PeachParts Mercedes-Benz Forum - View Single Post - Starter solenoid vs bad connection diagnostics

Jeremy5848 · #14 08-07-2009, 11:24 AM

Voltage drop test: Because the wire from the battery to the starter is big and heavy, it has almost zero resistance. Electrically, measuring the voltage at the starter should be the same as measuring it at the battery itself -- you should get the same results. This test is to make sure that you do. If not, you have a problem in that big wire.

With the vehicle power "off," measure the voltage between the negative battery terminal and the positive battery terminal. Use a digital meter, the best one you can beg, borrow, steal, or buy. Write down the voltage. Now measure the voltage between the negative battery terminal and the big wire at the starter, Write down this number. With the vehicle power off, the two numbers should be identical since no current is being drawn.

Now repeat these measurements with an assistant holding the key in the "start" position. The measurements should be lower because of the load placed on the battery by things in the car that are now drawing current and because of the starter (if it is drawing any current at all). If the voltage at the starter is significantly (more than a volt or so, probably) lower than the voltage at the battery, you have a voltage drop in the big wire from the battery to the starter. That voltage drop, probably due to dirty/corroded connections, could be preventing the starter from drawing enough current to start.

Jeremy