Testing Glow Plugs

[chas]

I won't bore with the background, but I am in a situation where I believe that two or more of the glow plugs on my T90 are not working. I get the same current drain showing on my ammeter that I have always had but starting is difficult and there is a fair amount (not excessive) of white smoke when it does start. This disappears after the engine warms up a bit. Wise old diesel chappie said that it could be some duff glow plugs and often when they failed, they still took the same amount of amps as they did when they were working (so my ammeter check is invalid). White smoke being caused by fuel in the unwarmed cylinders being expelled unburned.
Being a total glow plug testing virgin, can someone please tell me if there is a way of checking if the glow plugs are working without taking them out? I have one of those ogger instruments for measuring electricity but am not too familiar with it.

Any help much appreciated.

 

[nedmin]

I suppose you could disconnect them all but one,keep disconnected wires clear of earth,energise and I would have thought the outside should get warm of that one, then test the others same way. easiest way is to take them out and check across a battery.they are high current so you would need a really good low resistance meter to check.

 

[Talbot]

My car glow plugs are relatively cheap so cant believe that they will be that expensive for your engine. If you dont trust them, replace them.

 

[VicS]

Disconnect the wiring then test each one between its terminal and body with a multimeter on its lowest ohms range or continuity range if it has one. Their resistance is very low so they show up as virtually zero ohms if they are OK.

If you do not have a multimeter connect two long wires to a 12volt bulb. Connect one wire to a gloplug terminal and touch the other on the battery +ve. If the bulb lights the plug is OK. Repeat for each gloplug.

I do this regularly on my diesel car which for some reason eats gloplugs. Sometimes I use the multimeter sometimes I use the bulb.

 

[owen]

I have had exactly the same problem on my t90.i checked the old plugs out of the engine and they were fine but i replaced them anywayas they were ancient; cleaned out the plug holes as decribed in the manual , but no real improvemnt to starting. I am just putting up with it. hope you get an answer but it may not be the plugs.

 

[savageseadog]

They could be almost burnt out, you will only be able to tell by removing them and putting them across a supply. I really don't think it's worth the risk, your life might depend on it.

 

[alb40]

my old t90 used to be a right royal git to start. A blow torch down the manifold sufficed until i got a chance to take a look at the plugs.

Anyway, new ones from ASAP supplies went in (about 7 quid each i think), and with 30 seconds holding the button, then crank over, it then fired instantly with little white smoke.

Do make sure your starting it right before you bother taking the plugs out (there a bit of a sod to get to sometimes). They need a good 30 seconds on the plugs and full throttle when cranking to start well. (with the new plugs, mine would fire ok with 10 seconds, but took a little more cranking)

incidently, i took the old ones and held a wire to the battery to them on the floor, and they all went bright red hot still, but obviously, the new ones must get that bit hotter still to help more.

 

[ajt]

Had the same on a car years ago, a mechanic told me to remove them and connect to a battery, if they get hot they are fine. All but one glowed red hot so replaced only one engine then started first time and no smoke. New plug was only a few quid.

 

[2Tizwoz]

The engine and its problems and solutions is discussed in detail here at Canal Junction

 

[gorb]

Don't know about the T90, but on other engines I have had, the glow plugs are in series, so that if one goes, they all cease to work.

If you really doubt them, put an meter on the 'ohms' scale across the two terminals of each plug and it should read as near to zero ohms as doesn't matter (like less than 1 ohm). Do this for each plug. They are often 2V plugs so on a 4 cylinder engine you will usually find anothet one or two 'dummy' plugs in the circuit. Check them too.

 

[LadyInBed]

I have never seen any wired in series!
They have all been wired in parallel, so to test them, each one has to have the wire to the top cap disconnected then metered on a low ohms scale from top cap to engine block.
From memory, a good one is less than 400 ohms and when they start getting up to and beyond 600 ohms they need replacing.

 

[chas]

Thanks everybody. As normal, lots of helpful advice very quickly. Tiz, that a great site - thanks.

I am reluctant to take out all the glow plugs cos last time I did that two broke in the cylinder head and I had to take off the head to clean out the mess. Boat is now on the mooring waiting to be used and I was trying to keep the murphy factor to a minimum. If I can identify duff ones without taking any out, it would help no end, hence my query. I will try measuring the resistances as suggested.

 

[gorb]

Sorry, ladyinbed, but they often are! I've worked on Mercedes OM636 (50 HP) and BMC Commander (50 HP) and both have 2V plugs arranged in series, with a couple of dummy plugs elswhere in the circuit to match the 12V supply.

And doing the sums on your figures, a 400ohm plug across 12V is going to give about a third of a watt of heat. Don't think that would make much impact per cylinder!

 

[oldharry]

[ QUOTE ]
I have never seen any wired in series!
a good one is less than 400 ohms and when they start getting up to and beyond 600 ohms they need replacing.

[/ QUOTE ]


Never sen plugs wired in series either. But also never seen them with such high resistance - usually .5 and 1.5 ohms in my experience, so very difficult to tell betwen a good one, and one with an internal short. However multimeters quicjkly show up any that are open circuit.

 

[owen]

I for some reason thought mine were in parallel but they are obviously in series. live feed comes to plug 2 with wire from there to plug 1 and 3 ;and from 3 to 4

 

[Bodach na mara]

I do not know about the BMC Commander (which is based on the 2.2 litre engine) but the Captain (1.5 litre) had 2 ohm plugs wired in parallel. They cannot possibly be wired in any other way (and I cannot see how any other engine could be different) as they are all fed by a single cable, daisy chained (as described by Owen) to the terminal on top of each plug and they have a common (earth) return via the metal of the cylinder head.

As stated, the resistance is 2 ohms, rated current is 6 amps, power 72 watts each and total draw for the four plugs is 24 amps. When I got that boat, two of the plugs were dud. I got two replacements from a spares place patronised by black cabs owners. That was a few years ago and I cannot remember the price. Before replacement, starting was an "easystart" job, but I never liked using this stuff. Once they were replaced, 10 seconds of heater was enough for a start on the coldest day. No heater was required after the first start of the day.

 

[duncanmack]

Ehmmmm, you've just described parallel wiring.........

 

[Dave_Seager]

Re: Testing Glow Plugs (owen)

What you describe sounds like parallel to me. To be connected in series each plug would need to have two terminals with power fed to one terminal, passing through each plug in turn before connecting to ground.

My Volvos (AQD32As) are connected in parallel. The plugs each have one terminal and all are connected to a buss bar which supplies the power. The return circuit is through the body of the plug to the cyclinder head.

 

[owen]

Re: Testing Glow Plugs (owen)

yes sorry you would be correct. the connections at the top provide the common live and each plug completes the circuit thro to ground in the cyl head.

 

[gorb]

Re: Testing Glow Plugs (owen)

I can't speak for all engines, but my two cases LOOKED like they were in parallel. However, close inspection showed an insulator on the screw terminal between the wires which looked like daisy-chaining, but which were in fact in series, with the body isolated from the engine block.

Like I say, I am NOT arguing that all heaters are in series, but on Merc OM636 and BMC Commander versions I had, they were in series - fact! Tracing the wiring then showed, for the Merc, a dummy coil and then, in series, a 'glow-plug coil' which was meant to have been ignition panel mounted so that you could see it glowing as you ran the heaters. Simple visible verification that the series of heaters was operational. I am sure about this because I have just had to rewire a part of the circuitry.

Anyone elso out there found the same, after careful inspection?