6 replies to this topic

[tynor]

'99 NB

 

I was having trouble keeping my battery charged.  I suspected a voltage regulator problem since I was seeing 15.7-15.8V across the battery while idling.    I tested the alternator at the parts store and it tested bad, so I have replaced the alternator in with a Bosch reman.

While the old battery tested 'good' at the parts store, my charger would not charge it past 60%.  So I went ahead and replaced it "just in case" (it was 6 years old, so not inconceivable it would give up the ghost from old age soon anyway).  The new battery takes a charge and holds it.

HOWEVER!  The new alternator still shows 15.7V at the battery. I've since learned that the ECU is actually controlling the voltage regulation.   I had a spare (but possibly not legal) ECU, so swapped it and saw 15.2V at the battery.  Hurray - I concluded that in fact the voltage regulator in the first ECU was bad, so I bought a third one.  I installed it today and... I'm seeing 15.8V at the battery.   What are the odds of two ECU's with bad voltage regulators?   I'm wondering what else I'm missing.

Also: The ECU in the car is coded BP4W.   A friend has a spare coded BP5R which I'll swap and see if I get better regulation.  I can't find any info on the interwebs about differences in those codes - any reason to avoid the swap if I see better voltage with it?

 

Thanks!
Steve

 

[Jeff Wasilko]

AFAIK there isn't a 'voltage regulator' in the ECU. The ECU is commanding the alternator to charge based on voltage. Do you still have your stock dashboard with charging/alt warning light?

[tynor]

Yes - I have the stock instrument cluster.  Battery light goes out when I start the car.  

I haven't double checked that the voltages at the alternator connector are proper.  Factory manual says:

   P connector:  "below 1" when switch ON,  "approx 3-8" when IDLE

   D connector:  "approx 0" when switch ON, "*" when IDLE (whatever "*" means :-) )

 

I'll check that next time I get the car off the trailer.

[tynor]

alternator control voltages read good (I see 5.9V on P at IDLE - spec is 3-8V, and 1.8V on D at IDLE spec is "*" - not sure what a "*" means...).

 

I'm out of ideas.  Swapping ECUs isn't solving the problem.  The ECUs seem to be sending good signals to the alternator.

Help?

[Dave D.]

If you look up the voltage specs in the FSM, those numbers are in the acceptable range. 

[tynor]

If you look up the voltage specs in the FSM, those numbers are in the acceptable range. 

 

Yes - the control voltages look good. 

 

But I'm seeing 15.6-15.8V across the battery during idle (with a fully charged battery).  FSM says B+ should be in range 13-15.  - so I've been assuming I'm overcharging the battery by 0.6-0.8V.   

 

Am I overthinking this?

[tynor]

Best Answer

Got some advice from Mazda Motorsports that this is not an unusual charge voltage for a 99.  Who knew? 

 

I'm going to call this "good enough" and race the car.