bob928s3

Alternator is the 115 Amp
Part - Bosch AL170X or Porsche 928.603.011.05

Voltage Regulator is 14.5 V with 68 Ohm Resistor
Part - Bosch 1197311040 or Porsche 928.603.142.00

Bosch 1197311028 is the same Voltage Regulator except does not have 68 Ohm Resistor

What is the function of the Resistor?
- to prevent overheating of diodes?

Would it make a difference using the Voltage Regulator without Resistor?

Thank you for your insight.

dr bob

The external 68 Ohm resistor (in parallel with the warning light) is there to make sure the alternator has enough initial field current to "light off" as soon as the alternator spins.

I'm not sure what the difference is from your description though. Does the "Voltage Regulator is 14.5 V with 68 Ohm Resistor
Part - Bosch 1197311040 or Porsche 928.603.142.00" -include- a resistor, or -require- a resistor?

Also: You'll be looking for a 14.0V regulator

bob928s3

Hi Dr. Bob,

Thanks for your reply.

From reading on internet sites, specs on the Bosch 1197331040, which is the same as Porsche 928.603.142.00 is that it includes a 68 Ohm internal Resistor in the circuit. Though it is stamped 14V on the Regulator housing, specs are that it will cut off current at a maximum of 14.5 Volts The brushes are designed for 28 mm slip ring.

About a year ago, changed the voltage regulator with Bosch 1197331028 which unknown to me at that time, does not have the 68 Ohm Internal Resistor. It was about $35, which is a far cry in price compared to the Bosch 1197331040 and significantly less then the Porsche 928.603.142.00.

Bosch manufactured for Porsche, the Alternator AL170X with Voltage Regulator with internal 68 Ohm Resistor for a reason. Just trying to find out the reason and the benefits of including the internal Resistor. Do not want to swap out the Bosch 1197331028 for the Bosch 1197331040 with 68 Ohm Resistor, if it is not going to have any benefit.

dr bob

Just curious why you are looking for another replacement. The one installed in the AL-170X seems to work fine with the factory excitation (has 68 Ohm resistor in parallel with warning light in the dash). Are you chasing a particular symptom?

Reason I ask -- I spent a bit of time chasing what appeared to be sagging voltage and short battery life. I replaced the positive cable between battery and starter, cleaned and tested lots of connections and cable sections, replaced my original alternator with a Bosch AL-170X, but still had the same symptoms. Turned out that, even though the multimeter said otherwise, the ground strap at the battery had deteriorated enough that it was preventing the battery from fully charging and therefore it wasn't helping at low-RPM/high-load situations. Now all is well, even with the new alternator in there.

bob928s3

Hi Dr. Bob,

Will check my battery ground cable. Know there is some water getting through and may be affecting the conductivity. Least expensive option to repair.

Voltage Gauge sees dramatic drops as loads are applied. Blower Motor gives the most draw. When using turn signal, see gauge needle moving up and down. With Blower Motor, Wipers, Turn Signal, Rear Defrost, Headlights; gauge reads 12V and sometimes lower to about 11V in Drive and at Idle.

Battery is 6 Months Old and is Bosch S5 Premium, Group 49.
DMM Readings -35 degrees outside temp - taken in Engine Bay
Battery at Rest - 12.45 V
Idle in Park -14.45 V
Idle in Park with Load (Wipers, Blower, Rear Defroster, Radio, Turn Signal) - 13.9 to 14.1
2000 RPM with Load - 14.2

Idle RPM in Park is a bit higher then RPM when in Drive and Stopped, so would expect a lower Voltage Reading.

So was thinking Voltage Regulator, Alternator, but as suggested may be a battery ground cable issue.

dr bob

Your readings sound just fine. The idle capability of the alternator is an ongoing discussion point here, with some folks fitting up higher and higher current-rated alternators trying to maintain idle charging voltage above 14 with everything turned on. That's kind of why I asked what symptoms you are chasing.

Check that voltage in a few places. The gauge reads from the bus on the central electrics panel, references ground at the points just above the CE panel and on the left side footwell. I'd start by cleaning those (and all the other...) grounds before going any further. Current flows from the alternator forward to both the ABS terminal (left side front just forward of the power steering reservoir) and the jump-start terminal on the upper right fenderwall. From the jumpstart terminal there are smaller conductors that route in parallel to the "30" bus bar at the top of the central electrics panel. All of those need to be clean and pretty to function. The connection between alternator and jump post is prone to mechanical, heat and corrosion damage, especially at the end by the jump post. Put your meter between the alternator B-plus terminal and the jump post, in DC volts mode, and read the voltage drop under load for that section. It will inspoire you to at least clean the connections, and possibly replace that wire section. Handily, the wire is buried in the front-of-engine harness of course.

Lots to look at, but with 14V+ readings now I'll suggest that your alternator ad regulator are not the instant problem.

Meanwhile... Roger (928srus) and Mark/Tom (928 international) both offer upgraded ground straps, tinned-copper braid with shrink sleeve so less chance of ongoing corrosion continuing. IIRC the one I have now is rated at 2ga strap, a serious size uprate from the one that Porsche installed way back when. It's the biggest insulated strap I could get to pass through the relief in the battery well cover, FWIW.