7 days and still a weak spark - inspiration required please

7 days and still a weak spark - inspiration required please

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dwhitaker

Original Poster:

174 posts

122 months

Saturday 18th October 2014
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Been chasing this down for so long would you believe its getting a bit tiresome?

We have a spark that only a spark tester can show, with the garage lights off. Nothing at the business end of the plug. Plug is on a HT lead direct off the coil held to the engine with a pair of mole grips - to remove dizzy from the equation.

The story so far (very abridged). The car is an 290 S2.
Changed ignition module, and then dizzy plus ignition module thanks to a loan from Phillpot.

Changed coil for a brand new one off the "alternative parts list". VALEO 245025. Old one was duff for sure.
- Haynes manual for Granada 2.9 says primary should be 0.72 to 0.86 ohm, this is 1.5 ohm, and the secondary should be 4.5 to 7 kohms, and this is 6.8 kohms. Is this likely to be a problem? Anyone else got this coil on a 2.9?

Bridged coil positive direct to battery - to eliminate any resistance fault supplying power to the coil.

Removed "yellow" connector. And fixed a bunch of other wiring bodges - including a bit of thin co-ax used to extend the solenoid feed!!! - no wonder the solenoid has been a bit iffy.

Checked -ve coil with led/resistor tester, shows flashing while cranking.

Now becoming stumped. Tomorrow need to check the earth side, but don't know where to find the earth of pins 40/60 of the ECU. Does anyone know where it is? I assume the -ve of the coil is pulsed to earth via the ignition module, then the ground connection back the the ECU, then to the ECU ground on pins 40/60. Any resistance in this path would mean less voltage across the coil primary and therefor less spark from the secondary. As the coil is only an ohm or so, then even 1 ohm of resistance in the earth would be a problem.

Any further ideas appreciated.

Thanks in advance,
Cheers
Dave

Barkychoc

7,848 posts

203 months

Sunday 19th October 2014
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Got to be earth issue. I think you need to take a deep breath and go under the passenger side of the dash. Forget the resistance of the coil you will add an ohm or so with the meter leads it's very difficult to measure low ohm values accurately. When I looked at the V8S the loom ran pretty much independent to everything else on the car with a couple of power connections and earth's. You need to find those.

v8s4me

7,234 posts

218 months

Sunday 19th October 2014
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What is the coil mounted to?

tvrgit

8,470 posts

251 months

Sunday 19th October 2014
quotequote all
v8s4me said:
What is the coil mounted to?
Was gong to be my question. Try earthing the coil casing.

Tarmacshredder

135 posts

129 months

Sunday 19th October 2014
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I would start with just the coil and work backwards, as you have done run live to positive terminal on coil, earth the coil body, run the lead with plug to engine block then earth the negative terminal of coil momentarily to make the coil fire the plug. If you have no spark on the plug run a negative jump lead to the engine block and negative of battery to rule out engine earth strap. Once you have the coil firing then next step is to put the negative terminal on the coil wiring loom back on. If the engine still doesn't fire the plug when being turned over then remove the ignition amp plug and earth the coil wire there to make sure your wire isn't breaking down. Next step test the ignition module, then wiring back to ecu or distributor that sends the signal to the ignition amp.
Hope that helps.

dwhitaker

Original Poster:

174 posts

122 months

Sunday 19th October 2014
quotequote all
Measured earth wire from ignition module to ecu as .2 ohm, and from ecu to ground also as .2 ohm. However measuring from ignition module via ecu to ground was high res. Turned on ignition. Dropped but only to 195 ohms. I am guessing that this earth path is used to ground the coil so suspect the ecu. Does that make sense? This would mean a replacement ecu needed.

V8 GRF

7,294 posts

209 months

Sunday 19th October 2014
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Have you changed the Spark and HT leads?

dwhitaker

Original Poster:

174 posts

122 months

Sunday 19th October 2014
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Yes we have tried several leads.

v8s4me

7,234 posts

218 months

Sunday 19th October 2014
quotequote all
dwhitaker said:
.......I am guessing that this earth path is used to ground the coil so suspect the ecu. ...
Have you tried earthing the coil directly to the chassis as per TVRGit above?

Barkychoc

7,848 posts

203 months

Sunday 19th October 2014
quotequote all
Is there a smallish brown wire to the -ve terminal of the battery?
If the car uses a standard ford loom that's the negative for the engine loom.
V8S uses a standard range rover loom.

There is also looking in the Sierra manual a number of brown earths connected together near the ecu, one of those is the other end of the brown wire off the battery, another comes off the dizzy module.
Try measuring resistance between brown wire on dizzy module and brown wire if it's on battery neg otherwise just battery negative.

Edited by Barkychoc on Sunday 19th October 14:20

tvrgit

8,470 posts

251 months

Sunday 19th October 2014
quotequote all
v8s4me said:
dwhitaker said:
.......I am guessing that this earth path is used to ground the coil so suspect the ecu. ...
Have you tried earthing the coil directly to the chassis as per TVRGit above?
TBH I would start by earthing the coil case direct to battery negative. If that wprks, THEN try earthing to the chassis. If that still works then you know the engine block earth to chassis is ok, and chassis to battery earth is ok. (if either of them isn't ok, then "earthing" to the chassis might lead you up the wrong diagnostic path, thinking that "a good earth" makes no difference, IYSWIM)

v8s4me

7,234 posts

218 months

Sunday 19th October 2014
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[quote=tvrgit]...if either of them isn't ok, then "earthing" to the chassis might lead you up the wrong diagnostic path, /quote]

Good point.

tvrgit

8,470 posts

251 months

Sunday 19th October 2014
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v8s4me said:
tvrgit said:
...if either of them isn't ok, then "earthing" to the chassis might lead you up the wrong diagnostic path,
Good point.
I'm not having a dig, mate - I always try to make sure that diagnostics are methodical, because you see so much stuff where people leap from one conclusion to the next.

phillpot

17,105 posts

182 months

Sunday 19th October 2014
quotequote all
tvrgit said:
v8s4me said:
What is the coil mounted to?
Was gong to be my question. Try earthing the coil casing.
This is my car some time ago, coil quite happily mounted onto the fibreglass bulkhead, no earth, no issues.



Surely the only earth a coil needs is through the ignition circuit, this is the whole principle of how it works ????




"old fashioned" points and condenser but same principle.


Edited by phillpot on Sunday 19th October 18:12

tvrgit

8,470 posts

251 months

Sunday 19th October 2014
quotequote all
phillpot said:
This is my car some time ago, coil quite happily mounted onto the fibreglass bulkhead, no earth, no issues.

Surely the only earth a coil needs is through the ignition circuit, this is the whole principle of how it works ????




"old fashioned" points and condenser but same principle.
Well you would think so, and I'm not clever enough to work out why, but I can well recall when I was just a gitcub, taking the coil off a Ford Cortina (Mk1 at that!) and scraping at the paint (well, rust) under the mounting bracket to get a better spark when it wouldn't run properly.

phillpot

17,105 posts

182 months

Sunday 19th October 2014
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"Illogical captain"... smile




........ Sorry, this isn't helping our friend get his car sorted

Top Gear TVR

2,244 posts

153 months

Sunday 19th October 2014
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I had issues ages ago. I have decided that the approach when flummoxed is to pay a guy with the knowledge and equipment. In an hour the best can find what would take the fairly well briefed weeks. £90!an hour for the top guy in your area. In an hour you'll know.

My favourite is Clive Atthowe but he's in Norfolk.

Oldred_V8S

3,714 posts

237 months

Sunday 19th October 2014
quotequote all
phillpot said:
tvrgit said:
v8s4me said:
What is the coil mounted to?
Was gong to be my question. Try earthing the coil casing.
This is my car some time ago, coil quite happily mounted onto the fibreglass bulkhead, no earth, no issues.



Surely the only earth a coil needs is through the ignition circuit, this is the whole principle of how it works ????




"old fashioned" points and condenser but same principle.


Edited by phillpot on Sunday 19th October 18:12
When we assisted PotlessPaul on the Eurotour and advised he swapped the coil, his coil needed to be earthed before it would work.

dwhitaker

Original Poster:

174 posts

122 months

Sunday 19th October 2014
quotequote all
I believe most coils have the secondary earthed through the case. The one pictured may have achieved an earth through the case because the back of the fiberglass where its bolted is covered with a metal mesh which is itself earthed.

We have done an tap to earth on block from the coil secondary, and this does make a spark.


phillpot

17,105 posts

182 months

Sunday 19th October 2014
quotequote all
dwhitaker said:
I believe most coils have the secondary earthed through the case.
The coil is basically a transformer, converting 12 volt to several thousand volts
This means the primary and the secondary are connected together. When the points close (all "electronic" these days) 12V flows through the primary and makes a big magnetic field. When the points open, the magnetic field collapses and induces a very high voltage very fast spike through the secondary which is then distributed via the rotor arm rotor to the appropriate spark plug.

Those electrons are seeking earth which is the tab on your sparkplug and they jump across the gap, and hence we get the "bang" in suck - squeeze - bang - blow.
If the case was earthed those electrons certainly wouldn't choose to jump the spark plug gaps smile

The only purpose in earthing the case (which is often painted or wrapped in a plastic label so wouldn't earth through bracket) is to reduce radio interference.

Lessons over for today teacher