Where to get a decent tacho?
Discussion
I'm getting annoyed with the tacho in my Griff. It hasn't worked properly for a long time. This didn't matter 3 years ago when the car was standard (slow!) and it was only used on roads, but now it's become a couple of problems.
1. The RPM reading is erratic. It overshoots on acceleration, oscillates on gear changes and drops to zero on overrun before slowly settling to the correct reading.
2. The tacho isn't happy with running off an ECU. I have tried a converter box and while the tacho worked, it wasn't good - still no damping to speak of. I tried making my own converter box hoping to damp things down with some electronic trickery, but it didn't work all that well and all I ended up with was a sluggish tacho, but at least it didn't bounce and mess around.
3. Neither electrical solution has been all that reliable. The tacho spends as much time broken as the odometer spends stopped. (But that's another story)
So, where does one get a decent accurate tacho that match the other TVR instruments? (I like the look of them, and the car has been changed enough!) Changing the faces is not beyond my skill set if the mechanisms are compatible.
I haven't looked at a Cerb one - how bouncy are they? Or does it depend on the day they were made??
ETA: that the tacho was bad when driven by the standard means too. It's not merely an input conversion problem.
1. The RPM reading is erratic. It overshoots on acceleration, oscillates on gear changes and drops to zero on overrun before slowly settling to the correct reading.
2. The tacho isn't happy with running off an ECU. I have tried a converter box and while the tacho worked, it wasn't good - still no damping to speak of. I tried making my own converter box hoping to damp things down with some electronic trickery, but it didn't work all that well and all I ended up with was a sluggish tacho, but at least it didn't bounce and mess around.
3. Neither electrical solution has been all that reliable. The tacho spends as much time broken as the odometer spends stopped. (But that's another story)
So, where does one get a decent accurate tacho that match the other TVR instruments? (I like the look of them, and the car has been changed enough!) Changing the faces is not beyond my skill set if the mechanisms are compatible.
I haven't looked at a Cerb one - how bouncy are they? Or does it depend on the day they were made??
ETA: that the tacho was bad when driven by the standard means too. It's not merely an input conversion problem.
Edited by dnb on Monday 24th August 13:41
Yes, it was originally a coil driven tacho. First of all, I tried one of those "coil emulator" black boxes (I think they contain an inductor, a couple of resistors and a diode...) so that I could drive it from a +12v to 0v square wave output from the ECU.
In despair, I took the whole thing apart and made a circuit to drive the tacho coil directly using a frequency to voltage converter chip and various active filters. Obviously this took its input directly from the ECU.
In despair, I took the whole thing apart and made a circuit to drive the tacho coil directly using a frequency to voltage converter chip and various active filters. Obviously this took its input directly from the ECU.
You're using coil packs, I assume.
My MS does have a tacho output, and it came with advice to drive a relay from this to generate the EMF spike that the tacho needs (the relay is just a convenient source for a small coil, the switched output isn't used). I guess this is similar to your box of tricks.
In the end I didn't use this at all. I simply connected the tacho up to the coil packs with diodes to stop the four coils cross-feeding each other via the tacho circuit, and a pull-down resistor to clean the signal up slightly. It's not perfect, tends to wander around at the very top of the rev range but not enough of a problem for me to bother fixing it.
My MS does have a tacho output, and it came with advice to drive a relay from this to generate the EMF spike that the tacho needs (the relay is just a convenient source for a small coil, the switched output isn't used). I guess this is similar to your box of tricks.
In the end I didn't use this at all. I simply connected the tacho up to the coil packs with diodes to stop the four coils cross-feeding each other via the tacho circuit, and a pull-down resistor to clean the signal up slightly. It's not perfect, tends to wander around at the very top of the rev range but not enough of a problem for me to bother fixing it.
My box of tricks is the same as the MS idea, except mine used an inductor instead of the coil of a relay.
If it were simply a case of changing the drive, it would be easy. But the tacho really wasn't right even when it was driven by the original coil. The conversion from high voltage to low voltage merely makes an annoying problem into one I can't live with.
I therefore doubt that speedycables would help - they could easily convert the input but I would be left with the original "bouncy tacho" problem, which seems to be a common TVR tacho problem.
I am tempted after looking on Farnell to make myself a stepper motor driven tacho, but I would really like to avoid the work of doing this! (I have other projects that need to be completed)
If it were simply a case of changing the drive, it would be easy. But the tacho really wasn't right even when it was driven by the original coil. The conversion from high voltage to low voltage merely makes an annoying problem into one I can't live with.
I therefore doubt that speedycables would help - they could easily convert the input but I would be left with the original "bouncy tacho" problem, which seems to be a common TVR tacho problem.
I am tempted after looking on Farnell to make myself a stepper motor driven tacho, but I would really like to avoid the work of doing this! (I have other projects that need to be completed)
I would assume you've probably tried this but just in case:
My tacho and speedo both wobbled all over the place and would not even settle at constant speed. I 'cured' the problem when having to remove the speedo after the internal battery went flat. The contacts were a little corroded. Not much, just a bit. I cleaned them and stuck some electrical contact cream on them and they are really steady now.
Don't mean to patronise.
My tacho and speedo both wobbled all over the place and would not even settle at constant speed. I 'cured' the problem when having to remove the speedo after the internal battery went flat. The contacts were a little corroded. Not much, just a bit. I cleaned them and stuck some electrical contact cream on them and they are really steady now.
Don't mean to patronise.
spitfire4v8 said:
anything that causes a high resistance in the power/earth will affect its performance
Is that in the 12V/earth you're talking about, or the signal? Just wondering whether a dodgy earth might be causing my high rpm tacho funnies. Instrument power supplies generally seem to be all over the place on mine at the moment so I guess part of the original loom behind the dash is playing up. (I've replaced just about everything else.)
the bouncy tacho is far more reliable than the charge warning lamp .. typical tvr-ness 
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