No Power/Misfiring At Night W/Lights On - Alt. Diode (SUB0259AP)

NO POWER/MISFIRING AT NIGHT W/LIGHTS ON - ALT. DIODE

TECHNICAL INFORMATION TIP

NO POWER, ROUGH RUNNING, AND MISFIRING AT TIMES - LISTEN TO THE CLUES

YEAR(S): 1986

MANUFACTURER: Subaru

MODEL(S): Brat, DL/GL/RX & XT

DATE OF ISSUE: October 1997

ISSUE & TIP

This is a general tip, although in our case it concerned a 1986 Subaru 1.8 liter that came into our shop. The lesson we learned could be applied to any vehicle, however.

The customer had been experiencing complaints of no power, rough running, and misfiring at times. These symptoms always occurred while driving at night. Another shop in a distant state had scoped the electronic ignition and had found erratic traces. They replaced the distributor, and the customer believed the problem to be fixed.

When I spoke to the vehicle's owner, he expressed that he figured the replacement distributor had gone bad (again), and would need replacing. After listening to his complaint, however, it occurred to me that engine grounds could be the culprit. After inspecting them under load for a significant drop, I noticed a transient increase in AC voltage whenever he would switch on the lighting circuits. I also noticed that the battery was original (11 years old), and that the system voltage at 2500 RPM was 15.5 volts. Also, the positive battery post connection was nearly corroded through.

Despite all these checks and problems, the scope showed a perfect pattern from the distributor. After replacing the sulfated battery and cable clamp that was threatening to make him a pedestrian, I performed the charging system checks again. Under load, the alternator would put out a momentary high AC spike, then settle down. The problem was nailed.

However, when I loaded the battery with the carbon pile, the ignition traces on the scope went bananas. After repeatedly passing the AC voltage leak "diode/stator" test, the meter's needle suddenly left the "OK" box and nearly swung off scale whenever I loaded the system. This symptoms was accompanied by some strange-looking ignition patterns.

The tips that I want to pass along are:

• The customer's complaint (only acts up at night with the lights on) was a major clue as to what was really going on with this strange Subaru.
• Despite everyones belief that a new distributor would fix the problem, I couldn't condemn a unit that was functioning perfectly. Logically, I had to look elsewhere.
• Alternator diodes can and do produce some quirky behavior in a variety of cars, including this one.
• Repeated testing may be the only effective means of provoking an intermittent part failure to act up so that you can draw a bead on it.

Courtesy of Import Service Magazine

with thanks to:

John Wasilausky

D & L Automotive

Shelton, Washington

REFERENCE NUMBER

SUB0259AP