Voltage at glow plugs.

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Steviecops

Steviecops

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I just replaced the glow plugs on my 300 tdi because, even with 50/50 mix of bio diesel and regular diesel, it's not starting brilliant when it's very cold in the morning. I think at least one glow plug wasn't working because it was covered in oil when I took it out. The other three were also a bit oily but had small white/grey spots on the tips, which I thought could indicate that they were still getting hot. (Does that sound right?) When the new plugs were fitted I checked the voltage at each one and they were all showing around 10.5 until the relay dropped out, then returned to zero. Is 10.5 the right voltage? My searches have turned up various references to glow plug voltage but no definitive answer.
 
easiest way to test a glow plug is take a set of jump leads and put Negative to the outside of the plug and Positive to the tip where the wire would go, they should glow within 3 or 4 seconds, do not touch they ends once they have glowed as they stay hot for a long time and it will take your skin off, do not keep them on for too long either
 
The plugs are in now and I don't fancy taking them out again just yet to test them.

Should the voltage be the same as at the battery when the plugs are heating? The battery was showing around 12.5 v, but I didn't test it while the plugs were heating to see how much of a drop there was.
 
My plugs read at 11.5v which is the same rating thats stamped on them. Why not test the resistence. Discoverd that all my plugs were nacked. Just remove the link wire on the top of each plug before you test.
 
The plugs are all new, I fitted them today. I won't know if they've improved cold starting until the morning, but was just wondering whether the voltage to the plugs was right. I'm absolutely rubbish with electrics and dont know how to test resistance at the plugs?
 
Thanks, Alex. The voltage at the feed wire when I tested it yesterday was 10.75 v. The other three were all around the 10.5 v mark. I'll check the voltage of the battery while the plugs are heating later today.

Yesterday I gave the old plugs three cycles before starting because it was very cold and I'm running on it on bio/regular diesel mix. It took about 4 or 5 seconds to start. This morning with the new plugs I gave them two cycles and it started after about two seconds, but it isn't as cold this morning.

I know it wasn't the best move, but I'm skint so I put cheap glow plugs in. The ones I took out were Beru, so I'll test them all later to see if I can just replace one or two and put them back.
 
Well, either I'm testing them wrong or none of the old glow plugs were working! No wonder it was struggling to start in the cold.

Tested battery as the plugs were heating and it dropped from 12.8v to 11.6v. Went back up when the relay dropped out. Reading at the feed wire was 10.8v. Took off and cleaned body and chassis earths from the battery and cleaned relay contacts. No difference to readings afterwards.
 
you have a high and low voltage going into the relay next to the fuse box under the bonnet. The high voltage goes from the battery into the relay, then from the relay into the loom up to the plugs. chase the brown and oragne wire that goes from the relay back to the battery you will see the loom going back from there. mine wasn't even connected to the battery, as it had rotted off and the copper had gone black, which will cause alot of resistance. take a reading from the that wire see what it gives you. That wire should be giving you a reading of 12+ volts
 
alex8882 said:
you have a high and low voltage going into the relay next to the fuse box under the bonnet. The high voltage goes from the battery into the relay, then from the relay into the loom up to the plugs. chase the brown and oragne wire that goes from the relay back to the battery you will see the loom going back from there. mine wasn't even connected to the battery, as it had rotted off and the copper had gone black, which will cause alot of resistance. take a reading from the that wire see what it gives you. That wire should be giving you a reading of 12+ volts
Click to expand...

news to me - what is the high and low voltages then?
 
high voltage is the supply from the battery to relay then relay to glow plugs, the low voltage is the switch to close the circuit on the high voltage side refers more so to the thikness of the two cables
 
According to AutoData the glowplugs have a nominal rating V/A 12, and a activation time of 8 seconds.

If i'm right this means they are 12 volts and they run for 8 seconds before the relay cuts the power.

If your glowplug light is coming on for around 8 seconds and the car starts first time then i would say you are getting enough volts to them.

If you want to check out the ones you have taken out then using a multimeter set to ohms you should get a reading from the outside shell to the inside shell of 0.8 ohms approx, if you attach a jump lead from earth to the outside shell and attach the wire from your loom, switch ignition on and while the glowplug light is on check to see if they glow, if not chuck them in the bin, make sure you use pliers to remove, THEY WILL BE VERY HOT.
 
Last edited:
Thanks for that. I gave the heater plugs one cycle this morning and it started in two seconds. It wasn't as cold as it has been, but there was quite a severe frost.

To test the old plugs yesterday, I attached a wire direct from the battery + terminal to the bit of the plug that the harness would bolt onto and earthed the body of the plug on a known good earth on the engine. None of them got even slightly warm. I did the ohm test and got all sorts of wierd and wonderful readings. I'll do as you suggest and try bolting them up to the harness and using the ignition switch to power them up with jump lead to earth. If they are knackered, they are knackered, but it just struck me as odd that all 4 weren't working.
 
Once you "liven up" the glow plugs the volts on the battery will drop as the glow plugs are effectively shorting the battery and dragging it down-The voltage you see on the glow plugs appears lower than the battery,because of the resistance between the battery and the plugs-if you want to see full battery volts at the plugs you will need to fit thicker cable-ensuring all contacts are clean and tight will help.
10.5volts on plugs energised seems right about right to me,coming from a battery showing 11.5 volts
 
Cheers mate, that makes sense. Hopefully, these cheap plugs will see me through the last of this cold weather, then I'll replace them with a decent set before next winter sets in. Ie, in about July! :D
 
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