Using the low side readings will not get you a correct charge regardless of what kind of gauge you have.
High side readings are more acuurate, as are weight /charge ..but the sight-glass on that chassis is where you want to be looking when charging with only a low side gauge b/c it tells what is going on IN THAT system, not what the system Should be doing at pre-determined pressures.
When that glass clears up w/rpm around 1500, you are done.
Any more , and you are over-charging.
I watch the glass when aux cuts in too , just to get it right .

And on these two fan systems, you can up the K9 to 20 amp w/o any consequences. When a low fan blows 15A fuses, always look at the single wire connection on R15 Pre-resistor. [ that is the common weak link on Low Fan circuit] ....and I also spin the fans to make sure they are both free and smooth. Very common for that 15 to blow as the car ages and R15 feed gets fatiqued/corroded. The orig factory wires on the low fan circuit were borderline [ 2.5mm] to begin with.

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A Dalton

Just an additional point of information re: the blower. I was just tortured when my 1990 300e momentarily lost all electrical power while driving on a local street. It turns out that the culprit was R-15. The base of the resistor's receptacle had worn away and allowed one of the leads carrying voltage to make contact with the vehicles body. This killed the blower regulator and the ignition switch. I recommend removing the base for R-15 and inspect the underside for cracks and wear. The regulator you save may be your own.

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Achieving failure is no accomplishment, yet learning from it is.

Good tip , except I think you are talking about F22 .
R15 is the Dropping Resitor for the Aux fans , not the Blower.
F22 is the aux striop fuse holder for the blower and they were infamous for that problem you discribe.

Is that the part ??

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A Dalton

F-22 it is then.

__________________
Achieving failure is no accomplishment, yet learning from it is.

Yes..and a good Post/Tip ....that isa bad set-up .

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A Dalton

Arthur, clear something up for me please. Couldn't a bad R15 also shut down the ignition by shorting out, in the way that the blower fuse rodgiddens described does? As badly as the contacts on the R15 get corroded/rusted/welded, losing ignition seems like a real possibility, although I've never heard it happening. What's the scoop on that?

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't

Any short to ground as the R22 is famous for could possibly do that , but that is b/c R22 comes off the key and is HOT all the time the key is on.. thereby causing a direct short....whereas , the aux fan R15 circuit is only on when the pressure switch at the drier is calling for low fan, and that low fan feed to R15 is FUSED at the low fan relay, so they likelyhood of a burnt wire at R15 effecting the ign is remote , as far as I can tell from schematics and experience...but I will not say " absolutely NOT " b/c I know there are going to be threads that disagree with that analysis and claim "YES"
...but you know how that always winds up.....no where.

ANY short on aux fan circuits should simply blow the fuses, be it high coolant fan or low a/c fan circuits. And the problem on R15 is usually on the low fan feed side b/c those wires are undersized from the relay to the R15. [ 2/5mm, as compared to 4.5mm for high fan]

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A Dalton

Yes, I see that, thanks. You are gradually managing to teach me the fundamentals of electric circuits under the hood (Business Admin grad here), and I confess it's fascinating and gratifying to understand. So, here's my layman's read on what you just told me: a circuit that requires the ign key to be on could potentially shut the ign off if shorted, but in the particular case of R15, the relay isn't even operating unless there's adequate hi pressure at the drier, and even when it is operating, there's a fuse in the circuit that would blow before the ign would be affected. Ergo, not a hi likelihood that a short at R15 would manage to shut down the ignition. In contrast, F22 is on a circuit that's always hot with key on, and if the F22 creates a short, there's nothing left in the circuit to prevent the ign from quitting. If that's a correct layman's interpretation, I'm beginning to get the hang of this stuff.

__________________
1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't

<. You are gradually managing to teach me the fundamentals of electric circuits under the hood (Business Admin grad here), and I confess it's fascinating and gratifying to understand. >

Yes.... We have ID Nick-Names here for most posters , and yours is
"Car Learner"...so , seems approriate that you mention that...........


Anyway, you have the basics correct... a direct unfused short to ground will effect the systems voltage back to the battery source, whereas the aux fan circuits would blow the feed fuse if it incurred a short and the source would never know the difference. Itwould simply see a 20A draw on a 15 A protected circuit before that fuse blew...and a 20A draw on a system is nothing...the source simply sees that as an accessory draw..like you turned something on.
Whereas , a direct unprotected/unfused short can draw MANY ,MANY AMPS...
So many, that the wires burn up in a flash. Ever put a wire across a battery ???? ............ "POW"

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A Dalton

LOL. Well, I'll accept "Car Learner" b/c it's certainly appropriate. Actually, tho, my Internet ID is "Cal Learner", in fond remembrance of my halcyon student days.

__________________
1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't