The vehicle may lose vehicle speed signal, which can affect speedometer operation and transmission shift quality. Most often the Vehicle Speed Sensor is faulty, however, at times, the driven gear inside the transmission is at fault. Replace the Vehicle Speed sensor and inspect the driven gear upon sensor removal.
Car problem reports
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Most reported 1990 Mercury Topaz problems
Fuel pump failure can cause the engine to crank but fail to start. Diagnosis will show insufficient, or no fuel pressure, and fuel pump in the fuel tank will need to be replaced.
If the throttle lever is out of adjustment, or the grommets are missing / broken at the throttle or transmission, the automatic transmission can shift hard. Replace the grommets as needed and adjust the throttle lever.
Poor rubber material used in the motor mounts can cause them to fail, leading to a knocking noise or lifting sensation on acceleration. The motor mounts will need to be replaced with revised parts.
Internal heater blower motor issues can cause a ticking noise from the heater blower when the fan is on. To correct this concern, replace the heater blower motor with a revised unit.
O-rings in the quick connect fittings commonly allow refrigerant to leak from the AC System. The o-rings at system quick connect fittings should be replaced.
Outdated software in the engine control processor can cause poor engine performance, hesitation on acceleration, and possible stalling problems. The processor will need to be replaced to get the updated calibration.
Sludge build up in the throttle body can cause a poor engine idle, stalling, and/or hesitation. The correction is to install a new idle air control valve spacer kit to prevent the sludge build up, set the base idle, and clean the throttle body.