W210 Should voltage readings at battery terminals be same as at CCU?
 

My assumption would be that voltage readings at the battery terminals and readings available through the climate control panel should be the same. But I've read a number of threads indicating that the CCU figure is routinely lower....by about 0.6v. Why would this be? What have you all observed?

I have not checked this on my car but I expect it to be normal.

Wiring has what is called voltage drop, if you put a high load on the end of a wire, the wires resistance lowers the voltage at the other end. Take the load away and voltage will pop back up again. The climate unit is seeing voltage after a long run and probably through the ignition switch.

Think of it as a garden hose. With the house valve on and nozzle shut, pressure will rise and swell the hose. Open the nozzle and the hose will shrink a bit due to running pressure being lower.

Alternators and really good lab power supplies have remote sensing wires that carry no current and are there for voltage sensing. This keeps voltage consistent on the other end.

Also--who knows how accurate the volt-sensing chip in the CCU is? Couple that with differences between that sensor (it's a CCU, after all, not a DVM) and the OP's voltmeter and you could easily have a couple tents of a volt difference. Finally, the OP may be measuring system voltage at the special terminal under the hood (he doesn't say) while the battery is under the rear seat.

Also--who knows how accurate the volt-sensing chip in the CCU is? Couple that with differences between that sensor (it's a CCU, after all, not a DVM) and the OP's voltmeter and you could easily have a couple tenths of a volt difference. Finally, the OP may be measuring system voltage at the special terminal under the hood (he doesn't say) while the battery is under the rear seat.

Haven't pulled out the DVM yet but I did make the observation that the CCU readings on both the 98 and the 99 are exactly the same: about 13.3v at idle with no accessories on. While the alternator and voltage regulator on the 98 are original (or so I assume) at 88k miles, the alternator on the 99 was replaced 40k miles ago.

While possible, it seems unlikely that both charging systems are underperforming and to the same degree.

DVM reads about 14v idling, no accessories. So yeah, it seems like the CCU reads lower.

Any electronic engineers in the house??
 

All the components like solenoid, blower are probably driven by the nominal input voltage. They can accept a range of voltages, so any voltage drop along the wiring does not matter. The control is probably TTL logic driven at 5v ( I do not know for sure, don't have access to schematics. ). The voltage regulator in the CCU would regulate it down to the designated voltage in any event. So the voltage drops the OP observed is just normal wiring drop or caused by junction resistance.

The climate control on a W210 always displays a voltage 0.6-0.7V lower than the actual battery voltage.

From what I recall, it is due to the presence of a diode in circuit. (That perhaps protects the ECU from the car voltage)

If that's the case, then my cars are pretty close to 14v.