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John, ASE Master Technician
Category: Car
Satisfied Customers: 614
Experience:  ASE Certified Master Technician, ASE Certified Service Consultant, 22 years professional experience
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OEM..the battery, engine block to fire wall and alternator housing

Customer Question

My 1982 924 (standard)original miles 28,467 will not charge the battery. I have replaced the alternator with a new Bousch 75amp OEM replacement and have replaced the ground wires on the battery, engine block to fire wall and alternator housing to block. The battery has been tested by two local parts stores and they tell me its still good (it is however seven years old) The tension on the belt is at about 3/8" deflection. My voltage reading on the battery is 12.9v with everything off and 13.1v with the car running no load, With the lights on and AC on the voltage drops to 12.1v. All the fuses under the dash look good and intact and all electronics in the car works but the bousch fog lights.
Submitted: 7 years ago.
Category: Car
Expert:  John replied 7 years ago.

My first question is: Does the battery light come on in the instrument panel when you turn the key on (engine not started)?

Let me know on this and also what tools do you have to diagnose the problem (volt meter, ohm meter, etc..)?

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Customer: replied 7 years ago.

John, Yes the red battery light does come on before I start the engine.

I have a greelee digital voltage meter.

Expert:  John replied 7 years ago.
Ok. We need to check the output of the alternator right at the alternator. Connect your volt meter to the battery ground and the large wire coming out of the alternator. Compare this reading to the volt reading across the positive and negative battery terminals. Do they match or is the alternator reading higher?

Does your digital volt meter have an Ohm setting?


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Customer: replied 7 years ago.

Ok, at the moment I have the battery being recharged and disconected from the car. The volt meter has a Ohm setting.

Note this is all very difficult to see from under the car on my back,

The Alternator has a solid red wire that goes to the top of the starter and i am not sure but i think connects and goes up and thru the fire wall and into a fuse / relay box under the driver dash. There is also a small black wire that does the same thing from the alternator over to the starter. Top side there is a main black insulated wire with the positive battery connector on it coming from the starter and three red wires that attach to the end of the positive battery bolt coming from the fuse / relay box. Not sure which wire is the one you want me to check?


It's late, can we reboot this tomorrow eve when it will be light out and the battery is recharged to be able to start the car?

Expert:  John replied 7 years ago.
The red wire that goes to the starter (and then continues on back to the battery) is the B+ wire. This is the one that I want you to check.

If that checks out, I want to the check resistance to ground (Ohm reading) 3 different ways. Connect the meter leads to the following locations and see if the readings are all the same (which should be zero or very close to zero):

1. Engine Block to Frame.
2. Engine Block to Battery Negative Terminal.
3. Battery Negative Terminal to Frame.

Make sure you are connecting to clean, unpainted metal at each connection point. If One of them shows high resistance (open connection) then you will need to install a ground strap connecting those two components or clean the existing connections and make sure the connections are tight.

I won't be back until late tonight but I will try to check in.

Customer: replied 7 years ago.


Here is what I found out tonight:

With Engine Off battery connected and ohm setting on meter at 200 the readings were as follows:

1. +.6

2. +.3

3. +.3

With the engine running:

1. -5.8

2. -11.2

3. +2.4


Also I was able to determine that two of the three red wires attached to the nut on the clamp to the positive terminal were Ignition and fuel pump. The third I suspect is the alternator and I left it off for your test.

I also tightened up the pulley on the alternator it was more that 3/8" deflection ( may have not been tight enough and worked lose after yesterdays op)


The volt readings tonight were as follows:

No load, and car off:

battery= 12.7v

No load, and the car running


red wire=13.6v

Load (lights and AC on)


red wire=12.56v



Expert:  John replied 7 years ago.
As far as the Ohm tests to ground they are close but I think improving the grounds all three ways would help. Clearly the alternator is not charging enough (should be 13.9 to 14.4 volts running). I didn't understand what you meant by "The third I suspect is the alternator and I left it off for your test". Was this the volt test or the ohm test? Why did you leave it off? Also, originally you said you had 13.1 volts at the battery when running and on the last test you have 13.7. Something seems to have improved and is getting close to the minimum (13.9). But your test at the alternator when loaded dropped to 12.56? Are we sure we are testing the correct wire? That wire can not be lower voltage then the battery since that should be the charging feed to the battery. I was expecting to see a 1 volt difference between the alternator and battery with the alternator being higher. Possibly the new alternator is bad even though it is new. Finally, if you battery is actually bad it would not be the first time I have heard a parts store claim the battery is ok when it is not. I often see that they don't know how to interpret the readings properly. Do you have another good battery you can swap in just to try and see what changes?