Where does the Alternator's 3rd wire Control Voltage come from? - PeachParts Mercedes-Benz Forum

777funk · #1 08-09-2008, 09:27 AM

New Alternator and still no charging. I noticed that the blue wire (the small pin on the alternator's 3 pin harness has no voltage on it. I think it's supposed to have a signal that the car is running so turn the alternator's power on. But where does that signal come from?

Help me understand the alternator wiring.

thanks!
Nick

79Mercy · #2 08-09-2008, 09:30 AM

I think that blue wire is what controls the dashboard light. Check and see if it has voltage when the key is in the "on" position.

777funk · #3 08-09-2008, 09:39 AM

Quote:

Originally Posted by 79Mercy

I think that blue wire is what controls the dashboard light. Check and see if it has voltage when the key is in the "on" position.

Thanks! No voltage when the key is on.

79Mercy · #4 08-09-2008, 09:40 AM

when you turn the key "on"(not running) does the battery light on the dash come on?

777funk · #5 08-09-2008, 10:03 AM

Quote:

Originally Posted by 79Mercy

when you turn the key "on"(not running) does the battery light on the dash come on?

lol! Actually I am waiting to receive the gauges you'd sent so I've still got the cluster torn apart in the basement

I'm betting they'll be here today!

777funk · #6 08-09-2008, 10:08 AM

Oh... Also, to add more to help describe the situation... I do measure the 12.5V battery voltage at the 2 large terminals.

But of course no 13.5 or higher when the car is running.

I stuck the Red Multimeter probe into the back of the plug connector itself on the Alternator and measured 12.5V with the car running. That doesn't mean that the connectors are making a good connection on the alternator since I'm not touching the pins just the wires on the back of the Alternator harness. A bad connection may be the problem. But with the old Alternator I measured on the two large pins at the Alternator (not the plug) and didn't measure any voltage. I'll try it on the new one and see.

777funk · #7 08-09-2008, 10:16 AM

Ok... measured at the Alternator's 2 big pins and I didn't measure any DC voltage with the engine running with reference to the batteries ground post.

bbboomer · #8 08-09-2008, 10:29 AM

I am having alt problems myself. Others on the forum have given the following suggestions. So far it hasn't helped me but it may you.
Check the terminal block connections, if you have one.
Connect a ground wire from the alt housing to batt ground, the alt must be grounded to work.
Check the ground cable where it connects by the hood hinge.
Hope you find your problem.

SD Blue · #9 08-09-2008, 10:34 AM

Quote:

Originally Posted by 777funk

lol! Actually I am waiting to receive the gauges you'd sent so I've still got the cluster torn apart in the basement

I'm betting they'll be here today!

Without the instrument cluster installed, you won't see any charging. Power comes from the fuse, through ignition switch, then charge indicator to blue wire of alternator. The blue wire is the "excite" wire that measures the voltage and "turns on" the alternator to charge.

yellowbenz · #10 08-09-2008, 10:45 AM

I'm like a one trick pony when it comes to this problem but as I post on every one of these charging threads replace the belt and tension it well. However, you won't know if this is a problem until you complete the circuit for the exciter wire to the alternator.

79Mercy · #11 08-09-2008, 12:57 PM

just wondering, how did you run the motor with the cluster out of the car(oil pressure gauge)?

Diesel911 · #12 08-09-2008, 01:09 PM

I had thought 777funk had ask the question that I wanted to know about.
Why are there 2 (+) wires with battery voltage going to the Alternator (the 2 large slots on the connector)?

I bought the Bosch book on Alternators and Starters and still no answer to the above question.
In the Bosch book it said you need to have a least a 2 watt bulb in the circuit where the blue wire goes in order to "excite" the alternator to start charging.

lowriderdog37 · #13 08-09-2008, 02:04 PM

Quote:

Originally Posted by Diesel911

I had thought 777funk had ask the question that I wanted to know about.
Why are there 2 (+) wires with battery voltage going to the Alternator (the 2 large slots on the connector)?

2 10-gauge wires in my eyes provides a backup in case one breaks...it also provides the ability to contain much more current at any given time. Could be either/both

Quote:

Originally Posted by Diesel911

I bought the Bosch book on Alternators and Starters and still no answer to the above question.
In the Bosch book it said you need to have a least a 2 watt bulb in the circuit where the blue wire goes in order to "excite" the alternator to start charging.

The dash light could work as the 2 watt bulb in that situation. I haven't found any wiring diagrams though

777funk · #14 08-09-2008, 02:13 PM

Quote:

Originally Posted by 79Mercy

just wondering, how did you run the motor with the cluster out of the car(oil pressure gauge)?

Got the gauge today. I'm about to open it and give it a go! Thanks!!!

You'll laugh but I actually drove the car with NO cluster. I just put a piece of cut rubber (to the size of the hole in the oil pressure gauge) into the old gauge to plug it and off it went. The 617 runs fine with out power.

compress ignite · #15 08-09-2008, 02:14 PM

The first place I was made aware of this set up was on GSXR's instructions
about converting to the 143 or 150 amp Alternator.
(I believe he replaced them with a Great Big Honking single cable with the
diameter of your thumb.)

(Maybe Deutschland was short on 1.0 and 2.0 ?)