just did this on my 77 w200 yesterday.
there has been an aftermarket ammeter in the truck since i got it.
i drove it all morning yesterday running errands and i smelled some hot plastic, also noticed the ammeter was showing about 40amps charge when the rpm's were high enough for the alternator to put that out.
I agree with Rapid Robert on this.
Current climbing with rpms is a a battery with serious problems or some sort of short. Could be in the battery, or the wires crossing or grounding, or inside the starter relay?
Load test the battery and/or put it on a slow charge if its low.
thought about what i was going to do, decided to pull that connector where the black wire from the alternator goes into the bulkhead, it was melted pretty good.
i ended up taking a larger wire from the alt output to the batt+ on the relay,
also ran a 10g wire from that same post and went through a hole in the firewall right below the bulkheads. i cut the wire back where it feeds to the wiring harness for power and to the ammeter inside the cab, some of it was burned and corroded. just used a butt connector to attach my 10g wire to the original black 10g wire. so bypassed the bulkhead where it melted.
i thought i had a voltmeter hanging around to replace the ammeter but didn't so just left the ammeter. it's not accurate now but i don't really care. just needed to get the truck running again.
I understand now.
A voltemeter can not be used where an ammeter goes. Ammeters measure current running though them.
Voltmeter has very high internal resistance. It measures potential between two locations. No current flows through it.
Running the alternator output direct to the relay terminal might be useful if running a plow or winch off of the battery - and its a battery that can handle high charge rates.
One danger is no fusible link. If the alternator shorts internally - then the battery will send all the current it can down that new 10 gage wire - until it melts.
Anyway, to answer your other question, the ammeter is no longer accurate when there are two paths from the same point (starter relay) to the distribution point.
If the battery is good, then splice your alternator output to the A20 so the power flow is basically stock, but bypassing the bulkhead connector.
Then visually check over the battery feed/charge wire (S1 etc). While the battery is disconnected, use an ohmeter to see if there's connection to ground (shouldn't be, but that 40 amps indicates something is drawing current that shouldn't be)
