Trying to find out what my alternator is doing. I have a 91' NA 240 and for what I can tell the original alternator.

When I start the car my volt meter (aftermarket) shows about 12 volts until you start to drive or rev the engine, and it kicks up to about 14 volts.

When idling the power fluctuates even with no load on the system. it wavers between about 13.5-14 volts or so, enough to make the lights dim. As you speed up this stops and it stays steady.


I cleaned my voltage regulator and resurfaced the brushes.
I added a 6-gauge wire for both the positive and negative terminals in addition so what is there stock. (I did this because the voltage while running would peak at about 13.8 volts, now the voltage output peaks at 14.4 volts)

I’ve also noticed that it sounds as if the bearings in the alternator are going out, or at very least dry. The battery light never comes on and the battery is always charged, but I am thinking that the alternator is on its way out. If this is normal for this car/alternator, then fine no worries. If it is going out, I want to replace it at a time convent for me, rather than on the side of the road.

 

Meh, I'd get a junkyard alt and get it over with. I tried to get mine working again but it was a waste of time, junkyard alt has worked just fine since I installed it and it happens to be a slightly higher output unit.

 

also make sure the alternator body has a good ground. one way to test this is to take a volt meter, and measure the voltage between the alternator body (scrape a shiny spot) and the battery - terminal while the engine is running... any more than a small fraction of a volt is too much.

 

On the positive side I get .035 volts
On the narrative side I get .013 volts

This is at idle

 

I should have added, if you have an intermittent alternator problem, the time to do this alt. ground to negative test is when the charge voltage is low and not the proper 13.8-14.2 volts.

that said, I've had many older vehicles where if you start the engine without any throttle, the ALT light stays on, and the alternator doesn't start charging until the first time you blip the throttle, then the alt light goes out, and the alternator starts charging. My BMW motorcycle does this, too.

 

My alternator light never turns on with the key, so I'm wondering if I have an exciter circuit issue. I swapped the battery indicator light and the oil pressure light (because that one works and it's was close by) and there was no change. So I know the bulb is fine...

If the car is chugging to life after starting it the battery light will flicker a bit but I'm not sure why it doesn't come on like the other lights in key positions 2

I saw your post on testing it on anoher page, I'll give that a go and let you know.

 

yeah, check the D+ (smaller red wire on stud on back of altenrator) for about 9V (+/-) with the key on and engine off. if its close to 12V, then its likely a bad alternator, we just replaced one on my daughter's 240 with exactly that symptom (but that alternator wasn't outputting any charge at all)

 

0 volts with ignition off
1.4 volts ignition on
13.8 volts engine on

The light is coming on with the ignition again, I think it was just corroded. The bearings still seem to be going out however

 

1.4V is too low, it should be something like 6-9V. I think you have a worn out dying alternator.

 

So that being said, new, used or rebuilt?
Bosch or is duralast fine

 

I'd get a quality rebuilt Bosch alternator, if it was my car. I don't believe new Bosch alternators for these older cars are even being made anymore. beware of cheap rebuilt alternators, frequently, they are just cleaned up, new brushes+regulator, and quickly benchtested, then boxed. their bearings are likely nearly as old as yours.

there's 3 things that wear out on these alternators, the brushes, the bearings, and the commutators that the brushes press against. also, the diode board inside the alternator can fail, frequently from too much heat, but thats not a wear item.

 

Sounds like a plan, I am going to start looking because I just got in a full set of poly accessory bushings, so if I'm taking the alternator off anyway, why not replace it now. As I have seen so far, in most cases the bully is not included and it will have to be moved to the replacement alternator. If this is the case, is it a press fit? or once I get the nut off will it just slip free?


Just out of curiosity, I removed the D+ terminal, cleaned it up and checked voltage.

0 Volts with ignition off
12 volts with ignition on and Un-Plugged
1.5 volts ignition on and connected

So it looks like my ignition is likely fine but it is indeed something with the diodes in the alternator perhaps.

 

the pulley+fan are held on with a big nut, it takes something like an impact wrench to get them off easily. you probably could do it with a ratchet but you'd be swearing a lot, while the impact wrench just spins it off in a couple seconds.

 

oddly enough, ever since fixing the timing issue, the alternator has been working much better...perhaps a slipping belt? idk, still failing however.