Hello. Car is back on the road and running very nicely once it is warmed up. Took it out this afternoon to give it a bit of a real n and noticed that the tacho needle jumps around a lot if I accelerate fast and rev the engine out a bit. If I accelerate slowly then tacho is smoother. Recently had an issue with the alternator light staying on because it had got earthed. Any chance its all related?

 

It's hard to say. Most Rev counter misbehaviour is usually caused by the Generator on the back of the inlet cam or its drive as recently covered in another thread. Check the wiring from the Generator first.


https://www.jaguarforums.com/forum/m...ve-dog-106110/

 

Doubt it would be linked to the ignition light but stranger things have happened. More likely it is a problem as Glyn says with the RPM generator on the back of the inlet cam cover. Simple device that rotates with engine speed and generates an electrical current. The faster it rotates the more power so up goes the needle on the rev counter. The fact it is jumping when accelerating fast would mean there is a problem in the transference of that power. A good clean of the points inside the generator would possibly suffice.
I have another problem which is similar. Sometimes my rev counter sticks at 1500 rpm. Tap the screen and nothing happens, other times it helps and it starts again. Not quite sure if it is the generator or the rev counter. One day I will strip my generator down and give it a good clean.I actually have a couple of spares in my boxes of many things so might strip one down and clean it one evening during the lockdown.

 

Yes ~ The rev counter generator is traveling at half engine speed off of the cam & the Rev Counter itself is calibrated to display twice that.

Frequent issues. (Let's get it all in one place).

a) Rev counter generator bushing crimping loosens over time & the generator shaft withdraws into it's casing & disengages from the drive dog. No the visible circlip does not prevent this.
b) Dog fails
c) Generator fails. Read voltage at generator output ~ note it's AC. The Service Manual states 10 volts per 1000RPM ~ Set multimeter to 200 volts AC. Output needs to be stable at stable RPM.
d) The rev counter itself goes cuckoo due to internal bad connections or mechanical misbehaviour. Cass ~ Rob likely your situation if the rev counter generator output is stable at stable RPM.

 

WBO ~ see Edit.

 

post mentions Alternator, not Generator. Everytime an Alternator replaces the Generator there is a problem with the charging light at the tachometer, if not with the tachometer itself.

 

Note in all cases I'm talking about the REV COUNTER GENERATOR on the inlet cam. NOT the charging circuit. The rev counter generator outputs AC.

Never seen a rev counter issue with an alternator properly fitted. IGN light yes. Mainly due to people not observing the fitment instructions. WBO's alternator fitment is not new.

 

these cars consume too much money.

 

Apart from illumination, the rev counter system is independent of the car's electrical system. In fact, it's a beautifully simple arrangement: an AC generator and an AC voltmeter measuring its output.

 

I agee Pete a very simple system.
The ignition light is not a problem if as Glyn says the instructions are followed correctly. When I fitted mine it required a wire to go from the the new Alternator to the ignition light. As I recall there was already a wire that went from the Voltage regulator box RB340 to the ignition light so I just moved it from the regulator to the alternator.

 

I too had the problem of the bushing migrating so that drive was lost.
I fixed it with a spring pin.
Document attached
Bruce

 

Great advice Bruce.

EDIT: I know people that have tried epoxy, Loctite & peening/punching. All have ultimately failed again. Pinning the bush as you have is going nowhere. Should achieve a permanent repair.

I think the point is that these things are generally easily repaired. You don't necessarily have to replace them at a present cost of £156.55 in the UK or US$235.67 in the US.

 

I have had all of these problems in MK2 tachos. One area which does not seem to be touched on is the instrument its self. Sticking and strange indications are quite common and relate to the innards of the instrument needing cleaning and re lubricating.
Most people are not aware that these instruments need cleaning and re-lubricating. Using an artists paint brush and white spirit on the main needle pivots will get rid of the old lubricants and with another artists brush you apply light oil (singer sewing machine oil or similar)
It works every time

 

Yes ~ you are spot on and thanks for the good advice. I was going to move onto the instrument itself but got distracted.

IIRC there is also a 2uF electrolytic capacitor on the board in the instrument that dries out with age and needs to be replaced. Very simple to do. I'm sorry I don't have a picture. Did one on a friend's E Type & the instrument came back to life.

I'm sure Cass will easily repair matters with the symptoms he reports with his Rev Counter.

 

would you consider a parts trade? i might need one of those generators..im trying to put a 1987 4.2 liter out of an XJ6 Series 2 into a 1964 3.8S vehicle, and my cylinder head has no tach gen.... i do have an engine block that is .030 over with liners from 1972, and a bunch of parts like flywheels (heavy) and cast iron auto trannys (2 of them)

 

I have been told by several people that you don't need a tach generator in a 4.2. just take the signal from the coil. Run 1 wire to the tachometer. This was confirmed by a business called Distributor Doctor also.

I am thinking also of installing a 4.2 xj6 engine in my 1965 S type. I already have the engine but I also have a million questions. For example the 4.2 does not have the rear mount at the cylinder head either. Too many questions to resolve.

 

i am putting in a borg warner 65/66 aluminum transmission along with this rebuilt 4.2..got a intake maniforld, removed the water heated manifold blocks, shortened the studs and putting the two zenith stromberg carburetors as close to the manifold as possible for engine bay clearance..had to make original motor mount brackets as this shell came with no engine, and the 4.2 xj6 mounts are farther back on the motor...might mock up the bare block and trans in the shell, with my mounts, and add frame pcs to also use the xj6 mounts, having 4 points where the engine bolts down since the rear engine stabilizer is no longer used at the parting line of the tranny...

BUT I DIGRESS...the issue here is that the older dashboard tachometer input is looking for an AC signal ? and a pulse train from the negative side of the ignition coil wouldnt work unless you put in a newer tachometer in the dashboard?
right?

 

Yes ~ You need the newer 3 wire tach that picks up a pulse from the coil (2 white, 1 green excluding lighting). Your 4.2 Cylinder head (87) won't accommodate the AC Generator. You will lose your clock. The instrument is earthed to make the full circuit. Also fitted to 420 & Sovereign.

The Rev counter generator from a 3.8 puts out a perfect AC wave at 10 volts per 1000RPM. The tach on a 3.8 is a volt meter calibrated in RPM.





Some 4.2 rev counters were redlined at 5500RPM & some at 5000RPM.

 

I think most S type had a problem tachometer. I fixed mine with a wood toothpick inserted between some wound pickup thing and the case. Can't remember what I did but I do remember connecting the tach to my 12v DC power supply and messing with it until It stopped jumping.

heck I even dismantled the tach generator not knowing what I was doing, and it didn't fix it.

But the toothpick did.

I have just about given up on lumping my S type with the 4.2 engine, too many issues to resolve and who knows what other issues will surface afterwards. But mostly I do not want to lose the tach clock. I had it digitized by Jaguar Clock dot com and it works and ticks like a mechanical clock. Mike Eck is an electronics wizard. And warranted for life !

 

Yes ~ moving to the 3 wire coil pulse tach is why they had to mount a separate clock in the 420, Sovereign & 420G. Smiths did not have a suitable tach that included the clock available. My S Type tach works perfectly although I did re-lubricate it when I restored my gauges.