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Old 11-18-2013, 08:35 PM
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whunter whunter is offline
Join Date: Dec 2003
Location: Metro Detroit, Michigan
Posts: 17,360

Originally Posted by Zacharias View Post
About five weeks ago I had my wrench install a rebuilt alternator that I had sitting on the shelf for about four years. I got it from a place called Eagle Auto Electric, which sells rebuilt and upgraded OE alternators on eBay. This one is 70 or 80A, I forget which.

Anyway, when we installed it and started the car, we noticed it did not want to charge until the motor was revved pretty high. I am aware that these voltage regulators don't 'excite' until 1200 rpms, but this took well over 2000 rpm to come to life. Anyway, it was charging fine so I put the hesitation down to the unit having been sitting a long time.

Shortly afterward I replaced the battery with a Maxx H8 unit from Wallymart.

All has been well until yesterday, when I did the glowplugs and left the trunk and driver's door open all day. When I went to crank it over to get the last of the air out of the lines, the battery died in about 5 seconds.

I boosted it, drove it, seemed okay. This morning it had very little grunt. Later I drove for half an hour doing errands and when I tried to restart it after the last stop, wouldn't even crank the motor over. By the weirdest of luck that stop had been to buy a booster pack, so I got it going with that.

Sat for a while revving it with just the parking lights on (was dark by now). Noticed the dash lights were very dim, I don't think any charging was happening. Suddenly they brightened up.

Drove for a while and noticed the lights had dimmed. Also they are dimming and brightening a bit with engine revs when parked, which is not something that should be happening with a new alt and regulator.

Looks to me like it's taking very high rpms to excite the regulator, so on short drives it isn't charging at all.

Measures 11.58v at the battery at idle and 11.8 when revved right now. Was not able to get it any higher even with revving it high.

Voltage regulator bad here, or....?
The RPM issue = it should be charging as low as 550 - 600 RPM if the correct SMALL pulley is on the alternator...

I find many older MB have had the alternators replaced with WRONG (Large) V-belt pulley's = causes low RPM and idle charge issues.

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