no revs???

[fat325]

just a quick question
where abouts is the rpm sensor on an m20 b25?
thanks

[jlevie]

The crankshaft position sensor is mounted on the lower right front of the engine. It provides engine timing and speed data to DME. The DME generates a tach signal from that data.

[scrowner]

Jim, I believe the speed signal comes from the differential. But I have been always confused why the M20 has two position sensors coming from the bell housing, at least my two 87s and one 86 have two sensors on the crank - connecting on the driver side of the engine into a harness. There is a metal bracket here for the two connections and the bracket is integrated in the the starter mounting.

I have a problem with my instrument cluster where just the tach. does not work.

The E30 requires a rotation signal to continue running past the initial start of the engine. Which I assumed was one of these two sensors and my car idles and runs without a hitch. Could the second sensor be bad?

Some history. I have pulled the cluster, did the check on the battery and heated the tach board solder points to remove cold solder joints. No luck. Verified connections around the dash. But I have been always curious where the tach. signal originates on the M20.

[jlevie]

The road speed signal that feeds the speedometer/odometer and 13 button OBC is in the differential. But the tach is driven by a signal from the DME that is derived from the crank shaft position sensor.

The ETA cars have two sensors in the bell housing that provide timing data to the DME. one is used for engine speed and the other for timing. M20B25 engines use a CPS that reads a toothed ring on the harmonic balancer and a pulse generator on the #6 ignition wire.

[scrowner]

I do remember a sensor on the balancer on the front of the M20 engines. This sensor does not have a connector to allow replacement. I noticed that these sensors are sold with a pig tail. So, hack off the old sensor, soldering on the new, heat shrink connection, snap it back in place, and I should have my Tach back up working.

[jlevie]

I do remember a sensor on the balancer on the front of the M20 engines. This sensor does not have a connector to allow replacement. I noticed that these sensors are sold with a pig tail. So, hack off the old sensor, soldering on the new, heat shrink connection, snap it back in place, and I should have my Tach back up working.

The CPS used on an M20B25 does have a connector. The cable is about 2.5' in length and routes across the front of the engine under a plastic wire chase and connects to the engine harness under the forward end of the intake.

The plastic chase across the front of the timing cover is important. It keeps the CPS cable away from the water pump pulley, which will kill the sensor in short order. If you have to replace the CPS, have the two clips and chase on hand in case the one on the car breaks when you remove the sensor.

ETA engines also have a sensor on the front of the engine, but it is only for diagnostic purposes. The cable for that sensor terminates in the diagnostic connector. Timing reference data for an ETA engine comes from the sensors mounted in the bell housing.

[fat325]

i do believe the sensor i was looking for is connected to the ht leads whic supplies the rpm. well thats my guess anyway :)

[jlevie]

The sensor that is attached to the #6 plug wire is the Cylinder ID signal and is used by the DME for sequential timing of the injectors. The rpm signal for the tach is generated by the DME using data from the cranksahft position sensor. If the engine runs, the crankshaft position sensor is good. So if the tach doesn't work the problem will be in the SI board, tach, wiring or tach output stage of the DME.

[scrowner]

ETA engines also have a sensor on the front of the engine, but it is only for diagnostic purposes. The cable for that sensor terminates in the diagnostic connector. Timing reference data for an ETA engine comes from the sensors mounted in the bell housing.

Well, mine being a 325E makes this front sensor purely diagnostic. The car runs perfectly, just no tach signal. Actually, this is not true. It always sits at 2K. But as the car RPMs go up to some point the tach will slowly drop as the RPMs go up. Tach board was reflowed and the battery was good. So, that leaves some connection as bad? Painful. Still want to think a sensor is bad.

[nando]

there is no "tach" sensor - so if your tach doesn't work, yes it has to be a bad connection or something with the cluster.