apply normal working voltage and
test "voltage drops" with a voltmeter.
For example, in an ideal van with wiring from room-temperature
superconductors, suppose a circuit like this:
Battery pos ---relay panel---30-ignition sw-X-------X-light
sw-56------56-dimmer sw-56a------fuse-----56a-right 130 W super high
beam-31------star cluster grounds-----battery neg cable-battery neg.
If you started measuring voltage drops across each of those
components, you'd find (call it) 12 V between battery pos and battery
neg, and 12 V between high beam 56a and 31. Every place else would
be zero volts. Now to reality...
Battery pos --> neg 12.0 volts. For simplicity we'll say the current
is 10 amps, so the battery is supplying 120 watts of power and the
total resistance in the load circuit is 1.2 ohms.
wire to relay panel 50 mV (half watt)
relay panel 50 mV (half watt)
wire to ignition 30 mV (.3 watt)
ign 30 --> X 200 mV (2 watts)
wire to dimmer 50 mV (half watt)
dimmer 56 --> 56a 75 mV (.75 watt)
wire to fuse 60 mV (.6 watt)
fuse 300 mV (3 watt)
wire to high beam 50 mV (half watt)
Notice that we've wasted 8.65 watts so far...
high beam 56a --> 31 9.5 V (95 watts, considerably down from its 130
watt rating)
wire to star cluster 50 mV (half watt)
star cluster 1.6 V and the terminal is hot (sixteen watts, no
wonder..., and indicating a resistance of 0.16 ohms) <-- BINGO!
chassis to battery neg 35 mV. (.35 watts)
Fixing the ground terminal to bring the drop down to 50 mV (about
four milliohms) would change all the numbers slightly, but the big
one would be that the headlight would be getting about 11 volts, the
total circuit resistance would drop* to 1.044 ohms and the circuit
current increase to about 11.5 amps and the bulb output would
increase to about 125 watts.
*neglecting tungsten filament R vs T characteristics...bulb
resistance gets higher as the bulb gets hotter.
I made up these numbers, but they're all in the realm of
reason. Fuses always have a high drop because they *have* to act
like heaters if they are to blow properly on overload. Bosch
(Automotive Handbook, 4th Ed, p793) allows a drop of up to 0.3 volt
between the 30 terminal of a headlight switch and the lamp terminal,
with a permissible drop in the entire supply+ground circuit of 0.6
volt - so as presented this circuit (after you fix the ground
terminal) is still out of those limits. Fixing the headlight switch
and cleaning the fuse terminals would take care of that, maybe.
For other 12 V circuits they allow (first number as stated, second
number if present is drop in entire circuit, current at nominal
voltage which I take to mean 12.0 V):
Lights < 15 watts, 30 terminal of light switch to lamp or trailer
socket: 0.1 (0.6) V.
Lights > 15 watts, ditto: 0.5 (0.9) V.
Headlights, ditto: 0.3 (0.6) V.
Charging cable, alternator B+ to battery: 0.4 V.
Main starter cable, starter locked, 20C: 0.5 V with caveats
Starter control lead, switch to starter: 1.4 (1.7) V.
Other control leads (relay, horn etc.): 0.5 (1.5) V.
These are considered maximums.
By sticking a needle into the wire you can measure voltage drop
directly across a terminal or connector - this is where a great
proportion of our troubles lie.
Yours,
David
