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Q: Air pump, how to know if air pump would need to be replaced or not? on 1991 Mercedes-Benz 300SE

I have a question regarding to Air pump on 1991, 300SE.
The car has 200K miles. We are trying to see if we might have to replace air pump or not, because of miles and age. (The air pump seems like an original, has not been replaced.)
To turn pulley of air pump by hands. It's seems like hard to turn the pulley by hands. I am wondering if it is normal or not? How do we know, if air pump would need to be replaced? Thanks in advance for your help.
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It could be a bad or unsecured battery ground cable between the battery and chassis or chassis and engine. Turn on the ignition switch to the on but not run position do you see an icon on the instrument panel representing a battery, if you do not see this battery icon the alternator is not seeing power to allow it to charge. Start the engine does the battery icon go out. A more complex but good diagnostic test is checking "voltage drop" on the battery ground and power cables. You can find a video on how to do a voltage drop test on youtube. Put a volt meter across the battery terminals with the engine off, at rest the battery voltage should be 12.6 volts if the battery is fully charged. Now get a friend to start the car while you watch the voltmeter, the voltage reading across the battery should read approximately 14.6 volts if the alternator is charging correctly.
Jan. 17th, 2011
Hello Master,
Thanks for your replay. However, I am afraid but I am asking question on AIR PUMP not electric system (battery charging system) or alternator. Your answer might have been mis placed to a wrong place. Please send your answer to the right place.
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