450SL R107 Alternator voltage

[RaceDiagnostics]

A couple of time over the last week the car has only just started, the battery seemed almost flat and the car only just started. I checked the voltage at the battery today, it was 13.3 volts at idle and 13.7 at higher revs.

Does anyone know what it should be?

Cheers
Jim

 

[grober]

Alternator output voltage should run between 13.9 to 14.5 V when charging so seems a little low to me if the battery was almost flat? The alternator brushes might need renewing? A good auto electrician should be able to test its output for you. Automotive Charging Systems -  A Short Course on How They Work

 

[Ted]

Does the charge warning light come on full brightness, and go fully out?
If so, the alternator is probably ok, but the brushes and voltage regulator were a common failure point on those cars.
Easy replacement - two screws on the back of the alternator.

As Grober says get an auto-electrician to check it out, as voltage is not the only measure - if the voltage is low, it should be charging at (approx) 45 amps.

 

[RaceDiagnostics]

I don't see any charge light at all, I'll check the manual tomorrow and look again.

 

[Ted]

The initial excitation is via the small amount of current flowing through the charge warning light.
Chances are that either the bulb has blown, or the current isn't flowing to earth via the regulator/brushes.
Easy to check bulb - there should be a thin wire to the alternator (memory says that it is blue - but can't be sure). With ignition on, connect a test lamp between this wire and earth (the test lamp is only to limit current in case you have chosen the wrong wire or there is a short in the wire).
The ignition warning light should now glow (at about half brightness due to the test lamp in series with it).
If it IS the bulb, you can usually get over the initial excitation by revving the engine quite highly. The residual magnetism in the rotor will then provide enough current for charging to take place normally until the engine is turned off.

My bet is regulator/brushes.

 

[Druk]

A couple of time over the last week the car has only just started, the battery seemed almost flat and the car only just started. I checked the voltage at the battery today, it was 13.3 volts at idle and 13.7 at higher revs.

Does anyone know what it should be?

Cheers
Jim

What you're saying here is that the battery seemed flat; but a voltage of 13.3 at idle indicates there is not too much wrong with the charging although it is low. So maybe there is a fault with the battery, the starter, or the engine to body earth strap could be loose or corroded. Next time the car 'only just starts' try putting jump cables on the battery. That will isolate the fault to either the starting or charging circuits.
Edit... What Ted is asking is...does the ignition light come on full brightness when you turn the key to ignition, then go out fully when the engine starts?

 

[RaceDiagnostics]

I'll tell you the worry in my head,

The fact that I haven't had any isses starting from cold worries me.

I have not has to charge the battery with a charger at all.

The problem only seems to happen when the engine has been hot and then left for 30mins, then restarted.

It seems like somthing has almost seized and is restricting the rotation of the starter motor, ie a problem with the engine.

I hope my worries are unfounded, I'll carry round a charged battery in the boot with some jump leads to test this out, when it happens as well as the checks suggested above.

 

[grober]

Still could be problem with the alternator which isn't charging. In essence you are running the car sort of "on the battery" which is draining it, leading to very poor turnover. Leave it overnight and the battery recovers a bit- enough to get you started first thing??
p.s. any signs the engine is running very hot- is your fan belt slipping- could lead to overheating/siezure and/or poor charging??

 

[RaceDiagnostics]

Checked the ignition light tonight it operated completely normal, the battery earth strap was good, I couldn't see the connection to the sterter motor as it was too low and dark.

There are no signs of siezure, not running hot or having belts slip, maybe Im just thinking the worst.

I'm going to swap batteries with another car and see if I have the problem again.

 

[grober]

Checked the ignition light tonight it operated completely normal, the battery earth strap was good, I couldn't see the connection to the sterter motor as it was too low and dark.

There are no signs of siezure, not running hot or having belts slip, maybe Im just thinking the worst.

I'm going to swap batteries with another car and see if I have the problem again.

GOOD IDEA it might simply be the battery is on the way out for some reason.

 

[RaceDiagnostics]

I'll tell you the worry in my head,

The fact that I haven't had any isses starting from cold worries me.

I have not has to charge the battery with a charger at all.

The problem only seems to happen when the engine has been hot and then left for 30mins, then restarted.

It seems like somthing has almost seized and is restricting the rotation of the starter motor, ie a problem with the engine.

I hope my worries are unfounded, I'll carry round a charged battery in the boot with some jump leads to test this out, when it happens as well as the checks suggested above.

I put in a brand new 92Ah battery at the weekend, the issue is slightly better but not fixed, the bigger battery can turn the engine over and catch when hot, but only just. I'm sure the started will burn out soon with this slow cranking.

I'm going to stick a torque wrench on the crankshaft at cold and hot and measure the force needed turn the engine, I'll post up the results.

 

[grober]

Wonder if your starter motor brushes need renewing?

 

[RaceDiagnostics]

Im going to move this to the engine forum as I no longer supect the battery or alternator.