Cummins 6bta erratic tach

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One engine has had an intermittent tach for some time. Most time it worked normally but on occasion it would read erratic or drop out.

I finally decided to dig into this recently and after a few internet searches turning up nothing of value (disclaimer, I do not have accounts that provide access to some sites..having spent too much time in tech I have little interest and consider time in front of the computer to be time life is being sucked out of me), I got it in my head (silly me) that it had to be an issue with the alternator tach signal.

After checking the panel and cable connections, I pulled the alternator and opened it up expecting to find a bad connection. Everything looked good so after putting it back together with a new belt it was time to regroup.

Still stuck in my head it had to be the alt signal I traced the wiring looking to figure out where the orange signal wire changed to the grey indicated on the diagram and found the signal wire was dropped in a connector about 18” behind the alt.

At this point I finally woke up and caught on to the tach signal actually coming from the crank sensor shared with the heater control. Turned out the airsep installation had caused the wire to chafe to ground near the sensor. There really isn’t enough clearance for the hose and either at the time of install or service the wiring was pushed aside and the insulation was chaffed shorting the signal to ground intermittently.

It took over 2k hours for this to become a problem but I figured I would post this for anyone that might find it in a search someday:) If you have an airsep and a similar issue be sure to pull the filter and breather hose and check the wiring to the sensor. It is forcefully relocated to make room and can easily chafe if not rerouted carefully.