Sticky Tachometer
#1
11-09-2011 | 01:08 PM
Sticky Tachometer
For some odd reason, the tachometer needle keeps on sticking on my disco. When I start it up, it moves a little to less than 500rpms (less than idling rpm), then stops and appears to try to move to normal idling speed. After about 5 minutes of driving, it becomes unstuck only to get stuck again at about 2,000rpms. Yet again, it appears to move to where it should be. After a while, it works fine like it should. When it does work, it seems accurate.
any ideas?
any ideas?
Last edited by ESPN; 11-09-2011 at 01:11 PM.
#2
11-09-2011 | 01:19 PM
Super Moderator
Joined: Jan 2011
Posts: 16,322
Likes: 84
From: Savannah Georgia
Bad news.
The tachometer estimates engine speed from a signal that comes out of the alternator. This signal changes frequency of pulses as the speed increases. When an alternator fails, the tach drops to zero, and you are not charging the battery. If you check your battery at idle with a voltmeter, and have 13.8 - 14.2 volts, that's good.
That would leave a mechanical sticking tach. Pricey to swap out, parts and labor, etc. You might decide to instead budget about $80 for an "Ultra Gauge" which mounts anywhere, displays multiple things, with assignable alarm points, and it plugs into the scanner port. RPM is one thing that could be displayed. Temp is another important one.
Here is the first pix that pops up on Google images for rover ultra gauge, looks like a very crafty fellow installed it, and he gets the best mpg of any Rover, so we should listen to his advice...
The tachometer estimates engine speed from a signal that comes out of the alternator. This signal changes frequency of pulses as the speed increases. When an alternator fails, the tach drops to zero, and you are not charging the battery. If you check your battery at idle with a voltmeter, and have 13.8 - 14.2 volts, that's good.
That would leave a mechanical sticking tach. Pricey to swap out, parts and labor, etc. You might decide to instead budget about $80 for an "Ultra Gauge" which mounts anywhere, displays multiple things, with assignable alarm points, and it plugs into the scanner port. RPM is one thing that could be displayed. Temp is another important one.
Here is the first pix that pops up on Google images for rover ultra gauge, looks like a very crafty fellow installed it, and he gets the best mpg of any Rover, so we should listen to his advice...
#3
11-09-2011 | 01:38 PM
Well, the needle doesn't stay at zero, so it doesn't sound like a bad alternator. Even if it was, it wouldn't start after sitting in the garage all weekend. The needle moves to about 500rpm and tries to move a little more, even when im driving. Looks like a sticky tachometer. Im already checking into a ultra gage.
#4
11-09-2011 | 04:03 PM
Super Moderator
Joined: Jan 2011
Posts: 16,322
Likes: 84
From: Savannah Georgia
No, a bad alternator won't kill the battey while you are sitting in garage with engine off, it just will not charge it up while you are driving, with one totally out you can drive for like 30 minutes to two hours, then battery is exhausted. Of course, if you park it before total exhaustion, it won't start the next time.
Could also be a ground or corrosion on the wire from the alternator. So wiggle those wires and see if any are loose or appear to be corroded. I recall one fellow who had a cable issue between the alternator and the tach, but his was dead all the time. If there is a wire on the back of the alternator that unplugs (vs un-bolts) take it off and see what the tach does. Don't take off wires that un-bolt, unless wires off the battery.
Even if you fix it you will still want the ultra gauge.
Could also be a ground or corrosion on the wire from the alternator. So wiggle those wires and see if any are loose or appear to be corroded. I recall one fellow who had a cable issue between the alternator and the tach, but his was dead all the time. If there is a wire on the back of the alternator that unplugs (vs un-bolts) take it off and see what the tach does. Don't take off wires that un-bolt, unless wires off the battery.
Even if you fix it you will still want the ultra gauge.
#5
11-09-2011 | 05:28 PM
Baja
Joined: Mar 2010
Posts: 5,584
Likes: 7
From: Pittsburgh PA suburbs.
My tach has been acting up in a similar fashion for months now. I changed alternators, but symptoms were unchanged. So I went back to my original alternator.
Usually when I first start it up, the alternator is totally dead, drive it a few minutes, tach suddenly comes to life. Occasionally it will fluctuate a few times then work fine for quite a while. It does act like a bad wire or possibly a cracked trace on a PCB. Does the tach in these have a printed circuit board as part of it's internal workings. Trying to trace that wire is pretty difficult as it just disappears into the loom under the hood.
I plan to just get an Ultra Scan Guage and not worry about the tach.
Usually when I first start it up, the alternator is totally dead, drive it a few minutes, tach suddenly comes to life. Occasionally it will fluctuate a few times then work fine for quite a while. It does act like a bad wire or possibly a cracked trace on a PCB. Does the tach in these have a printed circuit board as part of it's internal workings. Trying to trace that wire is pretty difficult as it just disappears into the loom under the hood.
I plan to just get an Ultra Scan Guage and not worry about the tach.
#7
11-09-2011 | 08:57 PM
Super Moderator
Joined: Jan 2011
Posts: 16,322
Likes: 84
From: Savannah Georgia
If alternator dies, does that also change the RPM signal from the ODB2 port? I would think that while alternator dies and kills the tach, the scanner (Ultra Gauge as well) might be getting a signal from whatever the ECU uses for that (cam position sensor ?). Not having an Ultra Gauge (yet) can't test that, but if you pull off the little flying lead on the altenator the tach should die. Seems like if both ECU and tach were driven from same point, when alternator dies it would stop engine or throwe codes, etc. Agree that if they don't match it is bad tach.
#9
11-09-2011 | 09:23 PM
If alternator dies, does that also change the RPM signal from the ODB2 port? I would think that while alternator dies and kills the tach, the scanner (Ultra Gauge as well) might be getting a signal from whatever the ECU uses for that (cam position sensor ?). Not having an Ultra Gauge (yet) can't test that, but if you pull off the little flying lead on the altenator the tach should die. Seems like if both ECU and tach were driven from same point, when alternator dies it would stop engine or throwe codes, etc. Agree that if they don't match it is bad tach.
I guess I need to remove my belt and see what happens.
One thing though, the GEMS uses a generic default program to estimate info if one system fails so the engine will still run, it might run like crap, but it will run.