Hello,
Just wanted to share the debug steps that brought my tach back to life. This is all stuff I learned from other FABO members, but just to summarize...
1. Apply a 1.5V battery across the plus and minus terminals. The tach needle should go to roughly 2000 rpm. Mine did not.
2. Use a multimeter to measure resistance across the plus and minus terminals. You should get roughly 160 Ohms, give or take 20 Ohms. Mine was in the 140s, so good enough. If you get an open or short, you should be able to use some electrical debugging to investigate. There are a couple of diodes and resistors in the path. Brian at MrHeaterBox told me that these diodes and resistors are unnecessary if using a modern sending unit like his, or RTEs version. If the diodes and resistors are bad, you can bypass them and wire the plus and minus terminals directly to the internal meter.
3. OK, so my Ohm reading was good, but still no tach needle movement. In the picture below, the arrow points to an access hole for the clock spring tension adjusting screw. With the 1.5V battery applied to plus and minus, I started to turn the adjusting screw. At probably just 1/8 of a turn, the tach needle jumped to 2000 rpm! I guess the needle mechanism had gotten stuck from age and wanted a little more tolerance to allow movement.
P.S. I also included a picture of the aftermarket 5V voltage limiter that I use for the gauges. It’s about $9 on Amazon, so cheaper than the RTE version. Also maybe easier than the Radio Shack with capacitor and heat sink that some folks use, although there is nothing wrong with method. This Amazon unit is a switching regulator, so much less heat and power dissipation than the Radio Shack approach.
Just wanted to share the debug steps that brought my tach back to life. This is all stuff I learned from other FABO members, but just to summarize...
1. Apply a 1.5V battery across the plus and minus terminals. The tach needle should go to roughly 2000 rpm. Mine did not.
2. Use a multimeter to measure resistance across the plus and minus terminals. You should get roughly 160 Ohms, give or take 20 Ohms. Mine was in the 140s, so good enough. If you get an open or short, you should be able to use some electrical debugging to investigate. There are a couple of diodes and resistors in the path. Brian at MrHeaterBox told me that these diodes and resistors are unnecessary if using a modern sending unit like his, or RTEs version. If the diodes and resistors are bad, you can bypass them and wire the plus and minus terminals directly to the internal meter.
3. OK, so my Ohm reading was good, but still no tach needle movement. In the picture below, the arrow points to an access hole for the clock spring tension adjusting screw. With the 1.5V battery applied to plus and minus, I started to turn the adjusting screw. At probably just 1/8 of a turn, the tach needle jumped to 2000 rpm! I guess the needle mechanism had gotten stuck from age and wanted a little more tolerance to allow movement.
P.S. I also included a picture of the aftermarket 5V voltage limiter that I use for the gauges. It’s about $9 on Amazon, so cheaper than the RTE version. Also maybe easier than the Radio Shack with capacitor and heat sink that some folks use, although there is nothing wrong with method. This Amazon unit is a switching regulator, so much less heat and power dissipation than the Radio Shack approach.
