And...another successful Cluster repair (tachos mostly)

[casey]

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!

 

[Temujin]

Nice work, sure you could be inundated with work if you wanted. Be cheaper than sending overseas

 

[casey]

Thank you.

Two repairs (both tachos) successfully completed this month. Another enquiry came in yesterday. I'm not running a business, I'm simply a retired electronics engineer with a passion for rotaries, so applying my skills to help others where I can. If I get inundated, I will simply have to offer longer lead times. So far, it's not been an issue with typical turnarounds of a week or so.

 

[Temujin]

Prepare to be inundated I guess! Haha
Bit of beer money can’t be groaned at

 

[AlexG]

On a slightly related topic.....

I'm investigating my airbag light flashing on and off at the moment and came across info regarding re-capping the Airbag Diagnostics Module
https://www.rx7club.com/3rd-generat...capped-my-94-airbag-diagnostic-module-1153386

Is this also within your scope of expertise?

I wouldn't personally want to DIY it and would rather someone who is competent at this type of thing have a go.

Cheers

Alex

 

[noyzboy]

Well done Casey

 

[casey]

On a slightly related topic.....

I'm investigating my airbag light flashing on and off at the moment and came across info regarding re-capping the Airbag Diagnostics Module
https://www.rx7club.com/3rd-generat...capped-my-94-airbag-diagnostic-module-1153386

Is this also within your scope of expertise?

I wouldn't personally want to DIY it and would rather someone who is competent at this type of thing have a go.

Cheers

Alex

Hi Alex

Yes, I can certainly do that. I'll send you a DM.

 

[casey]

Sadly, full disclosure, the customer for this repair reported today that the tacho worked fine, but only for a short period. The tacho has now failed again, with the same symptoms.

My theory is that a bond wire, connecting the chip/die inside the plastic encapsulation of the (obsolete) integrated circuit to the solderable pins, has become detatched from the chip bond pad. This being the case, a reliable repair is impossible, as this case has demonstrated.

However, I now have a fully tested prototype circuit to replace the entire tacho PCB. It works brilliantly, using all new components. I'm currently at the stage of laying out and designing a suitable PCB to fit on the back of the tacho gauge, before getting a batch made. I have already invested in the components needed to make up the 10 boards, having seen reports of so many tachos fail with similar symptoms.

Realistically, it will probably take me until late January to have the first PCB constucted and fully tested. Watch this space.

 

[Temujin]

Please, for the love of tacos, put rpm needle sweep in it on power on if you are able.

 

[Jester*]

Love the work Casey. Amazing support for the community.

 

[casey]

Slightly ahead of schedule!

First pre-production replacement tacho PCB completed, benchtested and installed in a cluster. I'm very happy with the operation on the bench and also in a cluster on my test box - calibration is excellent across the full rev range, well within 5%.

I have 2 customers lined up to do some testing in their cars. Once I'm happy that all is well in those 2 cars I will offer these out to anyone interested. It'll probably be on the basis of ship me your complete cluster and I will replace, calibrate and return. Including return postage, I'm expecting to offer this service for £185. Excluding my pre-production PCB and those committed to test in their cars, I have 7 sets of PCB's and components available. I can easily get more PCB's and components if there is demand. Please post up in here if you are interested, so I can gauge demand.

I have adapted this circuit to replace faulty FD Speedos too. I have a prototype working on the bench, but no PCB designed for it yet. I'll work on that next year (2026 )

Happy New Year!

 

[Jester*]

Currently my rev counter & speedo is working as Mazda intended .

But I'd definitely want to go down this route if it fails. Have you reached out to any of the international FD clubs? There must be some interest.

 

[casey]

Thanks Jester, but I'm not going to take this any further than the UK (at least for the foreseeable), and only when a couple of modules have been tested OK in a real car environment, rather than in a cluster on my test rig. I'm not expecting any issues, but nothing like the extra stresses of a real car to be sure!

 

[Rob82]

Hi Malcolm, just to let know I fired my car up the other day and the odometer is still working perfectly
Hope you had a good Christmas.
Cheers
Rob

 

[MightyCondor]

I have 2 customers lined up to do some testing in their cars

I know we talked about it in quite a bit of detail, so hopefully my interest is registered.

 

[casey]

I'll pencil you in for one of the PCB's Alan

 

[casey]

Hi Malcolm, just to let know I fired my car up the other day and the odometer is still working perfectly
Hope you had a good Christmas.
Cheers
Rob

I had a great Christmas with family, thanks Rob. Hope you did too.

Good to hear your cluster is still working well. I'm very happy the odometer fault on your cluster is highly unlikely to recur after my repair. I have come to the conclusion though that I cannot offer any reliable repair option for "wayward needle" tachos, other than to replace the whole PCB assembly. I am now 100% convinced that this fault is due to an internal issue in an obsolete chip on the tacho PCB module, having seen so many now (and done much fault diagnosis). I'm really happy my replacement module is looking good in all bench testing though

 

[noyzboy]

Great Job Casey. It's great to know that our ageing cars can still be kept on the road when the OEM bits retire themselves

 

[casey]

The first cluster with my replacement tacho PCB design is now back in the customer's FD with the feedback,

"Cluster all back in and working perfectly, thanks again"

The cluster arrived with me on a Saturday, repaired same day, shipped back Monday, back with customer on Tuesday! I can't promise I can always achieve that timescale, but demonstrates what can be achieved

I'm now ready to take on the next tacho repair using the replacement PCB, cost £185, to include return shipping (Royal Mail Special Delivery, next day by 1pm).

I live in Colchester, so you are welcome to drop off/collect if convenient for you, cost £170.

Please DM me if you would like me to repair your cluster. Priority will go to those who have already expressed an interest.

BTW, I also have a prototype module to repair errant Speedos too. It replacing a chip on the speedo CPU board, which frequently fails - it's in the same family as the tacho chip (unsurprisingly, the same failure mode!). The module is fully tested, but I need to design and get a batch of suitable PCB 's made for a production version, but DM me if you might be interested in this later in 2026.....

 

[Gazfd]

I can confirm this as it was my cluster all good and reading spot on
Top bloke