Hi Everyone, my daughter's '92 240 stopped charging recently - the alternator was tested at an automotive store and they said it passed. I'm only registering battery voltage at the battery posts, and alternator B+ post to ground.

Initially I thought the issue was with the exciter circuit as the dash light for the battery wasn't lighting. I pulled the cluster and swapped the battery light with a known good light and no difference. At that point I did confirm continuity from the red wire at the connector on the back of the instrument cluster through to the exciter spade terminal on the alternator. I do see voltage at that spade connector when key is on.

I am now thinking it's a bad ground wire. Would the easiest way to test measuring voltage from B+ terminal to alternator body? If I get no reading then it's a bad ground, correct?

Thanks guys! It WAS a bad ground wire, even though it passed voltage, apparently it didn't pass enough voltage. The quick test was jumpering from one of the assembly bolts on the alt. body to the negative post, as indicated previously - instant voltage! And the Oil and Battery lights came back on.

Removed alternator, made a new wire, PM'd the voltage regulator contacts while I was there, and reinstalled the new wire on a visible more accessible post and she's good to go again!

Thanks much for your knowledge and patience!

Hi,

Glad to hear back that you had a good out and apparently inexpensive outcome!

It makes us all feel good that we can help each other and beat out those big dollar shop rates!

These cars and most systems automotive are just not all that complicated anymore, if ones takes the time to ask questions and read.

Thanks for giving us a shot or a dollar donation to the site.
The owner has been working hard to keep it updated!

Phil

Donated! We're tight on funds, currently, but I hope it helps!

Replace your regulator. Easy !

Hi,

To answer your conclusive question about the ground problem, it is yes!

With the key on position two the light of the cluster should all come on as it is a test for all the bulbs to burn.
The oil pressure light and the alternator lights are grounded through the engine block.

The alternator housing is unfortunately mounted on rubber mounts in three places. Two at the bottom and the adjustment bracket is also rubber suspended at the far end.

There needs to be a wire running from the alternator housing over to the engine block.
If it were connected you should be able to read voltage on the D+ wire.
If the D+ is disconnected from anything the light on the cluster will not light up.
If you touch that wire to the alternator's housing it should light up.
To a wire from the battery negative it should light up!
Same goes if you touch it ant what's a grounded point and with the use of a voltage meter get a reading. In amperage mode, (series) it will be a very low current draw due to the light bulb in the circuit.

If you are not getting any results then you have an open circuit and it's not working.
The light goes off after an engine start because the alternator's exciter diode circuit has gotten enough tickle current fron the light circuit.
When this happens, the opposing power now going into the alternator's winding. This creates current that is going towards the battery in to directions! B+ and D+.
Most of it towards the battery but a little goes towards the light bulb. This make the light, stop burning or conducting to ground.

It takes only a small amount to turn out the light. It does not mean everything is hunky dory!
It just means it's doing a little charging.
This is why is best to have a dash voltage meter so you can monitor the system better.
The dash light is called an idiot light, if you trust it!
It may not be enough if you don't see on your voltmeter at least 13.2 to 14.2 volts or more than one volt above the batteries resting voltage. It needs that much to push it into the battery.
The regulators job is to be the middle man and he works well ONLY with a good ground.
Resting, is after it has not been on a charging circuit for quite sometime or has been used recently to draw of residual charge.

I hope I have enLIGHTen your needs to know more about grounds!
(:-)

Phil

Hi Phil,

Thank-you for the details! I replaced her alternator bushings a couple months ago and recall that ground wire - which on hers goes to the alternator bracket from a post on the alternator body. I'll check that connection again, but last night I did verify voltage from the B+ terminal to the alternator body, so I think that one is ok. I have also read the articles I could find on here about that circut, but am still miffed about what's going on.

Her battery light does not come on - I did not notice if the oil light is coming on as well, I will check that tonight.

I do get voltage at the D+ terminal when key is ON, though battery light doesn't light. I did not test grounding that wire to the alternator body because I verified ground to through the alternator body, but will do that and see if the battery dash light works.

I am interested in adding a voltage meter, is one of those connectors behind the 2 dash spacers for that purpose?

@Nahtanha mentions the regulator and/or brushes. Would that cause the issues I'm seeing based off the dash light?

I understand the alternator can self excite above a certain range - I tested off the B+ terminal and did not see a voltage increase when we ramped up the engine RPMs.

Thanks much for all the help!

Pull your regulator out out the alternator. Clean the contact area that the regulator needs. If your not getting 13-14 volts at the battery when engine is running, replace the regulator. Check the small field
winding wire connection. It tend to get corrosion. If your not sure about the ground, use jumper cables and run the ground from the battery to the alternator housing..

OK I'll remove it and inspect brushes and clean them - hadn't thought to use the jumper cables for an alternate ground. Thanks!

pictures

that blue/aqua wire is the grnd. attaches to the motor mt bolt/engineblock.
Sometimes the wire gets torn away from those round-eye conectors under the red end insulation(aqua wire sheathing - RED eye-round ends)


https://www.brickboard.com/mobile/post.htm?id=1616444

It was raining last night so I couldnt' get back to it. Is it possible to have a partial ground? The night before I was able to test voltage from B+ to alternator body - is it possible it's a strong enough ground for that, but too weak for actual alternator function?

Will test tonight with a jumper cable ground on the alternator as that's easy enough to do.

" Is it possible to have a partial ground?"

Absolutely.

A lot of current (up to 80 amps!) needs to flow through that ground wire, and because it is subject to movement due to the rubber mounts and engine vibration, the wire as a whole (including the metal-to-metal contacts at the terminals) has a finite life which doesn't necessarily end suddenly.

You've used jumper cables before? Did you ever use jumper cables when they worked the first time you hooked them in place? Don't you recall having to wiggle the clamps, or re-position them on the battery terminals or ground point? Making a secure low-resistance connection needed to pass this heavy current requires forceful contact.

If you choose to use a jumper cable to substitute for the alternator ground, keep that in mind. It might not make good contact the first time you do it. The aluminum housing of the alternator itself is especially hard to make good connection because it quickly develops a tough, thin, insulating coating of aluminum oxide.

To test the alternator ground properly, connect your voltmeter between the negative post of the battery and probe the alternator housing at a bolt, being sure you are making forceful probe contact. Look for no more than 0.5V while the motor is running, heater blower on high, and headlights on.

--
Art Benstein near Baltimore

"Well, here we are, Mr. Pilgrim, trapped in the amber of this moment. There is no why." -KV

they Art thanks for picking up the ball.

aka the new Me is CeeBee1

The red sleeve is how the part number is applied to wiring.





--
Art Benstein near Baltimore

Eat, drink, and be thoughtful.

The alternator does require a ground to engine at the motor mount.
Dan