Vacuum Sensor Malfunction Diagnosis (MAZ0030AP)

WARNING: This page is about the 626 Base, 2D Coupe, Automatic, which is a different variant/trim than selected.

Publication date: 1970-01-01
Reference number: MAZ0030AP

VACUUM SENSOR MALFUNCTION DIAGNOSIS

TECHNICAL INFORMATION TIP

Reference Number(s): MAZ0030AP, Date of Issue:

VACUUM SENSOR PROBLEMS - MALFUNCTION DIAGNOSIS

YEAR(S): 1983

MANUFACTURER: Mazda

MODEL(S): 626

ISSUE & TIP

We repaired a carbureted 1983 Mazda 626 that was running very rich. I used a malfunction chart to find that a vacuum sensor malfunction will hold the air/fuel solenoid at 0 percent duty cycle (full rich). So I started at the ECU with a wiring diagram and found that Mazda does things a little differently than most automakers.

The vacuum sensor is supplied with 12 volts whenever the ignition switch is on. The sensor then drops the voltage to the ECU to indicate the engine's vacuum level. The sensor was obviously bad because I had the 12 volts from the ignition switch coming into the sensor and 0 volts leaving the sensor for the ECU.

I removed the vacuum sensor cover and wasn't surprised to find that it had fried. Enough water had gotten into the sensor to cause a short. This allowed full battery voltage to reach the ECU through the vacuum sensor input, so the ECU got fried too. I tested the ECU and found that several of its outputs were no longer working properly either.

I replaced the damaged vacuum sensor and ECU. The engine ran beautifully and the CO was also back within specifications. All of the system voltages were once again back to normal.

Courtesy of Import Service Magazine

with thanks to:

Gary Taylor

Brigham Street Service

Salt Lake City, Utah

REFERENCE NUMBER

MAZ0030AP

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