Why 1 bad glow plug can trash you (is my math right?) - PeachParts Mercedes-Benz Forum

#1 12-12-2007, 02:32 AM

The parallel resistance rule. ( I think?) I am asking you guys I had one plug come in at 67 kohms

so my guess which is usually always wrong is the equivalent R seen by the glow plug heater instead of being (R1xR2xR3xR4xR5)/(R1+R2+R3+R4+R5) where all the R's are say 1 ohm , then R eq = should be 1/5ohm.
That assuming the glow plug heater sees them in parallel ....
Now that I have one bad 67 kohms plug sucking up all the amps from the glow plug circuit. The 1 ohm plugs don't get much...because my horrible math shows R eq to be closer to 1 ohm vice 1/5 ohm so with the higher resistance the 12 V circuit heats them all with 1/5 the normal amount of amps? Is this right.

My car wouldnt even start at 35 degrees without a major scenario. Now it starts at 15 degrees like a brand new gasser! Thats because it can't read Shop Forum and it thinks everything is graet. ( I replaced them all anyway)

Does this mean after everyone debunks me temoorow, my car will stop running so well on very very cold dayz?

pmckechnie · #2 12-12-2007, 07:55 AM

I'm sorry, it doesn't work quite that way. In a parrallel circuit, if one circuit changes, the curent stays the same in the other circuits. It is the amperage (curent) that makes the glow plug get hot. The total amperage seen by the control module will be less but the amperage in the good glow plugs will stay the same and the amperage in the bad one will be less. Think I(current measured in amps) = E(Voltage) / R(resistance). The voltage to all circuits stays the same, the resistance in the good circuits stays the same, so the amperage stays the same in the good circuits. Only the bad one changes.
The problem is that a diesel will not start as well if even one gp goes bad even if the rest are working correctly.
I hope I didn't confuse you, but it is hard for me to explane things.

pawoSD · #3 12-12-2007, 09:09 AM

The more dead glowplugs you have, the more cylinders you have not firing off right away when you try to start....1 will make it start rough with smoke....2 will begin to take longer cranking and more smoke/roughness....3 or more and its probably going to be luck/or a very healthy engine that starts in real low temps with 3 or more out.

obwielnls · #4 12-12-2007, 09:19 AM

If they were wired in series.. I think your math is closer. (notice how vague I was on that)

networkboy · #5 12-12-2007, 09:49 AM

Also E=IR (Ohms law) The lower the resistance (for a fixed voltage, like 12 volts) the higher the current draw. Your 67KOhm plug isn't drawing anything at all practically speaking.
A 1 ohm plug will draw 12 amps.
A 12 ohm plug (corrosion?) will draw 1 amp
A 67KOhm plug (burnt out) will draw .000179 amps

#6 12-12-2007, 09:49 AM

One bad GP will not affect the operation of the others (unless it shorts out and blows the fuse). They all have a separate power supply and the good ones will fully heat up. I can tell if I have one bad GP at any temperature (rough starting) and it will give me trouble if it's very cold. Two bad GPs will make mine difficult to start, even in moderate temperatures.

ConnClark · #7 12-12-2007, 11:22 AM

I went all summer not knowing I had 4 glow plugs out.

when things started to get cool I replaced 2 of the bad ones and the one good one. A month and a half ago I replaced one more. The last one to be replaced is on the number 5 cylinder. That may wait until spring since I don't have a garage.

#8 12-12-2007, 11:34 AM

Quote:

Originally Posted by ConnClark

The last one to be replaced is on the number 5 cylinder. That may wait until spring since I don't have a garage.

Just wait for a nice day, it takes about minutes.

pleiades · #9 12-12-2007, 11:38 AM

Even if they were wired in parallel, one with very high resistance would make little difference. You can model it's resistance as approaching infinity, and it joins in with the other infinite resistance resistors in the circuit- the air between the glow plugs.

Having one GP out just makes that cylinder have trouble starting, which will cause some vibration, or in very cold weather affect the entire motor's ability to start.

Alberta Luthier · #10 12-12-2007, 12:17 PM

I have read the manual on this... -hate to admit it but it mostly just confused me. I tested with a continuity tester from the wiring harness plug and none are an open circuit. But i understand that they can also fail as a short. The next test using power on and testing amperage as the GP heats up confuses me and I don't think I have the right tester nor a full understanding of how to use it. I'm pretty sure I have a couple of duds. anyone got a simple way of discerning which one(s) are the duds?

vstech · #11 12-12-2007, 12:28 PM

http://www.dieselgiant.com/glowplugrepair.htm

Diesel911 · #12 12-12-2007, 12:37 PM

Out of curiosity I wired an automotive ammeter in series with 1 good glow plug. I put the wire going into the ammeter on the (+) post of the battery and the glow plug body on the (-) post of the battery. I was shocked to see that the ammeter went up to 30 amps hung the several seconds and eased on down to 15+ amps as it heated up and glowed.

vstech · #13 12-12-2007, 12:40 PM

yup, that's how resistance works.
it takes more amps to get the element hot than to maintain the heat. also the hotter the element, the more it's resistance increases, thus lowering it's ampdraw.

pimpernell · #14 12-12-2007, 12:44 PM

I do not know the layout of your engine, but if you can reach each plug, the easiest way is to compare the ohm reading on each plug with the ohm reading of a known good plug. Test the known good plug by putting one lead of your multi tester on the plug contact pin, and the other lead on the glow plug body. To test the ones in the engine, just remove the wire from each glow plug, put one lead of your multi tester on the glow plug contact pin, and the other lead on the side of the glow plug body. Should give you approximately the same reading on all plugs if they are good. Good luck....