I have an issue with the tachometer in my 1985 XJ 12 Saloon: When I start the car the tachometer doesn’t work. Every second time I use the car the tachometer suddenly starts working after 5 miles or more. Then it will be fine for the rest of this particular drive. Until next time. Any suggestions?
I thank you kindly smile:
try giving the face a tap with your knuckles and see if that makes the tacho needle spring to life.
Thanks. I have tried to tap. It worked but only a few times.
Morning sickness isn’t unusual in these gauges. I’ve never attempted a repair so have no firsthand know-how to pass along.
Cleaning connections can’t hurt.
Since tapping on the gauge sometimes helps perhaps the instrument needs to be opened up for a look-see and cleaning? I’m reminded of the days when taking your wristwatch to the repair shop for annual cleaning and ‘servicing’ was standard procedure.
Cheers
DD
You might have a point there. I’ll take that route …
Cheers
Lars
When I was training as a young bloke the instrument technicians is was with referred to this problem as “stiction”. I have a bad case in our XJ6 - works better with the tach rotated 60degs - the next bayonet mounting location.
Moving the tacho in the mounting may restore a bad ground, Paul…?
As Doug says, ‘cleaning connections can’t hurt’…
Frank
xj6 85 Sov Europe (UK/NZ)
**
I suspect, but am not certain the issue is related to sticking in the needle/bearing mechanism, rather than electrical contacts
have had this issue on other vehicles.
If it was electrical, on an XJ6, to the best of my knowledge, the tacho signal is in series with coil
If there was a failure of signal due to a poor contact, one may expect ignition issues?
If you unplug the tacho wires, the car will not start
Thanks. Very useful information…
Lars
**
On the SIII, the only commonality between the ignition and the tacho is that both use the ign amp for ground connection, Tony…
So disconnecting the tacho from coil neg will not affect ignition. In the early cars, the coil power was indeed fed through the tacho, which would cut ignition if tacho was disconnected from coil pos…
Quite likely the cause of the tacho sticking is, as you say, the needle bearing - or something mechanical inside the tacho. If the ign amp fails, so does the ignition - and the tacho also drops to zero. This is useful when the engine quits on road - if the tacho shows ‘0’ with the car moving; you have lost ignition…
Frank
xj6 85 Sov Europe (UK/NZ)
**
1 Like
Could well be, Frank. Haven’t investigated yet. Paul
Early AM observations:
-
Do the disconnect/connect protocol at the tach a few times. Many a device has been replaced when it was the process itself that restored function.
-
After the engine has warmed up a tad and if the tacho is still at “0”, rev it a bit, up and down. I’ve an idea that may get a read. So far, knock on wood, mine work just fine.
-
Indeed, the tach gets a make/break pulse from the ignition amplifier and translates it to a needle position. No ignition, “0” RPM!!! Useful in diagnostics on a no start engine. It should flick on crank!
-
Decades ago, I added a teeny little after market tach to my Tbird powered 57 Ford pickup. Fun for a while. Then the usually reliable old truck refused to start. After much travail, I got it to run. Tach disconnect did it. Tossed it in @@#%^%#$#@$#$
Not the truck, the tach!
Carl
Just tec the car a bit that will wake it up always work on my series.3
Rev i meant to.say spell check
Agree, as in 2. of my post. Mine behaves just fine. so far……
Carl
Update on transmission.swap: the transmission shifts has it should nice to feel 3rd gear at a normal speed around 35. When at idle in park then into drive it is a very smooth transition no more harsh forward motion of cclonking. And also believe it or not no fluid from trans on driveway. I drove it for 200 miles and parked it with no leaks. That’s a really good feeling other trans would leave big puddle after just a.few days of no driving. Just one thing my Speedo is not working will look into that this weekend. Thanks for everyone for your suggestions and info everything is stock, flywheel, torque converter, shift cable, and brackets.
**
Good work on that transmission, Sawyer - well done…!
Since the speedo is driven from the transmission, mechanically by wire or electronically from the speedo transducer - your work may have interfered? Actually, I ‘think’ on early electronic versions, the sensor was at the diff…
Frank
xj6 85 Sov Europe (UK/NZ)
**
That would be the speed sensor for the cruise control.
When the change was made to electric speedometers (circa 1983) the diff-mounted speed sensor was eliminated, the cruise control now being able to use the same electric signal given to the speedometer.
Not sure what year/engine is in question here. The speedometer transducer is on the RH side of the transmission on 6-cylinder cars, LH side on the V12 cars. On the V12s the transducer wires run very close to the exhaust pipe, so…that might be something to look for?
Cheers
DD
I know it sounds cliche about “Giving it a tap” but it does work. I’ve got a '86 that occasionally sticks at 2200rpm. I’ll give it a tap, and the needle drops (when idling at a stop light for instance). After I drive the car for consistently - then it doesn’t stick anymore. Then I’ll put the car up for a month or so, and it sticks again…
Just in case you didn’t already know…
The tack is driven off the coil / White / Slate / blue wire on a Series III. and grounded (black wire) under the dash behind the Tach and Speedo. The ground requires taking off the scuttle (I think that’s what its called - the bit that covers all of the unsightly wife; as my wires says.
OOPS!
unsightly wires…
Thank you.frank. I have a.cable.speedo. I found problem missing the square wire inside the angle piece that attaches.to transmission. Should have.check when I installed it good.thing I have a.box of these. They are.expensive! I’ll will post a picture of my mates exhaust job he.did on my series one.
Cheers