I recently picked up a fairly clean 1993 240 with 187K miles. It had the classic symptoms of a bad alternator. I replaced the alternator and battery as well and the symptoms went away and the car once again charged normally. Yesterday, a little over a month later the the idiot lights started to light dimly and the battery went dead.

I did multimeter checks and found that the new alternator is only putting out 2.3 volts. My question is could there be another problem that is causing the alternator to fail? Did I get a bad alternator? Are alternators an item that are covered by warranty?

Thanks

I'D RUN A 'TEST'(TEMP) GROUND FROM THE ALT CASE TO AN ENGINE BOLT AND TEST THE ALT OUTPUT.....IF IT JUMPS TO 13V+....I'D ADD A GROUND WIRE.....

OTHERWISE, BAD ALT'S OUT OF THE BOX 'HAPPEN'......

BUT THEN CHECKING THE HARMONIC BAL IS A GOOD IDEA....A LITTLE WHITE OUT OR ANY OTHER TYPE OF PAINT IS A QUICK AND EASY TEST........
--CARLOS

Check the exciter wire circuit (small red wire from back of alternator to back of instrument cluster). Should have continuity on that wire.

Check the bulbs in the instrument cluster.

Insert your key in the ignition, go to position II in the ignition and note what warning lights are on or not on; this is a basic diagnostic of the charging system. You might want to report back what lights are illuminated in this test.

And, last but not least [drum roll, please]... check your harmonic balancer. A failed HB will not transfer the belt rotation energy to the alternator and will result in low alternator outputs. If your power steering has recently required more effort, or your AC system has started to act differently, or if the alternator, parking brake, brake failure, and bulb failure lights are illuminated at the same time it is due to the design of the system and an indicator of charging system failure.

kourt

I did as you recommended with the ignition switch. The lights that were on were the check engine light, the alternator light, and the brake failure and parking brake lights. The ABS light is also on but it goes out when engine RPM's are increased. I measured from the alternator case to ground and from the alternator case to the negative battery terminal and in both cases the meter registered zero ohms so the ground appears good. When I measure from the small spade terminal to ground with the engine running I get 2.3 volts. Belt tension is good and there has been no indication of harmonic balancer problems. I called the autoparts store. They stated a lifetime warranty so no problems there. They will test it before pulling a new unit off the shelf. Bummer about the labor on my part but at least I have a warm garage to work in.

Thanks

I believe kourt99 got you to find the cause of your problem - or at least part of the cause. He just hasn't posted yet in response.

You have too many dead warning light bulbs. You need to replace the dead ones.
You reported seeing only alternator light, brake failure, and parking light, plus check engine.

You also should be seeing: oil pressure warning and bulb failure warning lights.
Oil pressure light looks like an old fashioned oil can with drip at spout.
Bulb failure is a small light bulb with an "X" over it.

Why does it matter?
The exciter wire mentioned in other posts is supplied via those bulbs. The circuit runs through the bulbs and then to the small red wire to alternator. Roughly 20% of the needed current is supplied by each of the 5 bulbs (check engine light isn't part of the system). If too many of the 5 are dead the current to the alternator is insufficient. It won't work.

In my 1990 manual the list of "pre-start" bulbs is on pg. 11. If you have a manual you should find it on a page close to that.

Hopefully someone else will chime in on how to get the cluster out. Not too tough really but it's dinner time here. Most importantly, UNLESS you have a tachometer, when you replace the cluster, DO NOT connect the red and white wire to anything. It's only for a tach and carries high voltage pulses from the coil.
--
Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.

Your comment below. You're saying that the bulbs are in series to feed the alt via the exciter wire. That would have never occurred to me. Why would they do that?


You wrote "Why does it matter? The exciter wire mentioned in other posts is supplied via those bulbs. The circuit runs through the bulbs and then to the small red wire to alternator. Roughly 20% of the needed current is supplied by each of the 5 bulbs (check engine light isn't part of the system). If too many of the 5 are dead the current to the alternator is insufficient. It won't work."
--
240s: 2 drivers and some parts cars

I can only answer partially/approximately.
If jorrel were in this discussion he could do it full justice.

Anyway, yes, the bulbs are in series with the exciter wire.
The bulbs themselves are in parallel with each other, more or less. Essentially those bulbs all ground via the alternator, for this function.

My understanding on the details of the alternator warning light is a bit hazy but here's what I have... If the alt is running and putting out more voltage than what's in the battery, the bulb won't light. But if the alt is not functioning for whatever reason the bulb lights up from the 12V it gets from the battery, grounding through the alternator.

There's a diode in the circuit somewhere, it's is a one-way valve for the current. The diode completes the "logic" so the circuit works properly.
--
Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.

Hello Dale,

I've found that the orig alternators last somewhere's between 150k and 200k. During that time, sometimes the voltage regulator's diodes go too. With a few 93's, I now always have a spare alternator on the shelf.

It makes great sense to check the rebuilt alternator on the Auto store's tester before you leave the door. Lifetime warrantied units are the only way to fly with these rebuilt units

Marty Wolfson

Sounds like you need a set of brushes or a regulator or a bad ground on the alternator frame. The brushes and the regulator are part of the same assembly and you can just replace it. The ground wire can also get eaten up by corrosion and you need one.

On the warranties, you can get a lifetime warranty on a rebuilt 100 Amp Alternator from Auto Zone. The only rub is you have to replace the entire unit even if all you need is a set of brushes. That is the Alt from a 740 but it fits and works in a 240 just fine.

You can also get a lifetime warranty from Advanced Auto Parts on a rebuilt unit.

Good Luck,

Paul