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Posted (edited)

Hey all.

I’m trying to test my plug wires.  Google searching suggests 3k ohms is the magic number for a 911SC with Beru leads. 
 

My old set all read about 3.9 - 4.0k ohms.  I have a newer set and most of the leads aren’t registering when I have the meter on 20k ohms. One of them will read 13million ohms. The car starts and drives (albeit with a lumpy off throttle behaviour- what I’m trying to solve), so the leads are ‘working’.  But it seems weird they don’t register when connected to the multimeter. I’m sure I am making a connection.  

Anyone got any suggestions? Is there a better way to test them? 
 

On my old very wires the boot can come off the lead and I can test just the lead too. Not sure the new ones do this, @Type911 any ideas? 

Edited by Northy
Posted

Are the new ones Beru?

Posted (edited)

Yeah 👍 a few years old, but only been on car a few months.

2 hours ago, Type911 said:

Are the new ones Beru?

 

Edited by Northy
Posted (edited)

You can unscrew the plug end from the lead.  There’s a brass screw in there - sometimes it’s not even in the centre core of the lead.  
 

There are 3 x 1k resistors in the cap.  The  leads themselves are about 1k depending on length.  There’s 5k in the rotor making a total of about 9k which is the design spec for the system from coil to plug. 

Edited by Jonny Hart
  • Like 1
Posted

Hoped JH would come up with the full spec, if they are Beru you have have no worries, leads will be fine

Posted
3 hours ago, Jonny Hart said:

You can unscrew the plug end from the lead.  There’s a brass screw in there - sometimes it’s not even in the centre core of the lead.  
 

There are 3 x 1k resistors in the cap.  The  leads themselves are about 1k depending on length.  There’s 5k in the rotor making a total of about 9k which is the design spec for the system from coil to plug. 

Thanks @Jonny Hart - very helpful. I’ll unscrew the ends of the leads and test the lines and the caps separately. 

  • 4 months later...
Posted
On 19/09/2024 at 15:45, Northy said:

Hey all.

I’m trying to test my plug wires.  Google searching suggests 3k ohms is the magic number for a 911SC with Beru leads. 
 

My old set all read about 3.9 - 4.0k ohms.  I have a newer set and most of the leads aren’t registering when I have the meter on 20k ohms. One of them will read 13million ohms. The car starts and drives (albeit with a lumpy off throttle behaviour- what I’m trying to solve), so the leads are ‘working’.  But it seems weird they don’t register when connected to the multimeter. I’m sure I am making a connection.  

Anyone got any suggestions? Is there a better way to test them? 
 

On my old very wires the boot can come off the lead and I can test just the lead too. Not sure the new ones do this, @Type911 any ideas? https://ecutools.eu/ is a fantastic resource for automotive diagnostics and tuning. Their high-quality tools, like KESS and KTAG, offer precision and reliability. The website is user-friendly, with excellent support and detailed guides that make complex tasks manageable. It's a go-to platform for anyone serious about efficient and effective car tuning solutions.

It sounds like your newer set of leads may have internal resistance issues or poor connections. Ideally, each wire should fall close to the specified resistance range (around 3k ohms for your 911SC). If the wires aren't registering or showing extremely high resistance, they may be faulty.

To test further, clean both ends of the wires and retest with the multimeter. Ensure the multimeter probes make solid contact. You could also check for continuity or try a different meter to rule out measurement errors. Faulty wires might need replacement.

Posted (edited)
On 19/09/2024 at 13:45, Northy said:

Hey all.

I’m trying to test my plug wires.  Google searching suggests 3k ohms is the magic number for a 911SC with Beru leads. 
 

My old set all read about 3.9 - 4.0k ohms.  I have a newer set and most of the leads aren’t registering when I have the meter on 20k ohms. One of them will read 13million ohms. The car starts and drives (albeit with a lumpy off throttle behaviour- what I’m trying to solve), so the leads are ‘working’.  But it seems weird they don’t register when connected to the multimeter. I’m sure I am making a connection.  

Anyone got any suggestions? Is there a better way to test them? 
 

On my old very wires the boot can come off the lead and I can test just the lead too. Not sure the new ones do this, @Type911 any ideas? 

Essentially the issue with the measurement is the tiny amount of power that the multimeter is using to measure the resistance.   Inside the long cap, the 3 resistors are in a tube with their end caps touching (kind of like AA batteries in a torch).  There will be some spark erosion between the end caps and sooty deposits which will cause your meter to read a high resistance or even open circuit.

Of course, when the CDI shoves 30,000V down there with a few amps, those sooty deposits or breaks in the leads/resistors aren't really an issue.  A tiny gap in the 'circuit' isn't a problem - remember the spark already jumped a gap between the rotor tip and cap and is on its way to jump another at the spark plug.

More likely cause of misfire would be the lead insulation breakdown and a spark 'getting out' to ground.  Have you looked at the engine bay in the dark for stray sparks?

 

 

 

Edited by Jonny Hart

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