Thoughts and advice on the care and feeding of fine automobiles from Machine Aficionado and bestselling author John Elder Robison, owner of JE Robison Service in Springfield, Massachusetts


We are independent restoration, repair, sales and service for Audi, BMW, Bentley, Jaguar, Land Rover, Mercedes-Benz, Porsche and Rolls-Royce automobiles.

Bentley archives

Land Rovers



Yesterday we took in a 2007 Porsche 911 with a chronic slow-crank problem.  The car tended to start fine in the morning, but later in the day, when warm, it struggled to crank.  The battery had been replaced three times, after one relatively new battery after another failed the Midtronics tester. The alternator was replaced 2,000 miles ago.  The owner now wondered if the car needed a starter, or if it had a wiring problem. In addition to that, I pointed out the engine could have internal drag when hot.

We had a few possibilities to chase . . .













Knowing this was a chronic problem that had already resisted several efforts at solution we started out with the Fluke 196 Scopemeter.  If you know how to use it, a scope is your fastest tool to get to the bottom of many electrical problems.  These tests could be done with a meter but the scope display adds another layer of information in the form of the pattern.  Newer digital scopes like this allow us to read voltage at any point on the graph, and also measure time differences between any two spots.

So we can see resting voltage, drop when cranking, how long the engine takes to start, and the operation of the alternator. On these cars the battery is under the hood and that was where we hooked the scope first. Here is what we saw.

On the left side of the screen you see the battery resting voltage, which is just under 12. The battery was just two months old. The resting voltage of a fully charged lead-acid car battery at room temperature should be about 12.8 volts.  The reading in this car indicated a defective or discharged battery.  The downward drop a few seconds into the trace is the moment we cranked the starter:


The battery voltage dropped to 9.65 volts, which is below the acceptable limit of 10 volts.  When crank voltage drops below 10 volts some of the computers drop offline, and set low voltage faults. Cranking starts to sound slow. This is an unacceptable reading, but again going point to either a defective or discharged battery.  Since the battery was new, and replaced another with the same symptom, we proceeded on the assumption this was not a simple battery problem. 

We relocated the scope to the alternator, which is at the opposite end of the high power cabling in this car.  The cable runs from the battery, to a junction box on the side of the gearbox, to the starter, and then to the alternator.  In principle, the voltage should be substantially the same at any point on this cable path.  That's not what we found. 

Crank voltage at the starter end of the car dropped all the way to 7.65 volts - a slow crank indeed. Alternator output rose immediately after startup to 14.1 and cut back to 13.7, because the alternator thought the battery was fully charged.

If you look at those numbers, you see we had 2 volts of drop in the cable when the car was cranking (9.65 to 7.65). We had just over 1 volt drop when the engine started and the alternator came on line (12.7 to 13.7) From those numbers we know the starter draw was a but more than double the alternator output.  That ratio tells us we don't have excessive starter draw, either from a bad starter or a tight engine.  It was good to get those possibilities off the table.

We're down to wire, as they say . . .


Resting voltage was the same, at 12 volts.  



Now we knew that came from a weak (mostly discharged) battery. We moved the scope meter from the alternator to the terminal block on the transmission, where the readings were the same as at the battery. That gave us our answer.  There was no significant drop from the battery to the transmission.

The voltage drop was worst on starting, which told us the issue would be in the cable from transmission junction to starter, or in the terminations at one end or the other. Getting that cable out for viewing wasn't easy, but the answer was clear when we saw it:



The cable termination at the starter connector was partly melted.  The photo below shows the whole cable, out of the car, and an undamaged termination at the other end.  Note that these terminations are not generic battery or ring lugs, but special connectors for this one application. So the repair calls for a genuine Porsche parts, not generic wire and connectors from the shelf.





As soon as the cable was installed the voltages rose to normal at the battery, and they were uniform at all points from front to rear.  That one failing junction led to all this trouble.  Next time you see a car with low battery resting voltage, and low charge voltage, remember it's not always the alternator or the battery. As cars age we need to be mindful of something we often take for granted: Sometimes the wiring itself will fail.

This problem was resolved by Bruce Caron at Robison Service. As an authorized Bosch Service center, this sort of thing is what we do day in and out.  Bosch designed and makes many of the electrical and fuel system components for the European cars we specialize in.  

John Elder Robison


John Robison is the founder of J E Robison Service Co of Springfield, MA. His company specializes in the repair and restoration of BMW, Mercedes, Porsche, Land Rover, Jaguar, Rolls Royce and Bentley motorcars.  John is also known as an author and advocate for people with autism and neurological differences.  His books include Switched On, Look Me in the EyeBe Different,and Raising Cubby. John has also written numerous articles on European auto service and repair. He's a tech consultant and track inspector for the Porsche Club of America, and he’s the technical editor for the Rolls-Royce and Bentley Club (the RROC). He’s always available to advise owners about the care and feeding of their fine motorcars.  www.robisonservice.com  413-785-1665



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