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ebmbiker

Intermittent starting problem

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Good day folks,

Hope someone can help shed some light on an intermittent starting problem I've got on my 1980 930.

When not in use the car is kept in my garage on a battery tender. Always starts strongly first time when I take it out of the garage.

No troubles when driving and the battery light doesn't come on and if I stop for fuel or 15 minutes somewhere it starts no problem.Trouble starts when it's been driven somewhere and left to stand...but not always.

When turning the key it won't start and there is a buzzing noise coming from the rear of the car. I'll try this a couple of times and no joy. Leave it for 5 minutes and then it will start but sluggishly. Once running and back on the road and all is good again.

Fitted a brand new Bosch alternator a couple of months ago as part of on going maintenance. 

Any ideas?

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Can you expand on the buzzing noise a bit? Is it more of a chatter?

You either have a tired starter or some tired high current wiring (Positive or Earths) or both. A fresh starter may relive the problems for a while but if the fundamental problem is with some of the high current wiring then it will reappear at some point.

Reason it only happens when the car is warm is that heat affects the resistance of all the wires and connections which in turn reduces the current available to power the starter. Repeated attempts to start the car when in this state will generate additional heat at the weakest part of the circuit.

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4 minutes ago, jevvy said:

Can you expand on the buzzing noise a bit? Is it more of a chatter?

You either have a tired starter or some tired high current wiring (Positive or Earths) or both. A fresh starter may relive the problems for a while but if the fundamental problem is with some of the high current wiring then it will reappear at some point.

Reason it only happens when the car is warm is that heat affects the resistance of all the wires and connections which in turn reduces the current available to power the starter. Repeated attempts to start the car when in this state will generate additional heat at the weakest part of the circuit.

Thanks for the reply jevvy. 

Hmm, it could be described as a form of chatter now I think about it. 

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What are you like with a multimeter? You can do a voltage drop test when cranking the starter to give an indication on how much power is getting to the starter but this test is only really of use when the car is failing to start.

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There should always be an audible buzzing from the engine bay of all 930s, and all SCs, from their CDI box when the ignition is on, so a buzzing may be a "red herring".

I would go with Jevvy's dodgy starter and heat soak as a starting point.

One added point, a std CDI box will not work if the voltage drops too low as it could do cranking a dodgy starter. @Jonny Hart is the expert on this aspect.

Mark

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When you say ‘it won’t start’ does it crank?  

It it is cranking but not starting I would suspect the timer relay that is unique to the Turbo.   This allows the ignition to run on for a few seconds after the ignition is switched on.  When these go wrong, there is no power to the CDI unit.  So next time it doesn’t start, see if the CDI box is producing its characteristic whistle.

 

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1 hour ago, Jonny Hart said:

When you say ‘it won’t start’ does it crank?  

It it is cranking but not starting I would suspect the timer relay that is unique to the Turbo.   This allows the ignition to run on for a few seconds after the ignition is switched on.  When these go wrong, there is no power to the CDI unit.  So next time it doesn’t start, see if the CDI box is producing its characteristic whistle.

 

Thanks for the replies guys. 

No, it doesn't crank and the buzzing noise is much louder than the usual CDI noise. I've took the car to Dave Griffiths Racing, my local Porsche indie, as I'm not that hot on electric side of things 

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On 4/29/2019 at 10:44 PM, Jonny Hart said:

Ok. so we are back at starter motor or solenoid or wiring. 

Thanks Jonny,

You came up in conversation I had with Dave Griffiths yesterday. 

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9 hours ago, ebmbiker said:

Thanks Jonny,

You came up in conversation I had with Dave Griffiths yesterday. 

Yes, 917 wiring diagrams aren’t that easy to come by!   

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19 hours ago, Jonny Hart said:

Yes, 917 wiring diagrams aren’t that easy to come by!   

What old man Griff doesn't know about the 917 isn't worth knowing!

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On 4/29/2019 at 10:44 PM, Jonny Hart said:

Ok. so we are back at starter motor or solenoid or wiring. 

Update on the problem. Ben at Dave Griffiths Racing took the starter motor off last Friday and one of the terminal eyes came off the wire and the other was loose. He's soldered new ones on and I pick the car up later.

Just in the nick of time as I'm driving to Hamburg next week in it for Roughneck Brigade TooCool2Cruise gathering.

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