Volvo tachometer erratic

[lustyd]

Hi all, I finally tackled the hour counter LCD this weekend and replaced the screen. This worked great and I can now see the engine hours.

Following this, in typical boat project fashion, I am now getting erratic RPM readings on the tachometer. Engine is steady and running as usual but the RPM readout will fluctuate and often give very wrong readings. I have no clue how to troubleshoot this so looking for pointers. I'm assuming I must have disturbed a wire somewhere along the line. Zero is happily reading as zero, I did manage to work out that applying 12V would turn it on and point to zero for replacing the needle.

 

[Martin_J]

The pulses that drive the tacho needle come from the alternator.

Apart from checking the wiring end to end, the only tool that will usefully check at each location on the path is an oscilloscope...

You will see from my short video (on OneDrive) that the oscilloscope clearly shows the tacho pulses closing up as the revs increase (but the voltage shown on the digital voltmeter are no so clear)..

OneDrive - Volvo tacho signal as seen on DSO203 oscilloscope and on a digital multimeter.



You might also have seen the replacement hours display with custom boot screen... A small OLED display and an Arduino Nano do that part.

 

[lustyd]

Ahhhh in that case could it have been my completely flat engine battery causing the issue? Very long story there, all my fault, currently charging so will try again tomorrow with it fully charged.
I also recently removed the Adverc so potentially did something there as I’m not certain I’ve run the engine and paid attention to revs since then.

Thanks for the info that gives me a couple of things to check

 

[Alex_Blackwood]

Ahhhh in that case could it have been my completely flat engine battery causing the issue? Very long story there, all my fault, currently charging so will try again tomorrow with it fully charged.
I also recently removed the Adverc so potentially did something there as I’m not certain I’ve run the engine and paid attention to revs since then.

Thanks for the info that gives me a couple of things to check

Perhaps you have just disturbed a connection when working on it. If it is erratic it must be getting a signal of some sort to read at all.

 

[lustyd]

Tested this morning with a fully charged battery and although it briefly went to 4000 revs it then settled and worked as normal so hopefully the battery was the issue.

 

[lustyd]

OK not so sorted as I thought! I've now looked at the wiring diagram and have one question - does the G wire on the tacho just go to negative/ground? I can see the W wire (grey/black) goes to the W on the alternator on the diagram, but no sign of where the other wire goes.

Considering buying an oscilloscope or multimeter with frequency to help troubleshoot, but will be useful to know what all the wires are first!

 

[Boater Sam]

Erratic readings could be alternator brushes especially if you were trying to charge a very flat battery.

 

[lustyd]

Ok it turns out this is due to the Victron Orion Smart charger. If I turn the charger off the rev counter goes back to where it’s supposed to be and is reliable. I turned it on and off a few times and the Orion consistently makes the rev counter change.
Any ideas why this might be? The Orion isn’t connected to the tacho wire so this seems odd.
@PaulRainbow you've fitted loads of these, any ideas? Seems to be a couple of posts online with the same issue but no answers or responses.

 

[geem]

Ok it turns out this is due to the Victron Orion Smart charger. If I turn the charger off the rev counter goes back to where it’s supposed to be and is reliable. I turned it on and off a few times and the Orion consistently makes the rev counter change.
Any ideas why this might be? The Orion isn’t connected to the tacho wire so this seems odd.
@PaulRainbow you've fitted loads of these, any ideas? Seems to be a couple of posts online with the same issue but no answers or responses.

I have noticed the same thing with my Victron Orions. Two connected. My revs fluctuate by a couple of hundred rpm for no reason

 

[lustyd]

Glad it's not just me!

 

[QBhoy]

glad you got to the cause of it. You think it's maybe best to turn the charger off, before the engine is started anyway?

 

[geem]

glad you got to the cause of it. You think it's maybe best to turn the charger off, before the engine is started anyway?

They come on automatically by sensing alternator voltage. That's how they are supposed to work. You can build in a time delayed start, but it makes no difference to the problem.
To be fair, it's not a major issue for me. It just seems to wander up by a couple of hundred rpm then back down again

 

[QBhoy]

They come on automatically by sensing alternator voltage. That's how they are supposed to work. You can build in a time delayed start, but it makes no difference to the problem.
To be fair, it's not a major issue for me. It just seems to wander up by a couple of hundred rpm then back down again

Ah..no worries. That's fair comment then. All the best

 

[lustyd]

Yes it’s completely off until the engine bank is switched on, but does nothing until the alternator or other charge source starts and voltage goes up for a set time. There’s also a setting in the app to turn off charging so I can get proper revs back but would be nice to get it fixed. I may raise it with Victron and see what they say.

 

[Alex_Blackwood]

Glad it's not just me!

As the Tacho is frequency driven. Obviously a "Ripple" on the output of the Victron, probably caused by the electronics. Switch mode?? This will be transmitted via the charging circuit or capacitive or inductive coupling of the cabling. Have you thought of mentioning it to Victron? they may be interested and able to incorporate a smoothing circuit to remove the ripple. That's what I think anyway!
Edit:- Not just output could conceivably be on the input from alt. as well, can't be ruled out.

 

[lustyd]

I’ve logged a support ticket

 

[lustyd]

Impressed with support. Logged with Victron and within two hours BMS tech (battery megastore) called me for further details, presumably they’re the distributor although I did buy from them too.
Response was essentially “yes that’s weird” and following a bit of a discussion he confirmed my setup seems correct. They will feed back to Victron and try to get to the bottom of it. He agreed it might be a next gen product kind of a fix.

 

[Alex_Blackwood]

Impressed with support. Logged with Victron and within two hours BMS tech (battery megastore) called me for further details, presumably they’re the distributor although I did buy from them too.
Response was essentially “yes that’s weird” and following a bit of a discussion he confirmed my setup seems correct. They will feed back to Victron and try to get to the bottom of it. He agreed it might be a next gen product kind of a fix.

That is great service, so far, Let us know if you hear anything further, be interesting to know.

 

[Martin_J]

It would be interesting to see the waveform on the signal wire to your tacho using the oscilloscope I showed in the video in my first post...

You could also connect the oscilloscope to the power feed to see if that was picking up noise instead..

Good that Victron have replied but (being a network engineer), I do like to see the waveforms myself.

 

[lustyd]

I’ll give it a go in the morning I just tried and it’s a pain to connect in without disconnecting the tacho so I had to make up a cable to get the oscilloscope in there.
Anything in particular to measure? I have a Hanmatek HO52 and a complete lack of training