MSefi.com read only forum (Jan.15,2018)

wrongear wrote:with 5 volts applied to Techselect I have .017 volts on both sides of R12.

Is TachSelect jumped to OptoIn? XG1 to XG2? (Don't confuse TachSelect with Tsel - easily done)

If so, and if the voltage either side of R12 remains the same, it means there is no current flow through it, so likely the LED in the optoisolator has failed. The LED is connected to pins 1&2 on the IC, so testing with the diode test function on your DVM would show this for sure.

But do check again that U3 is correctly inserted - the dot or half circle should be a the heatsink side of the board.

"Is TachSelect jumped to OptoIn? XG1 to XG2? (Don't confuse TachSelect with Tsel - easily done)"

A-Ok

"likely the LED in the optoisolator has failed. The LED is connected to pins 1&2 on the IC, "

There's a LED inside the U3 opto? I don't think so. Pins 1&2 on the IC??? What's the IC?

U3 is installed correctly.

Thanks Trakkies

Trying to reload the firmware Matt; this could take a while (month). I'm using Monitor_v2.905.abs.s19 but right click only offers to save the link, not the file.

The IC in question is U3, a 4N25 opto-isolator. It consists of an LED across pins 1 & 2, and a photo-transistor. When a voltage is applied across pins 1-2, the internal LED illuminates the base sensor of the photo-transistor, allowing current to flow through the emitter and collector. No voltage spikes can cross the boundary, as the internal circuitry inside the opto-isolator is physically separate.

ive gone through these steps with my problem (no rpm) i have direct short between tachselect and left leg of r12, tachselect to right leg is 388 ohms. also with no jumpers i have no voltage on tachselect but 5v on tsel