Tachometer Knowledge?
Discussion
Background: As a man with little mechanical experience whose main attribute is 'keen' rather than any measurable skill-base I like to book my S2 into a garage once each year so a pro can initially service and then cast an eye over any fettling I've been up to over the previous few months. It was the day before I was booked in for a full service at Central TVR in December that I had an unfortunate breakdown requiring an AA callout. The AA man was able to bodge a 'get me home' fix for the ignition problem that had prevented me from restarting the engine. The bodge involved running a fused wire straight from the battery to the ignition coil (the problem was diagnosed as a break somewhere in the loom between the fuse box and the coil itself). This allowed me to start and drive home safely (with the AA man in convoy as backup) but had the side-effect of 'blowing' the Revcounter/Tachometer.
Solution: The bodged repair allowed me to drive to Central TVR the next day where it was left for a week and underwent a full service and had an auto-electrician crawl over it for 1.5 days. The auto electrician is brought in as and when required for jobs so he wasn't there when I collected the car. Regardless I was happy because the faulty looming had been sorted/replaced, a number of faulty dash lights made good and the car is now running better than ever. The guys at Central gave me the auto electrician's phone no incase I wanted to fit a serviceable Tacho in the future as he had by-passed whatever makes the ignition circuit rely on a Tacho.
Christmas came and went and I was able to buy a NOS TVR Tacho from the bay at a good price. I've been trying to get in touch with the auto electrician but have had no joy despite several calls/texts. This is where I come to you guys with cap in hand....
On the left (silver bezel) you will see my broken Tacho with white wire removed from the female plastic connector (presumably removed and taped off by auto electrician at Central when faced with a broken Tacho?). On the right is the NOS replacement tacho (slightly later edition - black centre of needle and lines instead of chevrons in red-zone). The wires for this are bare and ready to be crimped for the female plug.
The second photo shows the profile of each. You can see that the new one is considerably longer in depth but has the same colour and number of wires emanating from the rear. You will also see a sticker wiring schematic on the old one which implies the, now disconnected, white wire is meant to take a feed from the ignition coil.
Note - my S2 now works with the Tacho completely disconnected. This is how I received it from Central and there is no longer the AA man's live feed direct from the battery. I obv have no RPM but my perception from noise alone is that the battery charging circuit (green light) now only turns off at a much higher RPM than it used to when original Tacho was connected and working.
Questions:
1) Despite difference in size (not a worry with space at that point of dash), should the new Tacho work if wired correctly?
2) Is your white wire in the 'empty' space within plastic female connector?
3) I do not have a picture now (and won't until weekend) but I think an additional cable now connects to my ignition coil. Is this likely to bypass the requirement to have a Tacho in the circuit for engine to run?
4) If I connect the new Tacho without addressing such a bypass in no. 3 am I risking blowing my new Tacho?
5) Any opinions on whether my 'delayed' charging circuit may be caused by having no Tacho in the circuit?
Sorry for the usual mass of info - just wanted you to have the full picture! As always, I look forward to any replies.
David
Solution: The bodged repair allowed me to drive to Central TVR the next day where it was left for a week and underwent a full service and had an auto-electrician crawl over it for 1.5 days. The auto electrician is brought in as and when required for jobs so he wasn't there when I collected the car. Regardless I was happy because the faulty looming had been sorted/replaced, a number of faulty dash lights made good and the car is now running better than ever. The guys at Central gave me the auto electrician's phone no incase I wanted to fit a serviceable Tacho in the future as he had by-passed whatever makes the ignition circuit rely on a Tacho.
Christmas came and went and I was able to buy a NOS TVR Tacho from the bay at a good price. I've been trying to get in touch with the auto electrician but have had no joy despite several calls/texts. This is where I come to you guys with cap in hand....
On the left (silver bezel) you will see my broken Tacho with white wire removed from the female plastic connector (presumably removed and taped off by auto electrician at Central when faced with a broken Tacho?). On the right is the NOS replacement tacho (slightly later edition - black centre of needle and lines instead of chevrons in red-zone). The wires for this are bare and ready to be crimped for the female plug.
The second photo shows the profile of each. You can see that the new one is considerably longer in depth but has the same colour and number of wires emanating from the rear. You will also see a sticker wiring schematic on the old one which implies the, now disconnected, white wire is meant to take a feed from the ignition coil.
Note - my S2 now works with the Tacho completely disconnected. This is how I received it from Central and there is no longer the AA man's live feed direct from the battery. I obv have no RPM but my perception from noise alone is that the battery charging circuit (green light) now only turns off at a much higher RPM than it used to when original Tacho was connected and working.
Questions:
1) Despite difference in size (not a worry with space at that point of dash), should the new Tacho work if wired correctly?
2) Is your white wire in the 'empty' space within plastic female connector?
3) I do not have a picture now (and won't until weekend) but I think an additional cable now connects to my ignition coil. Is this likely to bypass the requirement to have a Tacho in the circuit for engine to run?
4) If I connect the new Tacho without addressing such a bypass in no. 3 am I risking blowing my new Tacho?
5) Any opinions on whether my 'delayed' charging circuit may be caused by having no Tacho in the circuit?
Sorry for the usual mass of info - just wanted you to have the full picture! As always, I look forward to any replies.
David
David. My story is very similar to yours and perhaps we can learn from each other. I used to own a 400SE wedge which I very recently sold to go in search for a Tuscan. When I realised this search was taking long, I came across a 290 S2. I bought it as a rolling restoration thinking it would give me something to do while I search for a suitable Tuscan. I brought the car home and noticed that it had a Scorpian alarm/immobiliser fitted at some time, which had been disconnected years ago but with all the ancillaries still intact. I disconnected the battery to give it a charge as the previous owner didn't use the car much and told me there may be a charging problem. Once the battery was fully charged I connected it up the next morning and the car just would not start. The immobiliser had somehow activated and I had no way of switching it back on as the key and fobs to the alarm had been lost years ago. An electrician was recommended to me and I called him out. He spent hours sorting through masses of wire in order to find the wires the alarm had been cut into in order to rectify the problem. At one point I was beginning to have my doubts as to whether he would ever get this thing going. He was determined, and persisted, eventually the car fired up. He had to solder in wires to bypass the alarm system in order to get the car working again. After the work was completed we tested the car to see if anything was not working and we found that the Taco (rev counter) had stopped working and the ignition light would stay on. Otherwise everything else was working. The auto electrician patiently explained to me that the charging problem was not the alternator but another wiring fault in the system and the same applied for the rev counter. He explained that the way the alternator works is by receiving a live signal from the ignition to tell it to charge on start up and that somewhere there is a broken wire between the two. For now he showed me a trick where two wires can be shorted behind the alternator manually to get the alternator to start charging which I have to do every time I use the car until next week when its booked into his workshop where he will run a new wire to fix the problem. He explained that the same applies for the rev counter and that it is a wiring problem rather than a blown rev counter.
I will let you know what has happened when I get the car back from the electrician next week, but I am pretty confident that he will repair both as this guy seems to know his stuff well. All the best.
Tony. TCB.
I will let you know what has happened when I get the car back from the electrician next week, but I am pretty confident that he will repair both as this guy seems to know his stuff well. All the best.
Tony. TCB.
Barkychoc - I'm fairly confident my NOS tacho is for a V6 clicky. Also, I gather from this thread that it is probably from an S3 and that TVR may have had up to 3 different types in circulation over the years.... The one discussed by Norman turned out to be broken so I just wanted confirmation as to whether this NOS S3(?) one could be used in an S2?
Tony - I will be interested to see what your auto electrician says when he takes a proper look at your car. I'm not convinced it is an identical fault but it does sound like there are similarities which may crossover. For example I have a feeling the tacho governs the point at which the charging circuit is opened - mine does still open itself without a tacho or requiring any earthing tricks at the alternator, just at a higher sounding RPM than it used to.
Thanks to you both for your input so far - can anyone confirm that the white wire is definitely meant to go into the plastic plug on a 'normally' wired S?
David
Tony - I will be interested to see what your auto electrician says when he takes a proper look at your car. I'm not convinced it is an identical fault but it does sound like there are similarities which may crossover. For example I have a feeling the tacho governs the point at which the charging circuit is opened - mine does still open itself without a tacho or requiring any earthing tricks at the alternator, just at a higher sounding RPM than it used to.
Thanks to you both for your input so far - can anyone confirm that the white wire is definitely meant to go into the plastic plug on a 'normally' wired S?
David
David the tacho in these cars is a pretty simple device.
iirc they get their feed directly from the coil (cant check this on mine, as i dont have a coil on the 24v engine) so i was just wonderin g have you checked to see if its actually blown or has one of the wires come off/broken/shorted ?
wiring diagram here
http://www.tvrsseries.com/drawings/GeneralElecs290...
might be worth running a few fresh wires to see if that sorts it.
btw there dont seem to be any wires going from the tacho to tell the charging circuit anything - unless my read of the diagram is wrong (though electrics isnt my strongest point)
iirc they get their feed directly from the coil (cant check this on mine, as i dont have a coil on the 24v engine) so i was just wonderin g have you checked to see if its actually blown or has one of the wires come off/broken/shorted ?
wiring diagram here
http://www.tvrsseries.com/drawings/GeneralElecs290...
might be worth running a few fresh wires to see if that sorts it.
btw there dont seem to be any wires going from the tacho to tell the charging circuit anything - unless my read of the diagram is wrong (though electrics isnt my strongest point)
Edited by BountyHunter on Monday 4th February 21:02
BountyHunter said:
wiring diagram here
http://www.tvrsseries.com/drawings/GeneralElecs290...
Bear in mind that wiring diagram is for the early 2.8 cars with very different fuel injection and engine management systems.http://www.tvrsseries.com/drawings/GeneralElecs290...
You need this bit as well for the 2.9i engine....
supplement
phillpot said:
Bear in mind that wiring diagram is for the early 2.8 cars with very different fuel injection and engine management systems.
You need this bit as well for the 2.9i engine....
supplement
i still cant see the tacho controlling anythingYou need this bit as well for the 2.9i engine....
supplement
bottom line is its simply an instrument that reads current and then displays it via the needle, to me at least it doesnt seem to make sense for the charging circuit to be tacho controlled. (although i have been know to be wrong -
from time to time)from what i know there is usually a voltage regulator on the alternator itself - this is what tells the alternator whether to charge or not, the tacho plays no part.
edited
http://www.carparts.com/classroom/charging.htm
have a read here - its put quite well (well enough that even I with my basic electrical knowledge actually understood it)
Edited by BountyHunter on Monday 4th February 21:39
I agree and thanks for those two wiring diagrams. The way I read it the 3 wires from the tacho are Red:Live from ignition switch, Black:Ground, White:Ignition Coil. In addition there should only be 2 x wires from the ignition coil - the white one to the tacho and another to the distributor.
I'll have to see what the arrangement is from my ignition coil too - this may be where some kind of bypass has been added.
Regards,
David
I'll have to see what the arrangement is from my ignition coil too - this may be where some kind of bypass has been added.
Regards,
David
There are two types of tacho - current sensing and voltage sensing.
Current sensing tachos need to be wired in series with the switched ignition supply to the coil +ve. Any fault in the wiring would prevent power reaching the coil.
Voltage sensing tachos need to be wired to the coil -ve. Any fault in the wiring would have no effect on the coil, except for an internal failure which resulting in the tacho grounding the feed (this might also overheat and damage the coil).
The tacho in my V8S is voltage sensing. I don't know what sort was used in the S2.
The alternator also needs an ignition feed to energise the field coils. This is usually provided by the no-charge warning light (switched ignition supply at one side, the other side grounded via the exciter coils in the alternator). Once the alternator has started working it drives the coils itself so no current flows through the warning light, so it goes out. (This is also why it usually glows slightly when the electrical load is high and also glows a lot more after you switch the engine off as it spins down.)
Once there is enough magnetism to generate the current to power the coils it is self-powering but it needs some current to get it started. Sometimes the alternator will have enough residual magnetism to enable it to self-start if you rev it hard enough, so the revs needed to get it started depend on the exciter current and how much residual magnetism it has. My S2 used to need quite high revs (about 1700 rpm from memory) before the alternator would kick in, with the exciter circuit working. Maybe that's what you're seeing now. If it seems to need a lot more revs now, then the current through the exciter must be less than it was before. Is the no-charge warning light behaving as normal now?
Current sensing tachos need to be wired in series with the switched ignition supply to the coil +ve. Any fault in the wiring would prevent power reaching the coil.
Voltage sensing tachos need to be wired to the coil -ve. Any fault in the wiring would have no effect on the coil, except for an internal failure which resulting in the tacho grounding the feed (this might also overheat and damage the coil).
The tacho in my V8S is voltage sensing. I don't know what sort was used in the S2.
The alternator also needs an ignition feed to energise the field coils. This is usually provided by the no-charge warning light (switched ignition supply at one side, the other side grounded via the exciter coils in the alternator). Once the alternator has started working it drives the coils itself so no current flows through the warning light, so it goes out. (This is also why it usually glows slightly when the electrical load is high and also glows a lot more after you switch the engine off as it spins down.)
Once there is enough magnetism to generate the current to power the coils it is self-powering but it needs some current to get it started. Sometimes the alternator will have enough residual magnetism to enable it to self-start if you rev it hard enough, so the revs needed to get it started depend on the exciter current and how much residual magnetism it has. My S2 used to need quite high revs (about 1700 rpm from memory) before the alternator would kick in, with the exciter circuit working. Maybe that's what you're seeing now. If it seems to need a lot more revs now, then the current through the exciter must be less than it was before. Is the no-charge warning light behaving as normal now?
Edited by GreenV8S on Tuesday 5th February 01:36
Edited by GreenV8S on Tuesday 5th February 01:37
Morning Peter,
Thanks for that information, with regards to my charging light - No, it is not working now as it used to. Because the voltage indicated on the voltmeter rises when higher RPM is reached (now significantly higher than what I recall) I presumed it was simply the green bulb that needs checking. From what I read it seems the current through the exciter must have reduced and that high revs creates enough residual magnetism to get the alternator started..... does this sound correct? In which case is it likely I have developed a problem in the no-charge warning circuit between the alternator and switched ignition supply?
Regards,
David
Thanks for that information, with regards to my charging light - No, it is not working now as it used to. Because the voltage indicated on the voltmeter rises when higher RPM is reached (now significantly higher than what I recall) I presumed it was simply the green bulb that needs checking. From what I read it seems the current through the exciter must have reduced and that high revs creates enough residual magnetism to get the alternator started..... does this sound correct? In which case is it likely I have developed a problem in the no-charge warning circuit between the alternator and switched ignition supply?
Regards,
David
There must be some connection between the tacho and the alternator wiring because my electrician says the alternator is charging fine, but there is a break somewhere in the wiring between the ignition and the alternator, and the tacho has stopped working too. Its booked in with the electrician for this Saturday.
Tony. TCB.
Tony. TCB.
Thanks - that makes perfect sense. The car definitely runs with the white wire disconnected so I'll have a close look this weekend to determine where the bypass is. As I now know the tachometer is current sensing it obv needs to be connected to the white wire in order to receive the signal it goes on to display. I'll also confirm that the disconnected white wire carries 12V with the ignition on.
Assuming such a bypass exists, would it be foolish to connect the new tacho without removing it first?
Regards,
David
Assuming such a bypass exists, would it be foolish to connect the new tacho without removing it first?
Regards,
David
phillpot said:
BountyHunter said:
wiring diagram here
http://www.tvrsseries.com/drawings/GeneralElecs290...
Bear in mind that wiring diagram is for the early 2.8 cars with very different fuel injection and engine management systems.http://www.tvrsseries.com/drawings/GeneralElecs290...
You need this bit as well for the 2.9i engine....
supplement
Norman
An update for anyone who still has the will to read ;-)
I can confirm that my ignition coil +'ve terminal has an additional live feed to it from what should be standard. This has been put there as my ignition problem was that there was no live getting to the coil. The engine won't start with this 'new' wire unattached from the coil.
I have proved continuity from the standard yellow wire at the ignition coil to the white wire at the plastic connector for the tacho (agrees with Norman's wiring diagram I now have usefully laminated and in A3 format).
With my cheapie multimeter I can confirm that a current can be sensed at the ignition coil with the engine running and also at the connector for the tacho.
I proved that the red and black wires seem to provide the tacho 'power' as my replacement tacho needle re-set to zero when connected from the residual indication it picked up due to the vibration of postage.
Unfortunately this is where I think I made an error in my 'systematic' (read - illogical!) diagnosis. Enthused by the flicker of life at the new gauge I then connected the white wire...... no rev indication.
I say I think I made an error at the previous stage because I then decided to take my new tacho and connect it more simply/directly to see if it would work. This involved red wire to the +'ve terminal of the battery, black to ground/ -'ve and the white wire directly to the +'ve terminal of my ignition coil..... Surely this would work?? Unfortunately, no! I fear it may be because I tried the previous step first and as such may have 'blown' this tacho in the same fashion as the original.....
Is my logic correct though - would a serviceable tacho definitely indicate when connected as my final step above? (i.e. red and black to battery and white to ignition coil)
If so - I think I may have to go back to my guy on ebay and HOPE he has another NOS tacho available.
Thanks in advance for any words of wisdom,
David
I can confirm that my ignition coil +'ve terminal has an additional live feed to it from what should be standard. This has been put there as my ignition problem was that there was no live getting to the coil. The engine won't start with this 'new' wire unattached from the coil.
I have proved continuity from the standard yellow wire at the ignition coil to the white wire at the plastic connector for the tacho (agrees with Norman's wiring diagram I now have usefully laminated and in A3 format).
With my cheapie multimeter I can confirm that a current can be sensed at the ignition coil with the engine running and also at the connector for the tacho.
I proved that the red and black wires seem to provide the tacho 'power' as my replacement tacho needle re-set to zero when connected from the residual indication it picked up due to the vibration of postage.
Unfortunately this is where I think I made an error in my 'systematic' (read - illogical!) diagnosis. Enthused by the flicker of life at the new gauge I then connected the white wire...... no rev indication.
I say I think I made an error at the previous stage because I then decided to take my new tacho and connect it more simply/directly to see if it would work. This involved red wire to the +'ve terminal of the battery, black to ground/ -'ve and the white wire directly to the +'ve terminal of my ignition coil..... Surely this would work?? Unfortunately, no! I fear it may be because I tried the previous step first and as such may have 'blown' this tacho in the same fashion as the original.....
Is my logic correct though - would a serviceable tacho definitely indicate when connected as my final step above? (i.e. red and black to battery and white to ignition coil)
If so - I think I may have to go back to my guy on ebay and HOPE he has another NOS tacho available.
Thanks in advance for any words of wisdom,
David
junglieD said:
I say I think I made an error at the previous stage because I then decided to take my new tacho and connect it more simply/directly to see if it would work. This involved red wire to the +'ve terminal of the battery, black to ground/ -'ve and the white wire directly to the +'ve terminal of my ignition coil..... Surely this would work??
Yes that should have worked. Makes no difference whether the supply comes direct from the battery or via the ignition switch.When the tacho is connected to supply and ground do you see 12V on the white wire? I assume not.
It sounds like the current sensor resistance in the tacho is u/s.
Connecting as you say should not have damaged it.
Le TVR said:
The bypass definitely exists if the car runs with the white wire disconnected 
Shouldn't do any harm to wire up the tacho with the bypass in place, just that the tacho probably won't read anything.
Assuming white wire from tach is coil feed? Wouldn't doing this route 12v back into the tach output? I can see how this would kill the tach.Shouldn't do any harm to wire up the tacho with the bypass in place, just that the tacho probably won't read anything.
If this is an input though you may be ok?
May have missed something though.
D
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