My speedometer drops past 70 when I may be going only 35 mph 1989 Mercury Grand Marquis

My speedometer drops past 70 when I'm only going about 35 mph trying to figure out what is about to go wrong before I'm caught off guard.

My car has 179569 miles.
My car has an automatic transmission.

Experienced mechanics share their insights in answering this question :
Hi – this behavior sounds like a failed speedometer instrument (the portion in the dash cluster rather than the cable from the transmission). It will be necessary to remove the speedometer "gauge", and either have it repaired at a speedometer instrument repair facility, or replaced with a new unit. Remember, if you replace it, and the odometer mileage displayed is different that your current reading, that must be reported as different when you sell the vehicle. I recommend a speedometer inspection performed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, confirm this diagnosis, and give you an accurate assessment of damage and cost estimate for repairs.

How to Identify and Fix Common car Problems ?

Our sources include academic articles, blog posts, and personal essays from experienced mechanics :

The most common reasons a Mercury Grand Marquis speedometer isn`t working are the vehicle speed sensor (VSS), a faulty speedometer, or an electrical issue.
Fuel injector nozzles can become clogged over time, which can lead to a sputtering engine, slow acceleration and the car not having enough power. Fuel injectors can be cleaned if the problem is caught early, but as the conditions worsens, the injectors may have to be replaced.
Summary: Ford Motor Company (Ford) is recalling certain model year 2003-2005 Ford Crown Victoria and Mercury Grand Marquis vehicles manufactured October 3, 2001, to August 2, 2005. The affected vehicles have a lighting control module (LCM) that may fail, resulting in a loss of headlights.
Taller tires will cause the speedometer to indicate too slow a speed, and shorter tires will cause it to indicate too fast.
Your speedometer is directly linked to the speed sensor — meaning if anything goes wrong with the vehicle speed sensor, it`ll lead to a speedometer failure. Whether you have hall effect or digital speed sensors, here are some signs of a bad sensor that may accompany your erratic speedometer needle: Transmission issues.
A clogged fuel or air filter can make your engine sputter and your acceleration lag because the engine is not getting enough fuel or air, respectively. The combustion chamber needs the correct amount of air and fuel in order to generate combustion efficiently.
Fuel System – if you have a bad part in your fuel system, like a pump, filter or injector, then you could experience sputtering. When your vehicle doesn`t get proper air and fuel your engine won`t get the proper combustion in order to power your vehicle.
Mercury Grand Marquis chip tuning can be done in two ways – through the OBD2 diagnostic connector or through the engine control unit (ECU). Increasing the power of Mercury Grand Marquis is done by remapping car engine. There are no worries about compatibility or unstable engine operation.
The speed sensor is located in the rear axle. The speed sensor is removed from the rear axle housing. The new speed sensor is installed into the axle housing. The speed sensor connector is connected.
The exact fuse used by the speedometer is the instrument panel fuse. The instrument panel is the cluster of gauges on the driver`s side of the vehicle, and these gauges additionally include the odometer, tachometer, oil pressure gauge, and fuel gauge, among others.

Relevant Questions and Answers :

the most relevant questions and answers related to your specific issue

My speedometer drops past 70 when I may be going only 35 mph 1989 Mercury Grand Marquis
ANSWER : Hi – this behavior sounds like a failed speedometer instrument (the portion in the dash cluster rather than the cable from the transmission). It will be necessary to remove the speedometer "gauge", and either have it repaired at a speedometer instrument repair facility, or replaced with a new unit. Remember, if you replace it, and the odometer mileage displayed is different that your current reading, that must be reported as different when you sell the vehicle. I recommend a speedometer inspection performed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, confirm this diagnosis, and give you an accurate assessment of damage and cost estimate for repairs.

My car won’t go past 35mph
ANSWER : Hi there. From the description you provided, your vehicle has problems with the transmission again. You stated it will not shift into 3rd gear again, like before the transmission was rebuilt. If the transmission is again the problem, it should be covered under warranty. It could be a faulty shift solenoid. The shift indicator not reading properly could be a faulty transmission position sensor. If this is an overall lack of performance issue, according to your last statement, then possibly you have a restriction in the exhaust system. I recommend having your vehicle’s problems inspected and diagnosed by a certified technician from YourMechanic.

My car is hesitating / jerking at speed intervals of 20 mph, 30 mph, 35 mph and 40 mph.
ANSWER : This may be a sign of a dirty or failing mass air flow sensor. The mass air flow sensor is a unit in the intake system that monitors air intake as it is mixed with fuel prior to being injected into the motor. When this fails, this can cause an imbalanced air/fuel load to be supplied to the motor which results in an over fueling or under fueling situation depending on the conditions. This may cause a hesitation, loss of power or jerking reaction when this sensor is malfunctioning. When this is not working properly, you may also notice a strong smell of fuel. I would suggest having a professional from Your Mechanic come to your home to diagnose your mass air flow sensor.

My truck is stuck in 4L or 4H also the speedometer is saying I’m going 100 mph when I’m only going 20 mph. Is my GEM Module bad?
ANSWER : Hi there. The issue could be with the output shaft speed sensor on the tail shaft of the transfer case. The output shaft speed sensor is a sensor the registers the speed of the output shaft and lets the computer (ECM) know how fast the vehicle is going. If the output shaft speed sensor is working properly, then there is an issue with the electrical wiring from the output shaft speed sensor and the dashboard. If you need further assistance with the speedometer and 4WD acting up, then seek out a professional, such as one from Your Mechanic, to help you.

Unable to go past 70-80 Km when going up a hill. Code P0174 with flashing engine light
ANSWER : Hello. A flashing Check Engine Light is an indicator that the computer has detected an issue that may potentially cause harm to the vehicle’s emission system components. The code P0174 indicates a lean condition on bank 2 of the engine. A lean condition is when there is insufficient fuel being delivered to the engine. This will cause misfires, hesitation, loss of power, but is also dangerous to the engine, as a lean condition can lead to detonation. A lean condition can be caused by many things. A few things I would check are the fuel pump, injector, and the MAF and oxygen sensor signals. If you need a diagnostic to be done in person, a certified technician from YourMechanic can come to your location to inspect the loss of power problem and advise on proper repairs.

ok my 99 tahoe truck would drive over 35 mph got the problem fixd. no its a little over 10 months later and its doing it again but
ANSWER : It’s possible you have an exhaust restriction due to a plugged catalytic converter. To check that you could either measure exhaust back pressure and/or intake vacuum (latter gives only indirect indication of a restriction though). Among the other common possibilities for loss of power are throttle body faults (electronic throttle controls, mass airflow sensor, torn air intake hose), worn out spark plugs, faulty fuel pump or restricted fuel system, and large vacuum leaks (including EGR malfunctions, PCV system, leaking intake manifold, leaking EVAP purge valve). A scan tool would be used to look at real time conditions within the engine (particularly fuel trim, etc.) and identify the root cause. If you want these diagnostic tests performed by a certified Mechanic, dispatched by YourMechanic right to your location, please request a loss of power inspection/diagnostic and the responding certified mechanic will get this resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

Car will not go over 40 MPH
ANSWER : I have seen this same scenario many times. The problem I believe is from a clogged catalytic converter. When they become clogged they restrict the flow of exhaust gas and the engine can not operate efficiently. I would recommend having a technician, from YourMechanic, to come inspect your vehicle’s Check Engine Light and replace your catalytic converter if needed.

Battery light illuminates at 70 mph and turns off once vehicle speed dips below 60 mph.
ANSWER : Make a note of what rpm the engine is doing when you are at that speed causing the light to come on. With the car stopped in park, see if you can rev the engine up to that speed and cause the light to come on. If it does, a mechanic can check the charging system for the problem. It is possible that the alternator brushes inside the alternator lost contact at higher speeds and stop charging. A professional fro YourMechanic can come to your location and test the charging system and replace the alternator if it stops charging or overcharges.