[Engine] Glow plugs not drawing current - solved

[kerlaan]

Hello everyone,

2001 M49 VIN VF35BWJYF60439770  DW8 WJY

I have 12 volts to the glow plug wire line, all wiring, fuses, relays checked and without fault.  No OBD codes.  The dashboard display sensors and monitored behaviour of the final relay (cover removed to monitor activity) is all good, zero voltage drop and near to zero resistance to the glow plug wire line.  No dirty connections, all cleaned too while chasing the fault.  Battery health and charge is 100%, supplying over 13 volts.  I have a 0.15 volt drop between battery and the inlet manifold connector when manually activating the final relay or using the ignition key.

4 new glow plugs, new glow plug wire line (the old one was half broken, probably due to copper wire work hardening during engine vibration).

So, ahead of pulling the inlet manifold and glow plugs out, is this indicating that the glow plugs have burned out?  When I measured their resistance in situ though, I had 2 ohms across each one.

Reason for changing the glow plugs was firstly they were overdue and secondly we had a developing starting problem.

The engine valve clearances are all within margin.  So let's not distract any sensible thinking with that old chestnut, thank you.  When started, for which a quick spray of easy start is needed, the engine roars into life, ticks over nicely and I get over 50mpg too.  Not bad for 200,000 miles....

Any thoughts please?  Thanks in advance.

[Sol]

Hi Kerlaan,

A glow plug on average, should draw 10 to 12 amps per plug, equating to 120 to 140 watts (which you'll know if you already understand volt drop) so you're looking at 0.8 to 1 ohm each. 2 ohms per plug is 6 amps max and so 72 watts. But, if the element has burned out and become a hardened ball of nickel in the tip, the 2 ohms wont generate much heat.

Have you tonged the plug supply line from the relay? (I'll assume you definitely know what tonging is) as the whole set should draw over 40A.

Either test with a good wattage 12v lamp to see if current capacity is still there in the system and reaching the plug, or pull a plug and wire it across the battery, see if the tip actually glows. Dull lamp, high resistance somewhere. No tip glow, plugs are shot. Voltage without current does no work.

[kerlaan]

Hi Kerlaan,

A glow plug on average, should draw 10 to 12 amps per plug, equating to 120 to 140 watts (which you'll know if you already understand volt drop) so you're looking at 0.8 to 1 ohm each. 2 ohms per plug is 6 amps max and so 72 watts. But, if the element has burned out and become a hardened ball of nickel in the tip, the 2 ohms wont generate much heat.

Have you tonged the plug supply line from the relay? (I'll assume you definitely know what tonging is) as the whole set should draw over 40A.

Either test with a good wattage 12v lamp to see if current capacity is still there in the system and reaching the plug, or pull a plug and wire it across the battery, see if the tip actually glows. Dull lamp, high resistance somewhere. No tip glow, plugs are shot. Voltage without current does no work.

Hello Zion,

You made me laugh with that last sentence.  Good old V, I and R eh?  Also with tonging.  I had visions of getting my wife to put her tongue across the relay...  (I actually did that with the plug in the 1960's scalextric set as a kid after reading about Galvianni's experiments...  I flew up against the wall with a burned tongue.  "I wasn't doing nothing mom, honest".  How I survived I have no f*cking idea.

I'm just about to go and put a 100W lamp (yes, I have one still, although a relic) and see what happens.  I wish I had a DC clamp meter though.

I'll post what I find.  Your thoughts confirmed my aged, dementia ready, mindset.  I suspect the plugs are toast, for whatever reason and that ROOT CAUSE MUST BE FOUND.  It's the law you know?

Bye for now, thanks again for the smile.  I needed that.  

[GraemeT]

Hello - two ideas - battery maybe sulking? What happens if you add in a jump start/booster to the existing setup?
       - wee bit of a peely waly starter motor?

[kerlaan]

Hi Kerlaan,

A glow plug on average, should draw 10 to 12 amps per plug, equating to 120 to 140 watts (which you'll know if you already understand volt drop) so you're looking at 0.8 to 1 ohm each. 2 ohms per plug is 6 amps max and so 72 watts. But, if the element has burned out and become a hardened ball of nickel in the tip, the 2 ohms wont generate much heat.

Have you tonged the plug supply line from the relay? (I'll assume you definitely know what tonging is) as the whole set should draw over 40A.

Either test with a good wattage 12v lamp to see if current capacity is still there in the system and reaching the plug, or pull a plug and wire it across the battery, see if the tip actually glows. Dull lamp, high resistance somewhere. No tip glow, plugs are shot. Voltage without current does no work.

Hello Zion,

You made me laugh with that last sentence.  Good old V, I and R eh?  Also with tonging.  I had visions of getting my wife to put her tongue across the relay...  (I actually did that with the plug in the 1960's scalextric set as a kid after reading about Galvianni's experiments...  I flew up against the wall with a burned tongue.  "I wasn't doing nothing mom, honest".  How I survived I have no f*cking idea.

I'm just about to go and put a 100W lamp (yes, I have one still, although a relic) and see what happens.  I wish I had a DC clamp meter though.

I'll post what I find.  Your thoughts confirmed my aged, dementia ready, mindset.  I suspect the plugs are toast, for whatever reason and that ROOT CAUSE MUST BE FOUND.  It's the law you know?

Bye for now, thanks again for the smile.  I needed that.  

I used to say that if it wasn't for bad luck, I wouldn't have any at all.  That seems to hold true today.

I have received new glow plugs from what I suspect was a faulty batch.  The four all have resistances in the MΩ region, not open circuit but so high it beggars belief.

4 new BERU glow plugs on order...  
that my doubting, cynical mind will subject to resistance and glow test before they are assembled.

Can you hear my language over there in UK....  grrrr.

But you have to laugh.  Time for an early aperitif!

Thanks all.

[kerlaan]

Hello - two ideas - battery maybe sulking? What happens if you add in a jump start/booster to the existing setup?
       - wee bit of a peely waly starter motor?

No, it was 4 new glow plugs from what I suspect must be a faulty batch.  All of them in the MΩ level!!

[Sol]

Peely wally....now there's a good Scots saying!

And on the subject of tonging....in Scotland we use the word Tong (or tong-ing) for clamp testing, sorry I forget sometimes and lapse to colloquial terms. It means the same as clamping, not to be confused with tongue-ing :cry

[oilyrag]

Peely wally....now there's a good Scots saying!

And on the subject of tonging....in Scotland we use the word Tong (or tong-ing) for clamp testing, sorry I forget sometimes and lapse to colloquial terms. It means the same as clamping, not to be confused with tongue-ing :cry

Two weeks ago, one of our Irish Donkeys, Donal,  bit one of the LED lights off of his collar whilst I was looking the other way - heard the crunching as he munched the plastic bits - then he went nuts, bucking and kicking - presumably when his tongue had found the well-bitten coin type batteries. After a while he managed to spit most of the parts out, with the rest, well, 'recovered' shall we say, the following day.    
I'd like to think he wouldn't do it again, but he probably would.

[Sol]

Ouch! Good job they made a re-appearance!

[oilyrag]

Ouch! Good job they made a re-appearance!

I kept thinking of finding the sixpence in a Christmas Pudding whilst I searched ........