Coil experts

bahama68 · Post by **bahama68** » Thu Jun 21, 2007 6:05 pm

I have no idea whether to put this in wanted, here or technical. Or whether to pull my hair out and scream having just read the threads about the Brazilian silver coils.

My car won't start. The fuel pump's pumping, the starter's working (I've only recently had a new one fitted) and the engine's turning over but it will not fire up. It just goes and goes and goes until the battery dies. A guy tested it for a spark today, there wasn't one so his diagnosis was the coil.

However, (this has been going on for a month now) I've found that I can jump the car off a friends and it will start, eventually. And once it's started it runs fine, no problems. I can even stop and restart it - straight away and even up to a couple of days after - without any problems. But if I leave it for more than a week it won't start again.

So, does this sound like the coil or possibly something else? In the last twelve months I've had a new distributor and starter and, when I bought the car two years ago, the ignition was updated to CDS (Is CDi just a different way of describing the same thing?) Chris Turner checked my points last week, so that's not it and I've had new fangled expensive spark plugs twelve months ago when I got the new distributor.

Any thoughts gratefully recieved. As I'm looking to sell my car as soon as possible I'd rather not go through the stress of hunting down a new coil if it isn't actually the problem. My girlfriend lives in NY and I'm going there next week so I could just order one from Pelican, get it sent to hers and pick it up when I'm over there, then replace mine then I can eliminate the coil from the list of things it could be. Although that depends if they have any that aren't of the silver faulty variety.

John

impmad2000 · Post by **impmad2000** » Thu Jun 21, 2007 6:40 pm

If you'd like to try my coil. Please give me a shout.
I assume that the CDS system uses the same type of coil ... It may not.
Got any details on this CDS system
Cheers
Tim

Nick Moss · Post by **Nick Moss** » Thu Jun 21, 2007 6:44 pm

Its a standard LWB CDI system that I put on the car to cure the typical SWB high rev spark problem.

bahama68 · Post by **bahama68** » Thu Jun 21, 2007 7:30 pm

Thanks for the offer Tim. From what Nick said does it sound like we use the same coil?

This is the one I was looking at on Pelican - it's a 69-73 for use with a CD box:

http://www.pelicanparts.com/catalog/ima ... 1_coil.JPG

http://www.pelicanparts.com/catalog/sho ... htm#item14

chris68 · Post by **chris68** » Thu Jun 21, 2007 7:55 pm

Hi,

A friend of mine had a made in Brazil Bosch coil fail on his 911T (fitted with CDI). To get it going in a hurry we replaced it with a spare coil I had (for transistorised ignitions) from www.autoelectricsupplies.co.uk. The car started immediately and he's been running fine with it ever since.

At £13.00 (type 060705) it's a quick cheap way to check if your Bosch coil was the problem.

Cheers
Chris

Vic Cohen · Post by **Vic Cohen** » Thu Jun 21, 2007 9:19 pm

Dont want to spoil any progress here , but i ve had 4 faulty Brazilians, currently running on rare old German one ...............on each occasion they ve failed in the fast lane , but NEVER started again! Once they re gone, they re really ex! I also had minor dispute with very helpful RAC man who towed me in after the first one ,telling him it could nt be the coil it was new 2 months earlier!
Good luck , i am still awaiting Bosch [Denham] to tell me why they failed ?

bahama68 · Post by **bahama68** » Thu Jun 21, 2007 9:28 pm

Chris - thanks for that, I'll give it a try.

Vic - I read a couple of threads and saw the troubles you've been having. It's such bad timing for me as I'm leaving the country and need to sell my car. Maybe the Porsche God's are trying to tell me something. Do you know, once the coil's failed should I then be able to jump start my car? It seems I'm only having a problem starting but once it's running everything's OK.

John.

FMAN · Post by **FMAN** » Thu Jun 21, 2007 10:46 pm

Hi,

It could be the coil but in my experience they either work or they don’t. You could test it by disconnecting the wire to the CDI, connecting to 12V to the positive coil terminal, when you disconnect the 12 V you should get a spark , you will need to have the HT wire disconnected from the distributor and within a few mm of an unpainted part of the engine.

Other possibilities are volt drops in critical parts of the ignition circuit. Try measuring the following when you are cranking: - Battery voltage, positive feed to the CDI box referenced to the body earth (should be similar to the battery), voltage difference between CDI box earth terminal and coil earth (should be 0V), negative terminal of the coil to distributor earth (should be 0V).

What part of Birmingham are you in?

Regards

Neil

bahama68 · Post by **bahama68** » Thu Jun 21, 2007 11:01 pm

Thanks Neil. To be honest I only understood a little of what you mentioned. I'm in West London at the moment, I grew up in Birmingham and don't get back there as much as I used to, although my family's now moved to Walsall. I might be back up in about 2 and half weeks, although that totally depends on whether my car will get me there.

John.

bahama68 · Post by **bahama68** » Thu Jun 21, 2007 11:12 pm

Actually I understood more of it than I first thought.

Just to clarify. I disconnect the HT that connects the coil to the distributor from the top of the distributor then disconnect the live from the CDI and connect it to positive coil terminal, then disconnect it and I should get a spark? Should I have someone cranking the engine whilst I do this? And if there is no spark the coil isn't working?

John

Vic Cohen · Post by **Vic Cohen** » Fri Jun 22, 2007 6:41 am

I may now have the experience but am no electrician ............my point was they either work or they dont ,meaning dont think it is ur coil ...............last couple of post will help i hope

Gary71 · Post by **Gary71** » Fri Jun 22, 2007 7:42 am

Just a thought, but have you looked in close detail at the wiring going to your coil?

It may be that you have a high resistance connection on the low tension side and the additional current capacity from your friends car is holding the battery voltage up high enough as you crank to overcome this?

Once your car has started and the engine warmed up a little the drain to start the car is much less, hence you may be able to start it on your own.

This may of course be rubbish

Hope you get it fixed

impmad2000 · Post by **impmad2000** » Fri Jun 22, 2007 8:31 am

The symptoms that you describe seem to indicate a weak spark (though can't rule out a mixture problem)
Weak spark could be caused by poor connection between the CDS system and the coil, a fault on the coil that shorts out part of the windings (More likely to kill the coil dead) . Or the High Voltage prt of the CDS system not generating the full spark charge. A CDI type system is generally immune to voltage drop at start-up, more likely to effect a conventional kettering type (points and condensor).
The problem is that it is easier to detect a completely dead part of the system than it is to find something that is only "weak".
I reckon the best solution is to try substituting parts with known to be good.
On a friends car last week, we had to tow start it. Once running it was fine to re-start. It was set increadibly rich ! These symptoms could indicate a carb problem ! when slightly the warm the rich mixture will fire more easily. Give the engine more revs at start (jump start) it will also be more likely to start.
Just some thoughts.
Cheers
Tim

bahama68 · Post by **bahama68** » Fri Jun 22, 2007 11:48 am

Tim and Gary - thanks.

I realised I had a copy of the Haynes Classic Car Electrical Manual sitting on my shelf last night - I must have bought it when I was having some other electrical fault. It said to test the resistance across the coil to see if it was dead or not. I went down and checked it this morning. The primary coil was reading 0 ohms and the secondary coil was reading 0.6 ohms. Unfortunately, as my only point of reference was the manual and it said I should be getting 2 ohms and 5 ohms respectively, I have no idea what my readings mean

I think I'm going to buy a new battery this weekend and order the coil that Chris mentioned. If that doesn't help I'll take Gary's advice and check the wiring on the LT side of the coil.

Is there a way I can test if the CDS box (correct me if this doesn't make sense) is the problem?

Thanks
John.

impmad2000 · Post by **impmad2000** » Fri Jun 22, 2007 12:15 pm

bahama68 wrote: Is there a way I can test if the CDS box (correct me if this doesn't make sense) is the problem?

Yes, but not easy and you need some kit ! I have made myself a little test rig, coil, spark plug, and high frequency trigger circuit ! It will tests the CDI out at simulated sustained high RPMs. I am happy to try your CDS out on the rig if you want to post it up.
The Porsche CDI whistles if working, and given a manual trigger (Flick the points) should give a nice fat spark to the spark plug (Just hang a spark plug from the HT lead from the centre of the coil). That is really the simplest test.
As for the coil readings, the Porsche CDI coil uses fairly low impedance windings which most multimeters will not give a sensible reading of. A kettering style coil would probably give readings like your book says. I could take reading from my coil later, but I don't really think the readings would be much help.
Do the wiring check first, It may be the cheapest fix possible (i.e Free!)
Have fun
Tim

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