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MultiMeters, elect. trouble-shooting, Long Post
Regarding trouble-shooting automotive electrical/electronic
systems using a Multimeter. Here are some basic tips for
trouble-shooting electrical problems as well as some nerdy
stuff on DMMís. This is a very long post so strap yourself
in and lets go for a rideÖÖ
The majority of problems in automotive circuits are caused
by poor grounding or poor connections in the harness
wiring or connectors. The other batch of problems are caused by
poor internal solder connections or burned up components
(resistors, transistors, diodes, ICís, etc.) inside the "black boxes"
that electronically control all those wonderful gadgets inside your
The Audi engineers definitely can get a little over zealous and
seem to like complicated circuits with many connected devices
with various levels of interaction. Some of the dash light controls,
the three speed radiator cooling fan system as well as the 5 or 6
temperature sensors required under the hood come to mindÖÖÖ
Sometimes when a bulb burns out or a fuse blows, the current can
flow in other directions through the circuit and affect another circuit
totally unrelated to the one where the bulb blew.
Before running out and buying that new alternator or "black box" make
sure all the ground straps are clean and tight. Start at the battery
and work your way from the engine main ground straps to all the
centralized grounding points under the hood, dashboard, interior as well
as the ones in the trunk. Audi uses the body to provide grounding from
the battery to the rest of the areas in the car. In some cases running a
new separate ground wire to your circuit can fix the problem. I have
seen soooooooo many good starters, alternators, batteries, and other
electronic control units replaced because of a loose or corroded $2
battery ground strap or from some other bad ground connection
in the car.
I ran into one case where the accelerator cable was the only
wire providing a ground to an engines starter motor and alternator,
the owner kept replacing the throttle cable as the cable would glow
red hot when starting the car and this melted the outer plastic sheath!
Chrysler had a problem where the right front wheel bearing
would burn up over time in the FWD cars if the engine/ transmission
ground strap would break! If there is only one wire going to a
device and you measure +12V on the metal case of this device,
it usually indicates that the housing has no ground. This used to
show up on the blower motors used on the fiberglass bodied Corvettes!
The wiring supplying +12 V can also be poorly designed as
is found in the headlight wiring on most 5000/100/200 vehicles. Audi
chose to provide +12V to the headlights without any relays and this
can lead to smoked connectors inside the stalk mounted headlight
switch from the high current flow through this circuit. Adding high
wattage headlight bulbs is asking for trouble if you donít add any
relays. The headlight connectors can also go up in smoke with higher
wattage bulbs. Be careful when looking for a +12V connection for that
stereo or high wattage amp as much of the wiring underneath the dash
may indeed have +12 V but can not provide the current you need for
that add on device! Using a known fuse connection is likely a better
idea than tapping into one of the stray wires underneath
the dash. This applies especially to Air bag equipped cars.
Digital Multimeters or DMM's for short, with their high
input impedance (resistance) of 10 Mega-ohms, (10
million ohms) allow you to safely probe the majority
of automotive electronic and electrical circuits without
the risk of damaging any of the sensitive electronics or
disturbing the operation of the circuit under test.
The old analog needle type volt meters and the basic test light
with incandescent bulbs have a lower input resistance
and in some cases can damage an electronic circuit if used
to test for voltage. Even if they don't damage the circuit
under test they could alter the voltage output enough to make
you think the circuit is defective.
The DMM when connected to 12-14VDC in an automotive circuit
will draw around 1 uA, (micro-amp, 0.000001 amps) from the added
resistance of 10 Mega-ohms in parallel with the original circuit.
In other words probing a circuit with the DMM is like taking a
10M ohm resistor and connecting one end to the circuit test point
and the other end to ground on the vehicle. This very low current
draw typically does not alter the operation of the circuit under test
as the electronic circuit can easily supply the extra 1uA without
affecting its voltage output.
The basic test light draws about 200 milliamps, (0.2 amps)
from the same 12-14VDC circuit. This correlates to having
an input resistance around 60 ohms. This assumes the circuit
under test can actually supply that 200mA without being
damaged internally. Even if the circuit is not damaged,
it may not be designed to supply that 200mA and typically
the voltage output will drop way below the required
output. This is due to the output resistance of the
driver transistor causing a large voltage drop with the 200mA
current flow. This large voltage drop may cause the circuit
under to test to stop functioning correctly.
The older analog volt meters can have input resistanceís
from 50-200k ohms (50-200 thousand ohms). Others who use
these meters on a regular basis may want to comment on
these figures as it has been awhile since I have used one.
Many of the engines ignition/fuel control circuits handled by the
Electronic Control Unit (ECU) are driven with pretty robust transistor
circuits inside the ECU as they must provide a ground for solenoids
and other high current devices like electronically controlled
port or throttle body fuel injectors. As mentioned above these
circuits have +12V supplied from the battery or from the
ignition key being on but use the ground side of the circuit to
switch the device on and off.
Some of the newer low resistance fuel injectors are
driven by a transistor circuit that allows the injector to be
turned on fast with 4 amps peak and then this current is reduced
to 1 amp to hold the injector open. These are called peak and hold
injectors for obvious reasons. The other type of injectors that
have been used for 30+ years have around 10-12 ohms of resistance
and are driven with a single stage transistor that provides around
1 amp. The cold start valve,the Idle stabilizer valve, the Waste Gate
control solenoid are other examples where larger transistors are used..
Several people on this list have repaired ECU's that had these driver
transitors go bad.
The transistor in the variable speed blower motor controller used in
the A/C system is yet another example. It does not switch the
blower on and off but actually varies the voltage to the blower
and it must use a large heat sink to help dissipate the power from
the voltage drop across it and the current flowing through it..
Other ECU circuits may have very little current flow and can have
higher output resistance transistor drivers. Some examples of these
are the hall effect switch in the distributor and the signal that
drives the ignition coil power transistor. This power transistor
is driven from a lower current signal from a small transistor
inside the ECU but this larger power transistor can then drive the
ignition coil on and off with 5-10 amps of current (peak values).
Inside the ECU this small transistor is driven by the voltage output
one of the Integrated Circuits (IC) and uses a series current limiting
resistor. Depending on where you are probing in the above mentioned
circuit you might get away with using a test light at the ignition
coil but if you were to use it to probe the IC output signal driving
the small internal transistor it would likely go into current
limit or just plain burn up.
The nice part is that with a DMM with 10 M Ohms input resistance you
donít need to worry too much. There are only a few automotive circuits
that actually will be affected even by the high 10M ohm resistance and
most of these are not usually accessible by a technician. Even the
1 Mega-ohm input resistance on most oscilloscopes is sufficiently
high enough for most testing.
Some of the 5V output O2 sensors (resistance type) before being warmed
up have a high resistance that could be affected with the addition of
a DMM or scope to check the O2 sensor output. The majority of O2 sensors
(1V, voltage type) can be easily checked with a DMM or scope when they
are warmed up.
One down side of using a high resistance DMM is that in some cases
if you are measuring the voltage or ground output of a high current
circuit with the "load" disconnected you may see +12V but if the load
is re-connected the voltage measured drops way down to
6 or 9 Volts, this usually indicates high resistance between the "black
box" controller and the load. In some cases the internal components
(transistors, resistors etc.) in the black box are toasted and are the
source of the voltage drop. The after-run controller that turns on the
water pump for the water cooled turbo can be defective in this manner.
If your DMM has analog bar graph you can see the O2 sensor output go up
and down between 0.1 and 0.9V at a rate of 1 to 4 times per second
depending on engine speed and load. If you have a Relative or Delta
(zero) function on your DMM you can connect the DMM to the O2 sensor
output and then turn the ignition key on and you typically will see
around 0.45V which is used as a reference voltage by the computer. If
you then use the Delta or Relative function to zero out this 0.45
voltage and then start the engine, you can then watch the bar graph
indicate how the O2 sensor output goes above and below this 0.45V
Resistance and continuity checks with a DMM usually are done with one
or both ends of the wiring dis-connected unless instructed otherwise by
the shop manual. Leaving the wiring connected can add other components
to the circuit and usually will disrupt the resistance test you are
doing on the sensor or wiring. If you are testing wiring for basic
integrity, donít get too hung up on whether the wiring resistance
measures exactly 0.4 ohms or 0.8 ohms etc. You just donít
want to see tens, hundreds or thousands of ohms of resistance. Many
times when making low ohm measurements you should connect the DMM leads
together and use the Delta (zero) function (if available) to take the
DMMís lead resistance out of the measurement High current drawing
items can not tolerate much resistance in the wiring. If you look at
the thick cable providing +12 V to the starter motor, it can draw
150-250 amps when cranking the engine and having as little as 0.02 ohms
of resistance in this cable with 250 amps flowing through it will drop
5 volts before reaching the starter.
If your DMM has a Duty cycle measurement you can use it to look at the
CIS fuel injection frequency valve signal and see how well your idle
mixture is being controlled. This circuit has a transistor that provides
a ground for the +12V supplied frequency valve which means you are
concerned with the negative duty cycle or the percent of the time the
signal is low. Transistors are often used as switches to provide a
ground connection in many of the higher current
automotive circuits. If you have a turbo engine with computer
controlled boost you can look at the duty cycle of the signal driving
the Waste Gate control solenoid and watch the percentage change. An LED
test light can also be connected up to view this signal and adjust the
Waste gate spring tension when installing stiffer waste gate springs and
modified computers that allow higher boost. If the LED light briefly
flickers and then goes out you may have the WG spring tension set too
high for the boost map you have programmed into the ECU.
You can convert this duty cycle measurement to a Dwell reading by taking
360 divide by the number of cylinders. i.e. 360/4 cylinder = 90 degrees
of available dwell. Then multiply the duty cycle 40% (negative duty
cycle) times 90 degrees to get 0.40 X 90 = 36 degrees dwell.
Dwell can be converted to duty cycle using a similar calculation.
Measuring the Direct Current (DC) draw in a circuit can be done using
the separate amps terminals available on most DMMís. You need to break
open one end of the circuit and insert the DMM leads in series to
measure the current flow. You can check the parasitic current draw on
your battery this way by disconnecting one battery terminal and them
make sure everything in the car turned off. You should start with the
10A setting in the DMM and donít turn on anything or try and start the
car with the DMM in the series with the battery otherwise the fuse will
go pop!. Usually parasiticcurrent draws over 25 -50 mA can cause
problems with the battery going dead, but each manufacturer has their
own limits and this will vary with the size or capacity of the battery
used as well. Keep in mind that if the car door was recently opened the
interior lights may stay on for awhile and draw more than 25mA until
they time out.
One word of caution:
After you do current measurements with your DMM make
sure to switch your one lead back to the voltage input on the DMM,
otherwise you can blow the internal DMM fuse(s) if you try to do a
voltage measurement with this lead left in the amps terminal. Some
DMMís provide an audible warning if you have the leads in the amps
terminal and you switch the knob over to volts or resistance.
One of the current tests on the 86-90 5000/200 MC engines is to
adjust the idle speed which involves connecting a DMM set to amps
mode in series with the Idle stabilizer valve to measure DC amps
(average current flow) into the idle stabilizer valve. You then adjust
the idle speed until the current flow is around 430mA. +/- 20mA.
This centers the idle stabilizer control unit and allows it to
adjust the current level up or down to change the idle speed. The
control unit uses a pulse width modulated (PWM) voltage waveform to
adjust the current supplied to the valve. The frequency and duty cycle
of this PWM waveform can also be measured with the DMM.
The continuity tests can be used to trace out wiring faults in
the vehicle. The diode test can be used to test clamping
diodes that are used in the vehicles relays or wiring
to suppress voltage spikes that occur when inductive loads like
solenoids are turned off. You can also check out the diode
pack inside the alternator or check individual diodes in
the pack if you disassemble the alternator and remove
some of the connections.
If you have a capacitance checking DMM you can test the various
filter capacitors or "condensers" used to filter noise spikes or that
provide or sink extra current to smooth out + 12V voltage connections
through out the vehicle. These are found on the blower motor, the
alternator +12 output and often times at the ignition coil +12V
Thatís enough for now, have fun with that DMM!
By Scott Mockry Copyright 1997
Hell, if Ned can, why can't I.....