1994 tach dead

[Chad]

I have a 1994 VFR750f which I've owned for 21-years and the tachometer stopped working a few years ago. Right now the needle slightly twitches when rev'd, doesn't register anything more than that (luckly the rev limiter works😮). Looking to see if anyone has had a similar issue. 

 

Usually I fix my own problems but decided I'd take it to the local Honda dealership (Interstate Honda in Fort Collins Colorado) to have it fixed by the "professionals" plus regular mantanance stuff while they were at it. Well they butchered the bike breaking the fairing in multiple places, small dent the tank, brake fluid stains and left out 75% of the fairing bolts. They also didn't hook up the blinkers or the cooling fan, I was pissed when I got it back. In the end they told me it needed a new tach but the part is unavailable and I should shop eBay. They replaced the bolts and I've gone through it fixing most of their damage the best I can but not having a functioning tach is still the case.

 

Now I've recently picked up a 1993 CBR900RR which I've always wanted to complete my high school dream bike collection 😂. Riding that and having a working tach made me want to fix the VFR's tachometer even more now. 

 

Assuming it is the tachometer I've been looking online and found a new OEM tach on partzilla.com and bikebanits.com, I've never had experience with these online companies, the part run about $300 (if they really are available). 

 

Anyone have any experience with this tachometer problem? Or buying from these sites?

 

And I've never pulled apart the cluster so any heads up on that too is much appreciated! I've had the bike stripped down to where I didn't have much further to go.

 

Thanks for the help!

 

 

 

 

 

[Cogswell]

Wow - that's a nice looking pair of bikes.  Truly Honda's best days - the early 90's through early '00s - their build quality was fantastic - probably not to be replicated and they truly made some epic bikes. 

 

Sorry for your crappy experience at the dealership, but unfortunately I don't think it's all that rare.  With tight labor most are having trouble finding qualified mechanics and many of them just don't give a sh*t.  None of my bikes ever see a dealership for that very reason. 

 

On the tach - I don't recall how it picks up its signal - but it wouldn't surprise me that it's nothing more than a grounding issue, or maybe a contact issue in the unit itself.  If you can remove it possibly some contact cleaner could get it working again.  And some Oxgard on grounds / connectors might get the electrical signal coming through.  Other possibilities could be a loose connector or chafed wiring.   Whatever it is, I think there's a good chance it's not fatal requiring replacement of the unit. 

 

Regarding Partzilla, I have used them - no issues.  Lots of people like Mason City Honda - ask for Keith Dyball and tell him you're a VFRD member.  He'll give you a good price. 

[Marsman99]

Check the plastic coupler that connects the speedo sensor to the pulse generator.  See Item 20 below.  It gets stripped and doesn't turn with the flange bolt on the pulse generator, or does so erratically.  I replaced mine twice before buying an Al version...can't remember from who though.

 

[Dutchy]

+1 on Mars'    Just unbolt and fit new

[Grum]

2 hours ago, Marsman99 said:

Check the plastic coupler that connects the speedo sensor to the pulse generator.  See Item 20 below.  It gets stripped and doesn't turn with the flange bolt on the pulse generator, or does so erratically.  I replaced mine twice before buying an Al version...can't remember from who though.

 

 

Ummmm, sorry Marsman and Dutchy, but I think the OP has a Tacho issue not a Speedo. However that is good info.

Cheers.

[Marsman99]

Ummm, you're right.  Right solution, wrong problem.  Speedo coupler is such a common issue it is an instinctual response anymore.  Tach's aren't a common issue.  This gaffe acknowledged, I recommend the following:

1. First, run through the power check as described in the service manual on page 18-14.  Your dealers service dept. appear to be incompetent, so you may want to check this.  I can send the manual pages if needed.
2. If the tach fails this test, buy a complete gauge cluster on eBay.  I see two there, one for $100 and one for $145.  Sure, there is a risk the tach on this unit is bogus, but very unlikely.  Having a spare gauge cluster will help keep your bike running if you plan to keep it for the the long(er) haul.
3. The instrument cluster can be disassembled and the tach gauge replaced.  Be methodical and work on a very clean lint free bench with clean hands and it will be easy.  I've had mine apart twice to replace the speedo gauge and can advise you to be careful not to soil the gauge faces once the lens and cover are removed.  Any FOD on the surface will be highly visible.  There are four screws on the back that hold the gauge, and a small circuit card attached to the gauge that must be detached from the cluster.  You may want to check these contacts on your tach gauge first as Cogswell suggests - since you'll need to get to this level for a replacement anyway.

Again, I can send the service manual pages if needed.  It is pretty straightforward really, just a lot of crap to remove and reinstall.

 

[Chad]

Thanks guys!

 

Marsman99, that would be great if you could send those pages. 

[GreginDenver]

A motorcycle tachometer is a relatively simple instrument. 

The tachometer is fed with 12 volts from the bike's wiring harness (continuous 12 volt feed from the moment you key-switch the bike's ignition to "ON") and the tachometer gets its path-to-ground (negative) signal from the Ignition Control Box line connection to one cylinder's coil to induce current flow through the windings of a small electro-magnet inside the tachometer which pulls against a factory-calibrated return spring (the technical name of this combination is "Galvanometer").

 

As RPMs increase, this electro-magnetic circuit spends more time grounded (because the cylinder is firing more times per minute as you rev the bike), so more and more 12 volt power is flowing through it.  The tachometer-"galvanometer" receives a few milliseconds of path-to-ground every single time the Ignition Control Box provides a ground to the coil before abruptly removing that ground, the time the coil spends connected to a ground is known as "Dwell Time" (which is the time required to "charge up" the coil prior to firing its spark plug) and when the ground is removed this induced electrical field inside the coil will collapse which fires the spark plug.  The cylinder that the instrument ground wire is connected to, which means as RPMs increase it spends more time exerting a pulling force against the return spring which results in swinging the tachometer needle higher on the gauge's face.

The instrument (the tachometer) is probably still in its original factory-sealed state inside the bike's instrument cluster assembly and for this reason the actual instrument is probably in sound condition (not water-infiltrated) but you should take the time to inspect the wiring and the connections of the wires. I've seen corrosion on the connections on the back/bottom side of a Kawasaki EX250 instrument cluster assembly (on an otherwise very good condition bike).

 

So I'd bet that there's something wrong with either the 12 volt supply to the tachometer or the path-to-ground (as described above in this post).  You should look very carefully at the entire run of wiring that connects the tachometer to these things (its 12 volt supply and its path-to-ground through a coil control wire) and the connection at the backside of the tachometer.

[Chad]

GreginDenver, thanks for the details. I was wondering how the tach worked, lots of info here! 

[GreginDenver]

and... that's a good looking '94.  Both of those bikes are going 100 MPH just sitting still (actually, the CBR900 may be closer to 150).

[COS_VFR]

Looking at the wiring diagram, the Tach shares a common ground with everything else in the gauge cluster and is driven by a signal from the Ignition control module. The wire is labeled Y/G which I believe is Yellow with green stripe.

[GreginDenver]

Well, leave it to Honda to do it the opposite of what is the norm. 

 

So, same operational principle I outlined earlier, only with ground as the constant and an RPM-based voltage input (probably 12 volt, but could be less, maybe 5 volt, with Honda who knows?).

 

I've just looked up the Tachometer in my 5th Generation VFR Service Manual and the inspection procedure for the gauge shows that my guess is correct about the way Honda decided to run it.  To inspect/test the VFR's Tachometer you hook up a Multimeter (set to DC volts) to a peak voltage adaptor and put the red (positive) probe on the Yellow/Green wire at the 10P (blue) connector block.  Then start and run the engine at idle.

 

If you get 10.5 volts or better, but the Tach doesn't work, replace the Tach

if you get under 10.5 volts, replace the ECM

if you get 0 volts, check the wiring continuity of the Yellow/Green wire to the ECM

[Lorne]

12 hours ago, GreginDenver said:

A motorcycle tachometer is a relatively simple instrument. 

 

One might even say that a motorcycle Service Manual is a relatively simple instrument.

 

Great that you wish to help a fellow down on his luck, but do try to give accurate information 😉

[GreginDenver]

2 minutes ago, Lorne said:

 

One might even say that a motorcycle Service Manual is a relatively simple instrument.

 

Great that you wish to help a fellow down on his luck, but do try to give accurate information 😉

 

How's the view up there on the moral high-ground?

[Lorne]

1 minute ago, GreginDenver said:

How's the view up there on the moral high-ground?

 

Nah, I'm down at sea level like everybody else. We all err from time to time, and I am no exception.

[GreginDenver]

13 minutes ago, Lorne said:

 

Nah, I'm down at sea level like everybody else. We all err from time to time, and I am no exception.

 

So I was wrong about the operational polarity of the Honda Tachometer, everything else I stated about how the instrument operates was correct.

 

Learning is a messy, piecemeal process.  You don't always get a neat-and-complete package of knowledge right away.  Sometimes all you get it a hint or basic starting point.  Now it appears the "fellow down on his luck" took what I wrote and did exactly the right thing: he traced the Service Manual wiring diagram in an effort to confirm or disprove what I said.  And through his efforts he now knows the truth of how his Honda's Tachometer is driven (intermittent 12 volt DC, constant ground rather than the more common constant 12 volt with intermittent ground).  It would seem that we were making progress towards a solution or diagnosis... 

 

And then you showed up.

 

Your comment was meant to accomplish what?

[Stray]

2 hours ago, Lorne said:

 

One might even say that a motorcycle Service Manual is a relatively simple instrument.

 

Great that you wish to help a fellow down on his luck, but do try to give accurate information 😉

Come on Lorne, that’s not really fair. 

 

Greg went to a lot of effort crafting that post to help the OP. Other than the polarity he was bang on with lots of detail. 

 

We need to be mindful of how we respond to those who try to help or they will stop offering their expertise and this forum will be greatly impoverished. 

 

I’m sure you didn’t mean to offend so no real harm done. 

 

Stray

[Marsman99]

On August 11, 2018 at 8:32 AM, Chad said:

Thanks guys!

 

Marsman99, that would be great if you could send those pages. 

 

Will do.  Look for a PM later today.

[Dutchy]

On 8/11/2018 at 10:23 AM, Grum said:

 

Ummmm, sorry Marsman and Dutchy, but I think the OP has a Tacho issue not a Speedo. However that is good info.

Cheers.

 

Ah!  My poor command of the English language showing.......... I thought "tacho" was speedometer.......  ooopsie....

[Lorne]

Chad,

 

There are tons of owners and service manuals, along with wiring diagrams - and more - at the VFRD Downloads tab.

 

For the 1990-1997 VFR750 here are the:

  Service Manual      Wiring Diagram     Parts Fiche

 

 

 

[Lorne]

3 hours ago, Dutchy said:

I thought "tacho" was speedometer.......  ooopsie....

 

No Worries, Dutchy - so long as you can tell us if Glenmorangie is better than Oban 😉🥃

[Dutchy]

Caol Ila for the win! 

[Chad]

Thanks all for the info, this is great! I need to chisel out some time to tear into it, will update with what I find. Anxious to get it fix, want the bike to work as good as it looks!