Fisrst off, in my 92 RS, I am running a dual battery setup through a diode isolator. This setup has been working fine for at least 4 years. Last week I started off for work and noticed the voltmeter reading 10.8 volts (Dakota Digital gauges) where normally it would be 13.8 or so. This problem affects both batteris, so it appears that the alternator (new) is not charging. I noticed tonight when idling in the driveway, volts were up in the normal range, then a step on the brake would cause them to drop enough to kick off the heater blower, also noticed that the cooling fans did not turn on. My suspicion is the isolator is out or on its way out, or the fuse on the switched 12V excitation line to the isolator is open. A possibility of a fusible link has also crossed my mind. Before I quit for the night, I could only get the volts up to just over 12, and neither cooling fan nor the heater blower would turn on. Any other thoughts on where to look would be apprciated.

Al

 

You would have to provide more information on exactly how it's wired..
But basically, if you at one point had a battery (or both) up to 13.8V and then the brakelights made it (them) drop well below 13V then the battery(ies) concerned is (are) bad and also the charging system for it (them).
You will have to use a multimeter to provide more clues as to what is going on.

 

Thanks for the input. I have a 4-post isolator, 1 each for Battery A, Battery B and Alternator, and a smaller one for a sense/excitation wire which allows current to flow through the isolator. The sense wire is attaced to a switched 12V source (ignition) and also has a fuse in line. I suspect the fise on the sense wire has opened and no current flows through the isolator, and no charging occurs. Since I just thought of this, I'll check that out when I get home tonight.

 

Ok, I've figured this one out. It was none of the above. It's in the alarm. Have one of those with a remote start and it uses the brake light switch to disable the remote start function when shutting the car off. Pulled the wire off the brake switch, and no more voltage drop with the brake. I'll be taking this to my installer for some warranty repair this weekend.

 

Thought it was figured out. The reference wire to the alternator has no voltage on it to excite the voltage regulator. Haven't found the other end of this wire yet, but I'm pretty certain it goes to a fuseable link down by the starter. If not a fuseable link, then maybe an open in the reference loop.