Posted to Technical Discussion Forum on 1/3/2014
starter motor failure revisited
In reply to the thread http://members.iatn.net/forums/read/msg.aspx?f=forum2&m=362980&fv=0&ar=0 I will discuss a quick
and very accurate way to test not only the starter, but the
complete starting and charging system by using a lab scope,
a high amp current probe and a voltage test lead.
For this test, hook the scope voltage lead to the positive
and negative lead of the battery, hook the current probe
around the ground wire to the battery. If there is more than
one ground lead, hook your current probe around all the
ground wires. You want your scope to measure all the current
going in and out of the battery.
This quick and simple test will tell you about the integrity
of the cables carrying the current to and from the battery,
the condition of the starter solenoid contacts, the starter
armature, relative compression of the engine, condition of
the battery and the condition of the charging system, all
with only two scope hook ups, and less than 5 minutes stop
to finish of the test.
If you are using a Pico scope, put 5 minutes of time on the
screen, if you are using a Snap On scope (Modis, Verus, VP)
put abut 2 seconds of time on the screen. Set the voltage
trace to 20 volts, and the current trace so it will capture
600 amps (if you are using a 600 amp probe). Set a trigger
so the trace will be started when the starter is turned on.
You should see a scope trace something like this.
sulfated battery starting and charging system test
Starting at the left side of the traces, I can see the
starter solenoid contacts closed nice and clean, the starter
got the engine running in short order, then the generator
started charging the battery. Look at the right side of the
trace and notice the charging current the battery is still
absorbing after a three minute charge. It is still taking in
over 50 amps. This is putting the hurt on the generator, and
the battery will get very hot after it charges for a while
at this rate.
This is something I found on a vehicle that came in for a
routine service. In this case the battery is sulfated. There
are no starting complaints at this time, but just wait,
there will be several starting and charging complaints if
this problem is not fixed.
Step 2 in the testing is to zoom in on the starter turn on.
For you people using a Snap on scope, this will be where you
started, and to be able to display the previous waveform,
you will have to scrunch your waveform up. If you start with
a waveform like the one displayed, you will not be able to
zoom in for the detail of the starter turn-on. sulfated
Now to use a carbon pile and load test the battery.
sulfated battery test battery load test The battery
load test shows nothing wrong with the battery, but the
dynamic starting system test found the problem in short
Now on to a defective starter motor. This test found a
starter solenoid with defective contacts. starter current
test starter current test bad starter solenoid
contacts . In this case, the starter motor started turning
the engine a little slow. Here again, here were no customer
complaints, but the scope found the lurking problem. In a
case like this, load testing the battery or volt dropping
the circuits with a DVOM will not find a problem like this.
A picture is worth 2000 words in a case like this. Now a
zoom in on the starter turn on To capture this waveform, I
had my right foot on the throttle to put it in clear flood.
The engine was cranked for about 10 seconds, then the
throttle released and the engine started. I just performed a
relative compression test, a complete starter system test,
battery load test, then a charging system test, all with
only two scope hookups, in less than 5 minutes.
Sometimes a Tech must think out of the box to find problems
and make some easy cash.
Albin from Washington
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21 Replies Received