Speedo or Tach?

My 1985 xj700 has inconsistancy between the tach and the speedo.
When cruising at 105kph/65mph approx. the tach usually reads around 4500 rpm and sometimes reads 5000rpm. WTF?
The speedo is cable drive so I assume it's not fluctuating.
The tach is electronic it seems, if it driven by the alternator pulses like most e-tachs, is this an indication of alternator issues? Or is the tach getting shady?
Also I've seen info stating the XJ 700 turns 4500rpm @65mph and 5000rpm@65mph what is actually correct?
Any feedback is super appreciated.

 

My 650 does the same thing but I know in my case it's the Speedo. I need to take it apart and clean/lube it as well as lube the cable again. Might even be time to replace the cable. My tach seems to stay pretty steady while cruising and I can hear the difference in sound between 4500 and 5000 rpm

 

My tach is steady while cruising, just registers different @105 after accelerating from a stop.
The tach seems accurate by sound but with an open header it's so ignorant loud that I'm not 100% sure it right.
I'll try lubing the speedo and cable etc.
Thanks!

 

1. Check your speedometer against a GPS. You will likely find that the indicated 65 is not quite 65.
2. 65 MPH is aproximately 5,000 rpm on the tach.
3. The electronic tachs are susceptible to the effects of dirt, grime, and age just like mechanical tachs are. You may want to open it up and clean and lubricate the bearings for the needle.
4. Corrosion and bad solder joints may be at play here as well. That will cause the indicated RPM on electronic tachs to jump around. Clean all of the plugs in your wiring harness, and inspect the tachometer circuit board. Resolder any cold or suspect joints that you might find.
5. if the coil was at fault youd notice it as an intermittent (or possibly constant) misfire when warm.

 

Thanks k-moe I'll try that as well, one at a time and post what I found as the fault

 

i think the tach works off of the ignition, mine does

 

Yep. Grey wire on the 2/3 ignition coil drives the tach.

 

You "can't" open up the tach or speedo for the xj700. if you REALLY need to get inside, here's how---
http://www.xj4ever.com/repairing your 700 gauges.pdf

 

Thanks so much for the info! I found out today that I "Can't" get into my speedo lol! However I did lube the little gear inside thought the slot where (now I know what it is) the reed switch is. Saved me a question tks.
However, this didn't fix my funky tach disorder, I'll check n clean wiring for that first before I disassemble the tach.

Also, anyone tell me how to source those funky green bulbs, one of mine is burnt out.
Thanks again!

 

Okay so lubing the speedo did nothing, a speed trap confirmed I am travelling 80kph as my speedo read about 81-82 so it's fairly accurate. I cleaned electrical connections the other day and my tach is more consistent, showing about 4800 rpm at approx. 65 mph 105kph. It still varies a bit but nowhere near the 500 rpm range as before.
Thanks a lot to everyone who chimed in!

 

Actually, they are a funky-sized white bulb with a translucent rubber "sock" over them............

HCP3618A OEM meter illumination and warning BULB, for all XJ700 and XJ750-X models. Use a total of 10 bulbs per entire instrument cluster in non-X models, and 11 bulbs per cluster in the X-models. These STANDARD-DUTY bulbs last for a mean average of 500 running hours. NOTE: these bulbs have a tiny head, and over each head is a separate green "tube filter" that gives the gauge light and face its greenish tint. The soft rubber filters are very fragile and generally need to be replaced whenever the light bulb is changed. Each:

HCP3618B Aftermarket indicator warning BULB, for all XJ700 and XJ750-X models. Use a total of 4 bulbs per instrument cluster in non-X models, and 5 bulbs per cluster in the X-models. These STANDARD-DUTY bulbs last for a mean average of 500 running hours. NOTE: the original bulbs have a tiny head, and these replacement heads have a slightly bigger, more bulbous head, but they will fit fine for the warning indicator lights. And although they will also fit as replacement meter lights, their larger head will prevent the use of the original green tube filter (HCP10235). The soft rubber filters are very fragile and generally need to be replaced whenever the light bulb is changed. Each:

HCP10235 OEM meter illumination green BULB FILTER, this small green rubber filter fits over the end of the meter bulbs and gives the gauge light and face its greenish tint. These soft rubber filters are very fragile and generally need to be replaced whenever the light bulb is changed. NOTE: these filters will only fit the HCP3618A oem bulbs, and will not fit the aftermarket replacement bulbs HCP3618B. Use 4 on XJ700 non-X models, and 5 on all XJ700-X and XJ750-X models. Each:

 

Actually, for an old bike with mechanical driven speedo, mine seems wonderfully accurate...I thought it was reading slow v RPM's......but, at an indicated 85 Speedo (though it does 'float' a little) my GPS said 83MPH!! Perhaps when it was newer it was over reading more, maybe time makes them (slower/worn) and more accurate!!

 

Tks, looks like I'm adding that to my order...

 

That's good news. Most of the time the magnetic drive bucket gradually demagnetizes and throws off the calibration.