94 Mazda B3000 charging problem

94 Mazda B3000 3.0L ps/ac. When car is cold dash charging meter reads normal(3/4 scale), but turning on headlights or fan causes meter to flick. After running a few minutes meter reading begins slowly decreasing(about 1/3 scale) and battery light comes on. When this first started happening increasing engine rpms would cause meter to increase and sometimes stay in normal range for a while. Now increasing rpms does not cause meter to increase. Battery is 1 year old and just replaced serpentine belt with no change. Replaced upper idler pulley 1 year ago. No unusual noises from belt area. Alternator/voltage regulator are original..this model has regulator mounted on back of alternator. 270K miles, otherwise runs like a top.

 

Hi there, I had a similar situation with my 94 Cavalier. Turned out to be the alternator. One way of testing the alternator is to start the vehicle and remove the battery cables. If the vehicle stays running the alternator is ok, if it dies out replace the alternator. Good luck!

 

I'll try it. Could this be the voltage regulator? It is external and is much easier to remove/replace than the alternator. Also much less $. The brush assebly is external with the voltage regulator, too. Brushes maybe? Thanks for the help. Also curious as to why it reads normal the first few minutes after sitting overnight.

 

I'm a tech at the local Ford dealer ( The Mazda B 3000 is made by Ford, it is a ranger) your problem is actually quite common on the older 3.0L, replace the alternator. At 270K I would not even attempt to replace brushes, regulator seperately.

Edit: DO NOT remove the battery cables while the veh is running, one of the battery's jobs is to absorb voltage spikes produced during normal alternator operation. Removing the cables allows the voltage spikes to "hit" the computer and potentially create a problem within the PCM. If you need to test it, use a volt meter, turn on all accessories and check output voltage. It should be 13.5-14.5 V @2,000 rpm

 

Thanks for the reply, and thanks for the warning about removing the batt cable. Can the regulator be removed w/o having to remove the alt? If a replacement alt doesn't include a regulator/brush assembly, should I replace them first as I have to buy them seperately anyway? Really appreciate the help.

 

Any alternator I have put on those trucks came with the regulator already installed, I would just replace the whole unit.

 

Roger that. I just called the parts store and they said the new unit comes with the regulator. I'm looking at the repair manual and it says to remove the air cleaner housing. Sound right to you?...also, do you have any helpful hints for doing this job on this model? Once again, really appreciate the help.

 

It can be done without removing the air cleaner but it's not worth the effort

To be honest I haven't done one of those in probably 3 years so I don't remember the particulars without seeing it