Quote:
Originally Posted by TR65
I took a look at the tach signal that the ECM is putting out on the tach wire. This is for the E40 ECM so I don't know about other ECMs. The tach wire is used to drive the OE tach in the vehicle.
The ECM puts out a low voltage square wave that is two cycles or "tops" per engine revolution. That is the same as the number of times a 4 cylinder fires per revolution. So that is why we have to use the 4 cylinder setting to get the correct RPM. (V8 fires 4 times per rev.)
The first picture is the signal from the ECM. So the (-1.5 V) voltage is probably too low to fire the traditional tachs that are looking for around 12 volts.
The second picture is the signal with the pullup resistor installed. So now the voltage is up to 10V at the high. So the tach wroks fine.
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TR
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TR65,
I believe the 1.4 V signal was not from the ECM. The MCCDAQ USB1208FS has a low level output signal at 1.4V. Essentially you were using the DAQ as a pullup for the ECM Hall Effect Signal.
I made the same mistake, thinking my O2 sensor was outputting 1.4V. 100$ sensor later, I have confirmation from MCC that Single Ended readings on the DAQ output a 1.4V signal. This can be confirmed by reading the pins directly on the DAQ with a DMM. Mine reads 1.4VDC, 11 micro Amps.
I found this stray voltage does not happen when you use differential measurements.
Typically Hall Effect sensors ground when the magnet is lined up. If you aren't getting a reading, check the power being applied (pullup circuit). There may be a blown fuse or loose connection
Carl