PeachParts Mercedes-Benz Forum - View Single Post - After Vehicle Inspection-No Glow Plug Light

jkoebel · #2 10-02-2007, 03:13 PM

Quick and Dirty Glow Plug Electrical Testing. Adapted from Diesel Giant's electrical glow plug testing, to take advantage of not having to run wires across the entire engine bay.

(1) Make sure you've got voltage going to the glow plug relay.

Find a good metal screw that goes into the chassis, use that as your ground. (Battery Ground and Chassis Ground are the same, so pick the closest one that works.) Open the relay cover...test with multimeter between where the line comes in and that screw. Then test at each side of the strip fuse. It should be battery voltage at all 3. (Battery voltage should be above 12.5V with car off, above like 13.3V car on. If not, alternator problem.)

(2) Check wires going to each plug.

Remove the relay cap that goes to the individual glow plugs. Set the multimeter to check ohms, and test between Cylinder 1 on the connector and Cylinder 1 glow plug wire on the engine. Repeat for all cylinders. The number should be very, very small -- below 1 ohm optimally. Very high numbers mean the wire is broken.

(3) Check eack plug itself.

With multimeter in ohms, connect the positive end to the incoming power wire to each glow plug, and the negative end to the metal valve cover. This works because the valve cover is a grounded metal structure, so valve cover ground is the same as chassis ground and battery ground. You want a little bit of resistance, like between 0.2 and 1 ohm. 0 ohms means internal short (dead plug), very high/infinite means internal break (dead plug.)

(4) Test the voltage coming out of each relay while the glow timer is on.

This one can be done with 2 people, a lot of running, or two alligator clip extensions. Measure, on the relay itself with the plugs unplugged, voltage between pin 1 and chassis, pin 2 and chassis, etc. during the time the glow plugs would normally be on but aren't. It should be battery voltage. If any of the pins corresponding to a cylinder are, in fact, not -- then bad relay.

Cheers.