Experiencing a frustrating issue with your 1994 car where the RPM sensor seems to be misbehaving and your OBD2 reader can’t get a clear signal? You’re not alone. Let’s dive into diagnosing this problem, especially if it cropped up after some recent electrical work, like removing an aftermarket alarm system.
One common symptom of an RPM sensor issue, or related electrical glitches, is a bouncing tachometer, particularly noticeable above idle. If this is coupled with engine misfires that worsen under load, it points towards potential problems in the ignition or sensor circuitry. It’s understandable to suspect the recently removed alarm system as a culprit, as these systems often tap into crucial vehicle wiring to function.
When tackling electrical gremlins post-alarm removal, the first step is always meticulous inspection of the wiring. Carefully go over every connection point where the alarm was integrated. Look for loose wires, corroded splices, or “vampire clips” that may have damaged the insulation of factory wiring. Sometimes, these connections can become intermittent, especially with temperature changes or vibrations from the running engine. While you’ve already tried manipulating wires with the engine running, sometimes a more detailed visual inspection is needed to spot subtle issues.
The fact that the tachometer issue and misfires are more pronounced above idle is interesting. It suggests that the problem might be load-dependent. At higher RPMs and under load, the electrical system is under more stress, and any marginal connections or failing components might become more apparent. It’s less likely to be a purely mechanical RPM sensor failure if it’s directly tied to electrical system changes.
Finding cut factory wires under the steering column, unrelated to the alarm, adds another layer of complexity. While your car may have run fine with these cuts, it’s worth investigating further. Unfortunately, without color-coded wiring diagrams, tracing these wires can be time-consuming. However, consider if any of these cut wires could be related to the ignition system or ECM signals. It’s less likely they are the primary cause of the RPM sensor issue if the problem is recent, but they represent an unknown factor in the car’s electrical history.
On a 1994 model, the tachometer signal is indeed typically fed directly from the ECM. This means that issues with the ECM, the wiring between the ECM and the RPM sensor, or even the sensor itself can affect the tachometer reading. It’s less likely that a coil problem would directly “back-up” and cause tachometer irregularities, but electrical faults can sometimes have unexpected effects.
Regarding testing the ignition coils, your idea of using spare spark plugs to check spark quality is a good approach. By grounding spare plugs and observing the spark while disconnecting each cylinder sequentially, you can visually assess the strength and consistency of the spark from each coil. This can help rule out weak coils as a contributing factor to the misfire.
Image alt text: A close-up view of a spark plug and ignition coil, representing components that can be tested when diagnosing RPM sensor and misfire issues in a 1994 car.
The Check Engine Light illuminating during misfires and then extinguishing is normal for OBD1 and early OBD2 systems of that era. These systems often have limited memory for storing diagnostic trouble codes (DTCs). While the light going off suggests the misfire is no longer actively detected, it’s still worth trying to retrieve any stored codes. Even if the light is off, some basic OBD systems might retain a “freeze frame” of data from when the fault occurred, which could offer clues. However, don’t be surprised if an older OBD2 reader struggles to pull meaningful data, especially related to RPM sensor specific faults, from a 1994 vehicle that might have a transitional or limited OBD2 implementation.
In summary, for your 1994 car exhibiting RPM sensor reading problems and misfires after alarm removal, focus on these key areas:
- Wiring Inspection: Meticulously re-examine all wiring related to the removed alarm system and any other accessible wiring in the engine bay and under the dash. Pay close attention to ground connections as suggested in the original forum post, as poor grounding can cause a variety of electrical issues.
- RPM Sensor Testing: If possible, test the RPM sensor itself using a multimeter to check its resistance and signal output. Consult a repair manual for your specific 1994 car model for the correct testing procedure and values.
- Ignition System Check: Test the spark plugs, ignition wires, and coils to rule out any weak spark issues contributing to the misfire.
- ECM Connections: Inspect the wiring and connections to the Engine Control Module (ECM), as this is central to both RPM sensing and ignition control.
Troubleshooting electrical issues, especially in older cars, can be a process of elimination. By systematically checking these areas, you’ll be well on your way to pinpointing the cause of your 1994 car’s RPM sensor and misfire problems. If you’re not comfortable with electrical diagnostics, seeking assistance from a qualified mechanic experienced with older vehicles might be the most efficient path to resolution.
