Why does the speedometer quit when check engine light comes on?

A speedometer quitting when the check engine light comes on is not a typical or expected occurrence. The two systems are largely independent. The speedometer relies on the vehicle's speed sensor (which might be part of the ABS system, transmission, or differential), while the check engine light is triggered by the onboard diagnostic (OBD) system monitoring various engine and emission-related components.

If both are failing simultaneously, it points to a more serious underlying problem, likely involving:

* Faulty Power Distribution: A failing alternator, corroded battery cable, or blown fuse could deprive both systems of sufficient power. This is a common culprit for multiple electrical failures.

* Damaged Wiring Harness: A damaged or shorted wire in the main harness could affect both the speedometer's circuit and the OBD system. This could be due to physical damage, rodent chewing, or corrosion.

* Power Control Module (PCM) Issues: The PCM (also called an ECM) is the "brain" of the car and controls many systems, including the speedometer and the OBD system. A failing PCM could disrupt signals to both.

* Serious Engine Problem Affecting Multiple Systems: While less likely, a catastrophic engine failure could affect the car's electrical system in ways that cause both the check engine light and speedometer to fail.

In short: You should not assume a direct causal link. The simultaneous failure needs a proper diagnostic check by a mechanic to identify the root cause. Ignoring this could lead to more extensive and expensive repairs down the line.