How do you do this electrical stuff? - PeachParts Mercedes-Benz Forum

jbach36 · #1 04-13-2015, 12:45 AM

1) I was told to check the pins under the large harness at the glow plug relay to test for continuity. I think I have to take my multimeter and put the black (negative) wire onto a non-painted metal part of the engine and the red positive on each pin, is that correct?

2) I need to check each glow plug and I have one of those 6/12 V things that light up. To test those, I'd have to have the key in the preglow position so they're getting power, then put that to each glow plug to see if they're all getting 12 volts, correct?

3) While touching the glow plugs, if I touch the side of the metal engine I heard that will short out the plug, is that correct? There's not much space in there, and it's almost guaranteed I'll touch metal by accident, so what can I do to prevent that?

4) If all glow plugs are GETTING 12v of power, how do I know if they're bad? (they're receiving the power, but not heating up properly). By bad continuity numbers?

Maxbumpo · #2 04-13-2015, 09:04 AM

Your step (1) is the most basic check. Make sure your meter is set to measure resistance (aka ohms, continuity). Doesn't matter which color lead goes to which pin or to the engine, you just need one on each side (engine, pin).

Before you measure at a pin, put both leads together and observe that the meter reads zero ohms. If instead you read a stable value, then you will need to subtract that value during the next step. When you then test at a pin, you should be a reading of around 0.7 ohms, which is a fairly low resistance to measure and gives some meters a hard time.

Let's say you put the leads together (zero test) and get a value of 0.5 ohms. When you measure between a pin and the engine block, you get 1.2 ohms. 1.2 minus 0.5 = 0.7, which is about what you should get. Measure all pins that connect to a glow plug, and compare all the values because you are really looking for the outliers.

If your "zero" test is 0.5 ohms, and all the plugs read around the same value of ohms (let's say 1.2), then your plugs are probably good. If one reads 0.6 or 0.7 ohms, that plug may be a short to ground (bad). If one or two read a really high value, like in the kilo- or mega-ohms, or infinite resistance, then either you have a bad connection somewhere or a bad plug.

Any pins that give suspect readings may be a problem in either the wire or the plug, so the next step is to measure directly at the plug. If the suspect reading was close to zero ohms (short), you need to remove the wire from the end of the plug before measuring at the plug directly.

Maxbumpo · #3 04-13-2015, 09:10 AM

The ultimate test is to measure the current flow to each plug, but you need an ammeter that can measure 12-15 amps, which is a pretty high current. There are clamp-on DC ammeters, which make the job REALLY easy, as you just clamp the sensor over the wire to the particular plug, energize the glow plug relay, and watch the meter. You can also make an in-line meter using a $20 car ammeter from FLAPS. Find one with a 20 amp or 25 amp range, attach two wires to the input/output connections, push one wire into the pin for a glow plug, touch the other wire to the hot side of the 80 amp fuse, and watch the gauge. I think the initial current pull will be about 12-15 amps, and it should drop off to around 8 amps as the plug heats up.

Anther way to test a glow plug is to remove the plug, and then energize it using a set of jumper cables connect to the battery. Just a few seconds of power should make the plug get red-hot. Be careful to avoid shocks and burns!

Dan Stokes · #4 04-13-2015, 10:01 AM

Remember that volts are measured in parallel and amps are measured in series. Max described the amp reading process correctly - if you're using a cheap auto parts store gage it has to be in series (you have to break into the wiring). As he also said, there are clamp-on ammeters but those suckers are pricey!

To read volts you have to be concerned with polarity (+ to +, - to -) but you can read volts with the circuit hooked up as in use. If you get no reading make sure your polarity is correct. If that's good you probably don't have a good ground. In a 1 wire DC circuit (like cars use) the "-" side is (or should be) the same as ground and one of the causes of electrical issues is that there's some resistance between the - side of the item in question (say, a motor or bulb) and the chassis ground. If you look over my S-10 you'll find everything runs to chassis ground and many times with more than 1 wire. I hate electrical problems! BTW - most modern DVMs are polarity protected, even my Harbor Freight special, so if you hook it up backwards no harm is done. You might want to check the literature that came with yous just so you know.

To read ohms, the measure of resistance to the flow of electrons, polarity is not important, hence Max's instruction not to worry about which probe goes where in the one test. To test an individual component for resistance (say, a glow plug) you have to isolate it from the circuit just so that you know that you're measuring the resistance of that 1 item and not the resistance of the whole circuit.

Sort of "DVM101" - please forgive me if you're past this but it sounded like you were new to all this.

Dan

jay_bob · #5 04-13-2015, 12:01 PM

About the only positive result that comes from an ohm check of glow plugs is that it will definitely let you know when one has failed totally open.

You can't determine health of the glow plug from a simple resistance reading if you have continuity through the plug. Your meter cannot resolve the resistance accurately enough to determine if the plug is normal or partly shorted.

Only by pulling the glow plug, clamping it in a vice, and using a set of jumper cables, can you positively see the glow pattern. It's possible that the element can be half shorted and so the glow is only occurring up closer to the shank of the plug. The glow needs to be happening at the tip, that is the part that projects into the prechamber. If it's only glowing at the shank end, that cylinder will act like it has a dead glow plug, even though the plug is drawing current and showing continuity.

That said, here are another couple of quick check ideas:

- with your meter set to "ohms" check between pins 1 and 2, then 3 and 4, and then 5 and 6 (or 4 and 5 if you only have a 5-cyl) on the relay end of the glow plug harness. This checks the plugs 2 at a time and you don't have to hunt around for a decent ground, which makes it a lot easier to test by yourself. Now if you get an open circuit on one of the pairs, you will have to test the two halves independently to ground to figure out which one is bad.

- Use a 12V test light, put the clip on the battery + terminal and then probe into each of the sockets on the glow plug harness connector one at a time. If you find one that does not light up, it is definitely open.

- If you find a suspected open plug, test directly on its brass terminal before condemning the plug. You could have an open wire in the harness.