Hi All,

We are trying to fix a 94 850 (non turbo) for a friend.

Problem: The car hesitates intermittently at wide open throttle at 3500 RPM. Hesitation is coincident with the check engine light. When removing foot off throttle, the check engine light also blinks intermittently.

Luckily, I have access to an oscilloscope, so here are my findings with the electrical system:
*The ignition system (port A6) blinks code 214 - Engine Speed (RPM) sensor missing sporadically
*The impulse signal between pin 13 of the ignition computer and the power stage of the coil cuts out coincident with both the engine hesitation and check engine light.

Here's what we have replaced:
*RPM sensor
*Distributor cap, rotor, and plug wires
*spark plugs

We have also checked the throttle position sensor, cam position sensor, and primary/secondary coil resistance. All appear normal.

There are no codes from the fuel injection system. The transmission computer reports a fault in its temperature sensor.

Here are my questions:
*does the ignition computer normally cut the signal to the coil when you take your foot off the gas?
*if its not normal for the ignition system to briefly cease sending impulses to the coil, can any of the other sensors to the computers cause an interruption to the impulse signal?
*do you think a fault in the transmission temp sensor could cause this behavior?
*could a vacuum leak cause these symptoms?

At this point I am most suspect of the wiring between the RPM sensor and the ignition computer. Have any of you experienced similar symptoms with faulty RPM wiring?

Thanks in advance for any helpful advice,
-Michael in Portland

 

Issue is solved!

Mechanic had spliced the RPM sensor wiring due to a previous mehcanic routing the wires too close to the engine. They had melted. Unfortunately, the mechanic reversed the polarity of the RPM sensor when attempting a repair.

This inverted the signal to the ignition computer, causing a fault code and misfire under wide open throttle at 3500 RPM and also a misfire when taking your foot off the gas.