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My tachometer will only intermittently work. I will know when I turn the Bronco on. It will either work, or it won't. It doesn't drop out mid-running after start, just will sometimes not work at start. If it doesn't work at start, I have to restart the Bronco to roll the dice to get it to work again. I'm not sure why this would be, but I'll talk about what's been done and tried, and hopefully someone who knows a lot more about the tachometer can help me out?

The Technical Stuff:
I swapped a 4bt Cummins into my Bronco which required a few changes. For the tachometer sensor, a custom tone wheel came from Diesel Conversion Specialists (DCS) along with wiring and a 6.0 Powerstroke crank sensor. The crank sensor mounts off the block and senses the points on the tone wheel to send a signal to the tachometer. DCS assures me they've used this exact solution on hundreds of Broncos with no issues. I'm not at all certain that it is even an issue, since I can sometimes (~60-70% of the time) have the tachometer functioning at start.

Things I've Tried:
I recently replaced my tired battery with a dual battery setup. This seems to have improved the chances of the tachometer working, but it still sometimes doesn't. I read a post in the Bullnose section about glue becoming old, brittle, and conductive on the back of these, so I pulled it and scraped away the crusty old brown glue. This did not change the performance of or the chances of the tachometer functioning in any apparent way. I've replaced the sensor with a brand new one with no changes. The tone wheel was rebalanced and machined to ensure the tone points are all a uniform height, and that didn't seem to change the tach behavior either.

Noteworthy Stuff:
I've noticed a handful of things:
  • If I turn the key to on, but don't start the engine, and wait until the stereo turns on and my aftermarket gauges go through their startup, I have a much higher chance of the tach working at start.
  • If I just straight turn the Bronco on without waiting for any startup, the tach will rarely, if ever, work at start.
  • If I've been driving the Bronco a bit, the engine is warmed up, I stop, turn off the engine, and come back and start it again while the engine is still mostly warm, there is a very high chance the tach won't work at start no matter how I start it.
The shop that installed the engine suggested I get a module to help the tach signal, and they seem to believe the tach gauge isn't the issue, but that the solution from DCS is. This is what they suggested, though I'm not entirely sure what it's supposed to do. Dakota Digital Universal Diesel Alternator Tach Interface

My thoughts:
It seems to me like something to do with the circuitry directly related to the gauge itself. Maybe the circuit board for the gauge has a leaky capacitor or something (which I didn't see when I was cleaning the old glue off). However, the tach functioned flawlessly pre engine swap, so that takes the wind out of that sail a bit. If I should try to replace the gauge, I have no idea where to get another one other than a junkyard (I haven't looked for one super hard yet).
 

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If it is not a Bronco, it's just not worth driving.....
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My thoughts:
It seems to me like something to do with the circuitry directly related to the gauge itself. Maybe the circuit board for the gauge has a leaky capacitor or something (which I didn't see when I was cleaning the old glue off). However, the tach functioned flawlessly pre engine swap, so that takes the wind out of that sail a bit. If I should try to replace the gauge, I have no idea where to get another one other than a junkyard (I haven't looked for one super hard yet).
I have seen threads that have faulted the connections on the instrument clusters / gauges on the 92-96 trucks. I saw a thread, can't remember where, that suggested taking a soldering iron and reheating the solder joints to get the solder to flow.

I would try getting another tach and throw it in to see if it works.
 

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Premium Member
If it is not a Bronco, it's just not worth driving.....
Joined
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1,900 Posts
I found a video showing how to fix the circuit board and capacitors on the PSOM which should be similar for the tach.

 
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