450sl

[mbbilbao]

I have a problem with my charging system on a 1975 450 SL. I removed the alternator and had it bench tested and it passed with flying colors but when installed in the car its not putting out more than 12.6 volts. Any hints as to where the problem could be? The idiot light on my instrument panel does not turn on even when the ignition is on.

12.6 Volts DC, is the minimum required voltage. If your battery is good, and is fully charged, your alternator may not be receiving a command from the voltage regulator to increase voltage.......but............

An alternator during normal charging usually shows between 13.5 and 14.5 volts, and does not come back on until it reaches below 12.6.

It sounds like a voltage regulator issue, not alternator.

Early alternators did not incorporate built-in regulators. I'm not sure if your 107 did, or did not have a built-in regulator, if not, it is separate, and may be the problem. Mid to late seventies was the time when alternators began incorporating regulators, I just don't know when MB started their change. Hopefully someone here can clarify. ............BB

[dpatterson]

Your 450SL should have an external regulator. Mounted near the radiator , ie actually bolted to the radiator mount panel on the same side of the car as your alternator. Its a Bosch item and easily replaced for about $50. Later model regulators tend to be 2 pin and use the case as an earth whereas the early ones were three pin. Either will work.

[abe g]

On this model, the regulator is attatched to the alternator. I beleive can be ordered as seperate part to fit his model.Abe G

[mbbilbao]

what gets me is the fact that this alternator has been bench tested twice and was charging everytime with an output of 13.8 volts, but will not charge when installed in the vehicle.

[dpatterson]

In that case, if Abe is correct about your alternator having an internal regulator (which is contrary to all the early 350s and 450s I have seen but that could be a euro vs USA set up issue), then it sounds like you have a wiring problem. Check all terminations for poor contact. Dont just visually inspect but unscrew them, file clean any terminals and reattach. Plug type connectors (like the one into the back of the alternator) should be cleaned with an electrical contact cleaner. Especially check all the earthing points starting with the one from the battery to the engine. And dont forget battery terminals.

Its easy to waste time thinking you have a component failure when its just a poor battery earth.

[elbe]

Originally Posted by mbbilbao

I have a problem with my charging system on a 1975 450 SL. I removed the alternator and had it bench tested and it passed with flying colors but when installed in the car its not putting out more than 12.6 volts. Any hints as to where the problem could be? The idiot light on my instrument panel does not turn on even when the ignition is on.

The clue might be your "idiot light" not turning on. Some charging systems use this light circuit to excite the field coil in the alternator. Replace the light bulb and that might fix the problem.

Originally Posted by elbe

The clue might be your "idiot light" not turning on. Some charging systems use this light circuit to excite the field coil in the alternator. Replace the light bulb and that might fix the problem.

that is the problem, i have seen several cause a no charge when the light bulb is out, it seems to be 450 SL specific. i have seen 2 like that within the past 3 years. replace the bulb, or fix the wiring, that will be the cause.

[Carlos Serrano]

Is more common than you will think specially in German vehicles the light closes the circuit when it comes on. VW, BMW, need to know trick or get ripped off by unscrupulos shops,a simple light bulb, a well kept secret.

[mbbilbao]

Thanks for the help guys! I took out the idiot light and found out it was soaked with oil, probably came from the oil pressure line on top of it behind the instrument panel. Replaced the bulb and bingo!

[ctaylor738]

I would check the warnig light bulb and the wire. If that circuit (blue wire from alternator to the warning light) is bad, or if the bulb is burned out, the alternator will not get "excited" and charge properly.

I am not making this up.