a voltometer test on me 88 300e charging system has revealed a slight variance of charge at idle: the amps jump around between 14.4 - 14.7 at idle and smooths out to a constant at higher revs. the mechanic suggested the idle was varying, possibly due to a clogged injector, thus explaining the variation. Observing the tachometer, it reads approx 700 rpm at idle and barely varies noticably. I assume the voltometer to give more accurate readings than a dashboard tach.

Being of unsound mind regarding mechanical principles, I am wondering if the idle rpm "should" remain absolutely constant or if a variance is tolerable depending on engine age etc. and the %'age of this variance. perhaps i do indeed have a clogged injector, sticking valves or a deteriorating wiring harness...etc.

A volt meter is a very poor gauge of engine speed. Your readings are pretty high but not out of range.

Variations in system voltage have more to do with current usage than rpms. If your fan speed changes, the voltage at idle will change with no rpm change.

If your idle is smooth, whats the problem?

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

steve- is 700rpm at idle high?

There should be a plate with the idle spec on the radiator core support. Seven hundred sounds fine to me.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

actually, at idle, the rpm needle varies just slightly; regardless, should idle rpms stay constant or is there a range for modulation?

Alternator voltage isn't controlled in any way by idle rpm unless it goes so low that the alternator doesn't produce any current at all, and is totally useless as an indication of idle speed. Avoid any mechanic who tells you otherwise! 14.7 volts at idle could indicate a charging problem -- you may need a new voltage regulator/brush set.

Idle speed is 700 out of gear, 500 in, and is controlled electronically. Idle speed variations, rough idle, surging at idle, or stalling when coming to a stop can be caused by a bad or dirty idle valve, or vacuum leaks. Minor variations that don't cause the engine to "thump" or vibrate can be ignored.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Thanks for all the helpful info, especially the mechanical "principles" at work in these engines.

to review some general principles: 1) idle speed and alternator voltage are normally independant of each other;
2) both should remain relatively constant, aside from load factors.
3) an abberation in one should not affect the other, unless its causing a system-wide shutdown.

hence, I have a question: what is being measured when you attach a voltometer to the battery? is it the gross volatage simnply being produced by the alternator, or is it the net voltage subtracting load? wihat is the effect of increasing the rpm's on the volatge? if you run the rpms to 6000, does it not increase the voltage?

The alternator is an AC generator. The raw AC voltage varies with speed of rotation, but the elctronics that convert the AC to DC also control the voltage output. Thus an alternator in good order will produce an output voltage that remians more or less constant. The speed at which this starts to drop below spec is very low, which is one of the reasons why we ditched dynamos (DC generators).

I think you are confusing voltage (electromotive force) with amperage (current). Voltage is equivalent to pressure in the system, amperage is the flow. The current into/out of the battery is always a net figure, current demand from the electrical system less the current supplied by the alternator. Usually this is a flow into the battery which keeps charging during normal use. When you operate the starter, this turns to a large discharge. If you overload the system, e.g. with too many accessory lamps, you can produce a state of discharge in normal running. You get a flat battery when this happens.

Also, if the drive belt is loose, you can get a good voltage at idle and off load, but when you load up the electrical system this causes the alternator to start to stall, the belt to slip and to loose current.

Not sure if this helps, but I've done my best!

I hate to correct people but as a generator mechanic I can't resist this one...

Yes, an alternator is an A/C generator. The voltage does NOT change with rpm, the frequency of the alternation (positive or negative) does. The voltage is maintained by the (usually internal) voltage regulator. The voltage regulator varies the amount of voltage in the exciter (another part of the alternator) which increases or decreases the output voltage. 14.7 volts IS excellent output! In fact, any voltage above the battery's voltage will charge it. To measure battery voltage, with the engine off, connect the meter across the + and - terminals at the battery. If you then start the engine, this measurement becomes alternator output voltage. The current produced by the alternator is a function of demand. Its rating is based upon the wirings ability to handle the heat produced by a given quantity of current. An alternator will maintain its voltage (unless mechanically impaired) down to a very low rotational speed.

Hope this clears up any misconceptions.

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Michael
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71 MGB
76 450SL
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