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1996 turbo diesel tach is always been stuck on about 3500 I've tried many different things on this vehicle to get that tach to work I bought a 2-in tach to put on the pillar with two other gauges.
When I connected to the white wire on the alternator I get about 3,500 RPM this particular vehicle I just got done rebuilding totally ground up it was bored 40 over so everything is clean I like the alternator mounts are you know absolutely clean I can reclaim them it won't hurt anything to do that I'll try that but it's interesting that the tack the new brand new tach on that white wire or white wire is giving me exactly what this tach is stuck on but when I connect to my HP tuner scanner it does read correctly so thoughts?
 

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The signal your Scanner sees is what the PCM sees. The PCM gets its RPM info from the CPS. It does not use the Tach signal from the alt. There is no way to directly tap into the PCM to run a tach. You would have to plug into the OBD2 port like a scanner and use one of those bluetooth engine monitor screens. To use PCM output.
The alt signal is for the Tach only. If your tach is not working and wiring and grounds are all good. Then the problem may be in the alternator. There could be a short somewhere in the Connector, pins, or stator wiring to the signal output pin {white wire} messing things up. If you have another alt laying around you might try it.
An aftermarket tach will work on a 6.5 alternator set up. As the signal from the stator is similar to a gasser ignition signal. But it may not be an accurate RPM reading.
You said both tachs you have are stuck about the same point. So I am thinking that the problem lies in the alternator.
 
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