Questions and Answers
Prelude Charging Low
Q. Dear Sir, I have a 1990 Honda Prelude. It has a 2.0 liter engine and manual transmission. I have 180,000 miles on it also.
My problem is that the alternator is undercharging when the A/C and headlights are on and only while it is idling. When the A/C and lights are off the alternator is charging at 14.3 volts. When I turn on the lights and A/C it drops down to 12.5 volts. But when I rev the accelerator up to 2,500 rpms it charges above 14 volts. Also my battery warning light does not come on. My mechanic replaced the alternator and I installed a new battery but I still have the same problem.
Do you have any ideas what could be causing the problem or do you know who I can contact to resolve this issue. Any information you can provide would be greatly appreciated.
Jun
A. I don't think the problem is with the alternator or the battery. I think it's more likely the problem is with the engine control system.
From what you describe, the charging system seems to be working. Normally when you turn on more accessories, the voltage will drop. This is because of increased electrical demand. The fact the voltage comes up when the engine speed increases and the charge light doesn't come on shows that the alternator is operating properly.
There is an A/C Idle Boost System that compensates for the load the A/C places on the engine. The A/C Idle Boost Solenoid Valve, mounted on the right side of the firewall, is used by the PGM-Carb unit to control idle speed. When the compressor is activated, the PGM-Carb unit sends current to the A/C Idle Boost Solenoid Valve. As the valve opens, manifold vacuum is passed through to the diaphragm chamber of the idle boost controller. The idle controllers diaphragm rod is retracted to open the throttle valve a certain amount.
The amount of this throttle valve opening is adjusted with the idle control screw on the idle controller to maintain the original idle speed. When the compressor is off, the A/C Idle Boost Solenoid Valve is deactivated to close the vacuum passage and the vacuum stored in the controller is released through the filter on the solenoid valve.
If this valve is not working, then the idle speed will be too low for the alternator to put out enough to cover the extra load. I would have this checked and make sure if it is working and if it is, it probably just needs an adjustment.
Additional Information provided courtesy of ALLDATA
