[alfa] re: '88 Spider Quad Blowin' Fuses - Next Question!!!

["John Y. Hung" <jdaezhung@xxxxxxx>]

Dear Dan,

Winding resistance of motors that have brushes cannot be accurately 
measured with most ohmmeters (including digital volt-ohmmeters), 
because the test signal generated by the instrument is a very low 
level. The contact resistance of the brushes will cause the reading to 
be erroneous.  The typical method to measure winding resistance is to 
apply a voltage (no higher than the motor rating) to the motor 
terminals, and use an ammeter to measure the current with the motor in 
a STALLED condition; we call this a "blocked rotor test".  The winding 
resistance equals the applied voltage divided by the current.  A 
blocked rotor test is practically impossible on a sealed fuel pump. 
Furthermore most automotive fuel pumps should be tested with great 
care, because these motors usually rely on the the presence of fuel to 
cool the windings. Testing the fuel pump motor in a  "dry" state could 
overheat and ruin it.

Winding resistance of a motor is typically very low, because the 
current drawn by the motor during normal operation is not limited 
solely by the winding resistance. When a motor is running, a voltage 
that is opposite in polarity to the applied voltage is induced in the 
winding. The induced voltage is proportional to motor speed.  So, 
current draw is high when the motor first starts up, and then the 
current reduces as the motor speeds up. Current draw does increase with 
load.

There should be infinite resistance between the winding and the motor 
body. From the tests that you describe, it sounds like you have a short 
between the winding and the motor - not good.  On the other hand, you 
say the motor ran.  I would hook up the system again (with fuel in the 
lines), and try to measure current draw. Let me know if you need more 
help.

John Hung (EE professor)
Auburn, AL
'88 Spider Veloce
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