I have gone to two different shops in the past.
One was a truck shop with a manual tester and teh guy had to remember how to do the testing... reading as he went. They charged me for half an hour's time (35.00).
I then took a set to a Bosch shop in No California and they threw them on their tester, ten minutes later told me one was weak but not too bad and thanks for stopping bye. No charge...

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique