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[Webby the Bear]

hi guys, want to check on my mates rover 25 that he has good spark. if I take plugs out, attach them to the leads and earth them on a part of the engine is that safe? ive seen my teacher do it but is it a habit I should not pick up? 

[Andy B]

hi guys, want to check on my mates rover 25 that he has good spark. if I take plugs out, attach them to the leads and earth them on a part of the engine is that safe? ive seen my teacher do it but is it a habit I should not pick up? 

Nothing wrong with that as far as I'm concerned. I did just that the other week with my Dad's car. Use Molegrips/Vicegrips so you avoid getting a shock!      It's not a habit you'll be able to pick up for too long anyway, because more & more cars use coil packs   

[mantahatch]

I have only seen it once but someone did this on flooded car. Interesting to see flames shooting out of the spark plug holes 


That said it used to be a very common way to check the spark. Some very good people I used to work with could diagnose quite a bit just by looking at the sparks colour/duration. And they where right  a lot more times than they where wrong. But I suppose that is the days of mechanics who repaired things rather than parts fitters you find in many places today.

[mantahatch]

hi guys, want to check on my mates rover 25 that he has good spark. if I take plugs out, attach them to the leads and earth them on a part of the engine is that safe? ive seen my teacher do it but is it a habit I should not pick up? 

Nothing wrong with that as far as I'm concerned. I did just that the other week with my Dad's car. Use Molegrips/Vicegrips so you avoid getting a shock!  It's not a habit you'll be able to pick up for too long anyway, because more & more cars use coil packs 

Thats a good point. Out of interst is any car still being made with conventional HT leads ? or dare I even say a distributor 

[martin42]

Use a spare plug instead of removing plug,and do 1 lead at a time

[Webby the Bear]

cheers boys

just checked the car and its the weirdest thing..... cyls. 1 and 3 has a coil pack each that and two leads going in to it for each cylinder 

presumably I can pull them out, leave them plugged in and earth them? if they'll stretch to the nearest earth point ??

[mantahatch]

I used to use one of these http://www.ebay.co.uk/itm/1-X-GENUINE-LUCAS-HT-LEAD-SPARK-PLUG-TESTER-YDB116-/111095539253?pt=UK_Diagnostic_Tools_Equipment&hash=item19ddcf6635


Still got one, somwhere, I think, gawd memory not so good these days


We also used to make one. Spark plug. jubilee clip and large croc clip. Flatten out end of crop clip and using jubilee clip attach to spark plug. Pull of lead clip on your nice home made tester and clip to good earth point. Worked very well indeed. Now def know where that is.

[henryd]

cheers boys

just checked the car and its the weirdest thing..... cyls. 1 and 3 has a coil pack each that and two leads going in to it for each cylinder 

presumably I can pull them out, leave them plugged in and earth them? if they'll stretch to the nearest earth point ??

Quite common Webby

[dbug]

cheers boys

just checked the car and its the weirdest thing..... cyls. 1 and 3 has a coil pack each that and two leads going in to it for each cylinder 

presumably I can pull them out, leave them plugged in and earth them? if they'll stretch to the nearest earth point ??

Quite common Webby

Use a jump lead to earth Webs

[Marks DTM Calib]

It tells you very little though as we have discussed many times in the past.

[Entwood]

You might get what looks to be a decent spark .. but it is only jumping through 14.7 psi, (29.92 " hg, 1013.25 hectopascals) (atmospheric pressure) .. a very much easier task than at the top of the compression stroke when the pressure is some 10 times higher and the "atmosphere" it is working in is also full of fuel vapour. All it will tell you is that the circuit is complete, nothing at all about the quality or capacity of the circuit.

TBH .. it is often better if you get NO SPARK when doing this test, as it tells you the circuit is broken and proper fault finding can be done .... IMHO ..

[Marks DTM Calib]

Indeed you only need about 1-2kV to jump a 1mm gap at atmosperic pressure, in a fuel/air mix at 10x atmospheric pressure you need 10-20 times this at least (with many systems operating at 35-40kV).

And given how the system generates this voltage (e.g. by a collapsing magentic field), a spark that jumps a 1mm gap when tested as above gives no indication that something is good.

Considering also that a coil rarely fails open or short circuit these days and that the general failure is related to the iron core in which the magnetic field is generated, its a pretty poor test.

[chrisgixer]

As Mark said, not that I understood 3/4s of it, given these can fail at various rpm and not another rpm, or fail under various load conditions and not others, I fail to see how a simple electrical test on a bench can give conclusive proof of a good or bad coil.

But then, I am utterly useless with electrics. So...  .... Listen to what Mark says


I wonder... Say we had a brand new tested ok omega 3.2 pair of coil pacs. And then loaded it under excess conditions for which it was not designed, say an 8 litre v6, if such a thing existed... ....would it eventually fail, or fail to provide a spark under such conditions eventually, or fail early. Even though there is nothing wrong with it, as such, given the purpose it was built for.


Manufactures only build stuff to be for for a purpose, not fit for all purposes , to my mind.

[Marks DTM Calib]

It would work fine in the conditions you describe.

It would be more intersting to see if the ECU coil driver would be happy driving it