The voltage gauge in my car is wrong, always wrong. It reads below 12-volts most of the time and it’s like this on a lot of the 928’s out there. It’s one of those things that all new 928 owners ask questions about when they get there car.

The voltage gauges in 928’s are notorious for not being accurate, or at least, not what you think it should read. Most of us would think that the voltage gauge is a reflection on the charging system and should show how many volts are being returned to the battery and we are typically used to seeing 13.9 – 14.4 volts for a good functioning system. This is useful to know if you want to understand how well the charging system is working.

But like so many other things on the 928, the engineers were given the opportunity to innovate on many of the common automobile concepts. In some cases, like flush bumpers we get trend setting and in other cases we just get complicated solutions.

Subsequently 928’s voltage gauge actually reads the voltage being delivered to the Central Electrics Panel (aka Fusebox) system which powers all the devices on the car. This is equally, or potentially more, useful so you can understand if there is an issue with the electrical supply system to the car. On the 928, the vehicle electrics are powered directly off the alternator and do not rely on the battery once the car is running. The battery / charging is actually on a different circuit. I am not sure if this is somewhat unique to the 928 or not.

The voltage available to the CE panel is usually lower than the voltage being returned to the battery because there are a series of connections and devices powered between the alternator and the CE panel that inevitably cause voltage drops. They include: Alternator. Starter, ABS pump, and the 14 Pin Connector that connects the engine harness to the chassis harness which intern supplies the CE panel.

On top of that, resistance in the wires increase with age and heat cycling from the engine. The 928 engine harness is not shielded well from heat and suffers badly with age.

Subsequently, voltage readings at the CE panel are lower than at the alternator. In my case 14.2 volts are being supplied by the alternator. Here are the test results at the respective locations:

Location No Load Full Load
ABS post 13.98v 13.75v (good no load values are 14.0 – 13.0V)
Jump post 13.98v. 13.71v (good no load values are 14.0 – 13.0V)
CE Bus 13.89v 13.56v
Gauge 11.9v 11.5v

The next step is to check for resistance in the rest of the circuit into the gauge and out to ground. The gauge is grounded under the dash. I cleaned that and it had no effect.

I removed the gauges from the dash pod and noticed the gauge has a 68-ohm resister. I tested that and saw 68.9-ohms, which is well within the 5% variance specification.
 

I cleaned the +/- posts that supply the gauge and powered up the gauge and saw no change.


The gauge can be calibrated using an adjustable potentiometer. I measured the voltage being supplied to the gauge at the gauge and saw 13.86v which is very close to that at the CE panel. So I adjusted the potentiometer to reflect the actual voltage. 014_one-volt-low

Now I am much more comfortable with the volt gauge and I know what it represents even though nothing has really changed in the electrical system some good testing has given me some good confidence.

Benno oct 2018.