Which sensor feeds data to the tacho?

nut3y

('99 s40 T4 AW50-42LE) There appears to be two sensors on the bell housing for engine speed. Parts diagrams don't really specify what they are other than "sensor". Googling the part numbers 30896814 and 30618272 state vehicle speed sensor and engine speed sensor but I thought the vehicle speed came from the ABS system?

Just curious what does what and if anyone has some info.

jamesinc

Both are right, because you're mixing together two different concepts. The engine speed (what you see on the tacho) comes from the ECU and relates to signals from the crank position sensor and the cam position sensor. The vehicle speed (what you see on the speedo) comes from the tone ring sensors on each wheel via the ABS system.

nut3y

jamesinc So the second crank position sensor is exclusively for the TCM then? Counted a total of 4 sensors; 1x camshaft, 2x bell housing, 1x signal from ABS unit.

jamesinc

nut3y I'm fairly sure, but not 100% sure, that a '99 S40 T4 has one crank trigger and one cam trigger. Can you post a photo of what you're looking at?

nut3y

jamesinc It's a bit hard to get a photo of but this parts diagram has it albeit in a different location to what i can actually see on the car.

Image description

On the car sensor no.1 is directly atop the transmission, removing it gives access to I'm assuming the flywheel or torque converter, big disc with evenly spaced holes to determine the rotation. Plugs in at just below the windshield in that little pod of connectors along with knock sensor, O2 etc

Sensor no.2 is leaning towards the firewall, removing it shows nothing but it is covered in transmission fluid. Plugs into a little harness that sits underneath the airbox. Looks very similar to sensor 1 except there is a tiny little metal disc at the end, looks to me magnetic as it had picked up some ferrous goop

jamesinc

The crank signal comes from a third sensor that is not shown in that diagram (it won't appear on gearbox diagrams, it will be on engine diagrams).

The two that are pictured, I'm not 100% sure but likely the front one is output shaft rpm and rear is picking up an axle speed. I'm speculating though.

It sounds like you found one of these two sensors and the crank trigger sensor.