THERMO Switch Causes Poor Acceleration (VOL0106AP)

THERMO SWITCH CAUSES POOR ACCELERATION

TECHNICAL INFORMATION TIP

THERMO SWITCH CAUSES POOR ACCELERATION

YEAR(S): 1985-90

MANUFACTURER: Volvo

MODELS: 740

DATE OF ISSUE: December 1990

ISSUE & TIP

A faulty radiator cooling fan thermo switch can cause poor acceleration on 1985 and later Volvo 740s, equipped with B23F or FT (turbo) engines. The thermo switch serves two purposes on these models. Not only does the thermo switch control cooling fan operation, it also sends a signal to the ignition system control unit.

If the ignition system control unit receives a signal from the thermo switch when engine speed is below 1750 RPM and the throttle switch is closed, the control unit will advance the ignition timing 13 degrees. This helps the engine return to normal operating temperature and also helps the alternator handle the extra electrical load. The control unit returns the ignition timing to its normal base setting when the thermo switch opens.

Engine performance will suffer if the ignition timing is adjusted while the thermo switch is closed and the radiator cooling fan is operating. The ignition control unit will retard the ignition timing 13 degrees as soon as the throttle switch is opened.

The control unit thinks that it's returning the timing to the normal base setting. But the timing will actually be retarded 13 degrees later than the correct base setting instead. This will cause very sluggish acceleration, although the engine may idle normally.

An inoperative radiator cooling fan, a thermo switch that has failed in the closed position, or a thermo switch that has been intentionally by-passed can also trick you into misadjusting the timing.

Courtesy of Import Service Magazine

with thanks to:

Mike Allen

Mike Noury

Internal Combustion

Newburyport, Massachusetts

REFERENCE NUMBER

VOL0106AP