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Discussion Starter #1
My tach is going crazy... at idle it bounces between 3 and 5000 rpm, if i give it gas it will bounce up to redline even though it's not turning more than 5000.

i've looked in my haynes but all i can see is a wire from CPU.

is this a problem that shows up a lot? any fixes???


thanks
 

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1999 748 with a 2000 engine, did the search... can't find anything
No you didn't, there's literally dozens of threads covering it. But since you're new:
1) Welcome to the site
2) There are three brass screws on the back of the tach. Tighten those up, they tend to vibrate loose and they hold the circuit board together within the tach.
3) if that doesn't do it, then you need to check the wires. Follow the wires coming out of the tach.
4) if THAT doesn't do it, then look at some of my old posts and you'll see several threads covering it.
 

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Discussion Starter #5
you are right sir.... i did not do a search on this site, but on another, thanks for your reply. I will try the fixes tomorrow and keep the board posted as to what i find.

Thanks for you help
 

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This board tends to have a LOT more tech info on it then others. I would suggest searching.
 

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Old Wizard
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Erratic Tach

Electronic tachometers give erratic readings when the base voltage is low, usually caused by a loose battery connection. The worse situation for low voltage is at idle when the alternator output is at it’s lowest. You may just need a new battery or a good charging.

If it's not the battery:

Most likely - Intermittent power or ground connection
Next likely - Intermittent rpm signal connection
Least likely - Tachometer internal problems

Check for a loose or corroded electrical connection at the battery, the fuse, and any in-line connector. Check the two battery connections first, next the connector at the instrument cluster, then at the fuse, and lastly at the wiring harness connections.

Next, since the bike is running OK, the RPM sensor gap, the sensor itself, and wiring back to the computer are not the problem. So you need to check the connectors between the computer and the tach.

Finally, try to borrow a tach to verify that it’s actually the problem, before buying a replacement.
 

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Agree with everything but "Least Likely". The solder on the board is weak and the three brass screws come often more then anything else.
 

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No you didn't, there's literally dozens of threads covering it......................
But I did, which is why it brought me here.:D

So I'll check the screws on my 2000 ST4 since this started happening on my Duc two days ago. The three rubber stud mounts are broken on my instrument set so I'm sure it vibrates more than normal. Need to replace those while I'm in there......:rolleyes:


:eek: Mark :eek:
 

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Good man!

But I did, which is why it brought me here.:D

So I'll check the screws on my 2000 ST4 since this started happening on my Duc two days ago. The three rubber stud mounts are broken on my instrument set so I'm sure it vibrates more than normal. Need to replace those while I'm in there......:rolleyes:


:eek: Mark :eek:
 

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grand poobah
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I remember my tach went wild right after my voltage regulator/rectifier connector fried.
 

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I've already confirmed mine to be loose connection at the gauge assy. Tach will be spot on and steady if I apply slight pressure at the edge of the bezel. :D


:eek: Mark :eek:
 

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Sadly mine is not the brass screws being loose, those are tight. It's the circuit board. If I apply angled pressure to the base connector attached to the circuit board it works just fine. Which means it a loose solder joint on the board. So now gotta figure out how to get the bezel off without buggering it up so I can get the circuit board out to fix it............. :(

Or just upgrade the whole dash to the hi-tech unit? :rolleyes:

Decisions, decisions.............


:eek: Mark :eek:
 

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I got my rubber isolator mounts from Commonwealth the other day. So now I need to do this while I'm in there>>>>>>>>

Tighten the three brass screws on the back of the tachometer. If that doesn't work, then you will need to cut the bezel off (about a 1/4" below the bezel around the body of the gauge) and take the microboard off. Most likely, a resistor has vibrated out of it's solder footing and will have to be soldered back.

^^^^^^^^^^^^^^^^

I'm having a hard time visualizing exactly where, how much, how deep, to cut for this. Can I get a clearer description or perhaps some pics! :)

I'm debating on shopping ebay for a used one before I end up butchering mine...


:eek:Mark :eek:
 

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I'm having a hard time visualizing exactly where, how much, how deep, to cut for this. Can I get a clearer description or perhaps some pics! :)
I followed Jimmy's description and it was easy to do. The white plastic shell is only ~1/8" thick, so don't cut too deep. Just take a Dremel and cut all the way around the circumference ~1/4" below the black bezel (the outer black ring that's visible from the rider's perspective when it's mounted on the bike).

My wonky tach also responded to pressure on the face, but I was unable to fix it after multiple disassemblies/assemblies. I checked all the brass screws and the solder points. Everything would work fine after I reassembled it, but after 5 minutes of riding the tach would quit again. :mad: I eventually ended up replacing it with a used one. Just a fair warning - hopefully you'll find a loose solder point and it will be an easy fix. Good luck!
 
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