- Joined
- Apr 24, 2018
- Messages
- 66
Just reporting that I had another FD owner send me their cluster to look at last week. The tacho was reported to be unreliable, only working about 20% of the time. On testing, the tacho needle simply wanted to sweep hard anti-clockwise against the zero stop, reading nothing on the gauge when a 10-300Hz signal applied.
Tacho removed from cluster. Same result when signal applied direct to to the tacho gauge, confirming it was not the processing of the signal through the speedo board at fault.
A hot air gun set to 100 centigrade was applied for about 10 seconds around the components on the PCB and the tacho sprang into normal operation. Conclusion: either a dry joint or an internal IC1 bond wire failure.
Gauge needle carefully removed. Always a hairy moment as there is a danger of the shaft pulling out of the gauge air-core motor, which cannot be repaired! This one was tight, but with the correct technique, came off fine.
Gauge face removed (lift and 90 degree twist) and two screws removed, so the PCB could be accessed.
After waiting for the PCB to cool (and fail again), I used an oscilloscope to check that the input signal was getting to a pin on IC1, which it was. There is no known datasheet for the obsolete IC1, but knowing how these class of IC's operate, I was expecting to see a processed "ramp" signal, the same as the input frequency, on another pin on IC1. There was none BUT, with heat applied, the ramp signal appeared on another pin, and disappeared when cooled! Conclusion, internal IC1 fault, probably a bond wire detachment.
Nothing to be lost now in reflowing solder on all 22 pins on IC1. There is just a chance the heat transmitted through the pins to the bond wires, can "repair" the fault.
Wow, it worked! Picture of cluster on test rig where I soaked tested the tacho at 6k rpm for a few hours, plus a pic of a tacho PCB for reference.
Customer put the cluster back into his car yesterday and reported the tacho is working fine
No knowing if this will be a permanent repair, but at least it's working for now. I know others on here have gone through the same solder reflow process to restore working tacho's, with a proportion failing again after a few months. Others have had no recurrence. fingers crossed for this one!
Tacho removed from cluster. Same result when signal applied direct to to the tacho gauge, confirming it was not the processing of the signal through the speedo board at fault.
A hot air gun set to 100 centigrade was applied for about 10 seconds around the components on the PCB and the tacho sprang into normal operation. Conclusion: either a dry joint or an internal IC1 bond wire failure.
Gauge needle carefully removed. Always a hairy moment as there is a danger of the shaft pulling out of the gauge air-core motor, which cannot be repaired! This one was tight, but with the correct technique, came off fine.
Gauge face removed (lift and 90 degree twist) and two screws removed, so the PCB could be accessed.
After waiting for the PCB to cool (and fail again), I used an oscilloscope to check that the input signal was getting to a pin on IC1, which it was. There is no known datasheet for the obsolete IC1, but knowing how these class of IC's operate, I was expecting to see a processed "ramp" signal, the same as the input frequency, on another pin on IC1. There was none BUT, with heat applied, the ramp signal appeared on another pin, and disappeared when cooled! Conclusion, internal IC1 fault, probably a bond wire detachment.
Nothing to be lost now in reflowing solder on all 22 pins on IC1. There is just a chance the heat transmitted through the pins to the bond wires, can "repair" the fault.
Wow, it worked! Picture of cluster on test rig where I soaked tested the tacho at 6k rpm for a few hours, plus a pic of a tacho PCB for reference.
Customer put the cluster back into his car yesterday and reported the tacho is working fine
No knowing if this will be a permanent repair, but at least it's working for now. I know others on here have gone through the same solder reflow process to restore working tacho's, with a proportion failing again after a few months. Others have had no recurrence. fingers crossed for this one!
