PeachParts Mercedes-Benz Forum - View Single Post

micalk · #7 11-29-2009, 12:37 AM

50 is the start circuit. Power is applied to this circuit when the ignition is in the Start position (Position 3).

15 is the run circuit. Power is applied to this circuit when the ignition is in the Run (Position 2) or Start positions.

The Starter Lockout and Backup Switch (part of circuit 50) is just a switch on the shifter to allow the starter to energize, and the same switch also senses reverse to engage the backup lights, hence its name. This is simply where the connection is made, since you must be in Park or Neutral to start the car, and you continue to glow if you're starting the car. When you release the key from start, this kills the glow. So this provides a signal to the glow plug relay circuitry (transition from 12V to floating/open) that tells the glow plug relay to shut off.

FYI, just because the glow plug light doesn't light up doesn't mean that your glow plugs aren't being energized, it just means that something is not up to snuff. You're system could be working at 90% and you may not get a light.

Question: When you apply power to pin 1 of the 4-pin connector of the relay, are you applying ground to pin 4 at the same time? If not, this may be why the relay drops out immediately. If circuits 15 and ground check out okay to the 4-pin connector, then I would leave the plug connected to the relay and move on. Circuit 50 tells it to turn off, so even if it doesn't check out it shouldn't enter into the equation for this situation.

Even though the relay pads look good, drag a two-sided nail file or sandpaper back and forth between the contacts anyway. It's for good luck. And to make sure the smoke can get out easier. And to remove the relay pads as an area of doubt.

Also, pull the 80amp fuse and make sure it is intact. These fuses can develop a microfracture in the center and look good and may pass voltage checks but won't pass any significant current. If you pull it and it's bad, you will be able to tell immediately.

Assuming that your connections to the four pin are okay, your relay connections are fresh and your fuse is good, pull the other plug to the relay. Turn the key on and see if you have voltage at the pins on the relay that connect to the glow plugs (you probably have 30-40 seconds to perform this check before the relay times out). If not (i.e., the relay is still dropping out immediately), then it is most likely that the electronics in the relay are bad and you'll have to replace it. If you have voltage at one pin, then you should have voltage at all pins. If you have voltage at all five pins your problem is in the glow plugs or in the wires to the glow plugs.

Leave the connector off and check the resistance of the glow plugs by connecting one end of the ohmmeter to ground and the other end to the connector pins. Resistance should be on the order of .6 to 1 ohm IIRC. What you may find is that glow plug 1 has a different resistance (probably higher?) than the other four. This is a fault that will cause your glow plug light to remain unlit. I suspect this is your problem as you state that your car starts even at cold temps. I live in SoCal, and I had a relay fuse break late afternoon as I was getting ready to leave. It was a warm day and it took a while for the engine to start. The engine only has 109KMiles, so it's in good shape and it still took a while to start. I can't imagine trying the same thing at 6 degrees. You guys in Wisconsin still use Fahrenheit even though you're up there next to Canada, right? If you find you have high resistance in one of the glow plug lines, check the resistance with your ohmmeter between the tip of the plug and its body (not the engine block - we're trying to rule out a bad connection between the plug and the block).

Let us know how these checks turn out, and if you find anything. These cars are amazing when they are well maintained, and the folks at this site can help with just about any problem you encounter. Good luck!