[MadforitKev]

My rev counter is dead. From the wiring diagram it looks like its fed straight from the ECU?

I believe the power feed to the meter is OK as the other linked indicators (on same power feed) work.

Any suggestions? Common faults?

 

[Pim205GTI]

The signal comes from the Ignition Unit, located under the seat. Check connectors

 

[MadforitKev]

As the same feed goes to the left coil and that is working I am hoping it is a busted wire. Will get the scope out......Cheers

 

[MadforitKev]

I scoped the line all the way through to the rev counter and it looks good.

The gauge has a good 12v and earth so I must have a faulty gauge. Has anyone ever tried open heart surgery on a rev counter?

 

[MadforitKev]

So I've confirmed the ECU signal is getting all the way so its time to fix the gauge.

These Tacho's are basically a frequency to voltage generator that drives a DC coil meter.

@1200RPM the frequency is 20hertz, as the tacho goes up to 10,000rpm, I need a 0 hertz to 166 hertz frequency converter. These can be bought but the challenge is getting the voltages right (12v in and 5v out), so I thought I would start by trying to repair first.

Console cover off (lots of small screws) exposes the meters.

The tacho is held in with 5 screws from the back. 2 are mounting screws and the other three are 0v, 12v and signal from the ECU.

The the gauge comes out......

The small PCB has the converter.

The long black chip half way up the board is the Toshiba 2295A frequency to voltage converter.

First I checked all the other components for damage and correct readings. All good so I think its the chip.

They are not easy to find now but good old Ebay has come to the rescue.....https://www.ebay.co.uk/itm/115056402414

So standby for the next report when the bits arrive.

 

[elrodicater]

These tachs historically have been pretty reliable. I can't say I have seen this on the forums before. I have my popcorn and I'm ready for the show!

 

[MadforitKev]

While waiting for a replacement T2295a chip from China, I decided to review the design and come up with an alternative should the Toshiba chips finally run out (which seems possible).

In doing so I have been playing with tacho ECU feed and have noticed that it is very susceptible to interference. Basically any load on the tacho line from the ECU will cause a mis-fire.

There is also a lot of ignition spiking on the tacho feed on top of the normal square wave. These high voltage spikes can damage electronics and obviously the original design deals with this fairly well although it could be a result of my failure.

Initially I tried just changing the capacitors on the board as they are normally the first to fail when old but with new ones in the failure pointed to the main chip being dead, hence the Ebay purchase.

So looking at a new design that doesn't rely on the old chips, I decided to break it into 2 parts;

1. An opto isolator circuit to protect against the ECU spikes.
2. A frequency to voltage converter to provide the 0-4.5v DC needed for the RPM meter.

I selected a simple F to V chip in the LM2917N. An older devise but fairly easy to get hold of. It also has an inbuilt voltage regulator circuit to handle the bikes varying voltages.

The opto isolator is a TLP2962. One minor disadvantage of isolators is that most run at low voltage (5v) so need a separate regulator which I have added. At the end of the day it all aids stability.

So my design so far is;

I've done some basic testing and it seems OK. I am just waiting for some new LM2917 as I blew a couple up testing the ECU spiking issue

If it all works, the PCB will look like;

It fits in the original space and when I unsolder the 3 brass lugs that connect power and tacho feed from the original PCB, I can re-use them.

I will still play with the replacement T2995 when I get them but the new design is probably a better long term solution.

 

[MadforitKev]

I made a proto of the new design and it tests really well. Very liner response and the opto isolator sorts the ignition noise.

I made a few adjustments to the circuit and PCB......

So all good!! 👍

Unfortunately short run PCB's are n't cheap £30+ but at least we now have an alternative to dying and obsolete tacho chips.......

The small brass contacts on the PCB are riveted and soldered in place but can be carefully drilled out and reattached to the PCB

I will also check the Chinese T2995 when it turns up for those not keen on a whole new PCB build.

 

[wjvh]

Very clever stuff.

 

[elrodicater]

... and I thought wiring in a headlamp relay to a remote switch was genius... Nicely done. We need a video or it didnt happen

 

[MadforitKev]

How do i insert a video to the thread?

 

[nolimitz]

upload to you tube and post a link

 

[MadforitKev]

ah ha!! Here it is;
This is using my prototype board at the moment so not the fancy new PCB.
Home made tacho board

 

[MadforitKev]

Quick update on the Chinese chip replacements for the original T2295A........Don't bother!! I just received and tested a couple. They are either reused (and broken) or counterfeit. They basically don't work and don't look great either. It was worth a shot but best avoided.........

 

[elrodicater]

I'm thinking you had WAY too much fun solving this problem Are you considering a small run of new PCBs or just running your proto? You may look at marketing these! Murray has a small store for some CX bits he sells- could be an option. Forum name MurrayF

 

[MadforitKev]

Agreed....it has been fun. All the better for being a success!

Yes I probably will make some PCB's and assembly notes, if anyone wants one. I could do with a few orders to reduce the PCB upfront costs, so lets see where that goes.

Someone else will have a problem someday, so happy to help.

 

[titan joe]

This is pretty old now, but in case it helps someone: PCBWay is out of China, but it a really affordable way to get some prototype boards made. They are really quick too, considering that they are half the world away. We use them at work.