Rev counter won't go past 4500rpm
Discussion
I am running my 400SE on an Emerald K6 ECU with wasted spark coil packs. Since the rev counter normally gets its feed from the positive coil lead, I kept the same wiring and just switched the positive from the old coil to the new coil packs, splitting it to feed both coil packs. The rev counter works as expected until I reach 4500rpm when it drops to zero although the engine continues to run as normal and accelerate. When the revs. drop back below 4500 the rev counter jumps back into life. Any ideas what might cause the rev counter to stop above 4500 rpm?
P.S. the rev limiter is set to 5700, so well above 4500rpm.
P.S. the rev limiter is set to 5700, so well above 4500rpm.
I'm surprised you got it working at all off the positive .. you normally run the tacho signal from the negative side. If doing that on wasted spark you need diodes in the wires to isolate each wire you take off. You'd normally do that off all 4 signal wires, diode each wire on its return to the tacho signal input on the ecu.
by far the easiest way of doing this job is to buy the tacho module from emerald, it just works!
by far the easiest way of doing this job is to buy the tacho module from emerald, it just works!
Hi Jules, thanks for the quick reply. I have a wiring diagram for the 400SE which shows the rev counter on the +ve coil lead. So far all the electrics on my car that I have looked at appear to agree with this diagram.
I thought that I would have a problem with the rev counter when I was thinking of fitting coil packs so I contacted Karl at Emerald. This was his response:
"Hi Leslie,
No, the ECU cannot drive that tacho type and the tacho driver module won't work either.
To keep the RVI tacho (current loop pickup) working just wire the coil +12v supply through the tachometer current loop, i.e. +12v to tachometer, through current loop and to the coil positive. It does not matter that you have coil packs - there are the same number of pulses per cycle so the tachometer reading will still be correct."
So I appear to have an odd rev counter (Stewart Warner I think) which runs from the +ve coil and should work ok. But I have this problem when I hit 4500rpm. Very odd.
I thought that I would have a problem with the rev counter when I was thinking of fitting coil packs so I contacted Karl at Emerald. This was his response:
"Hi Leslie,
No, the ECU cannot drive that tacho type and the tacho driver module won't work either.
To keep the RVI tacho (current loop pickup) working just wire the coil +12v supply through the tachometer current loop, i.e. +12v to tachometer, through current loop and to the coil positive. It does not matter that you have coil packs - there are the same number of pulses per cycle so the tachometer reading will still be correct."
So I appear to have an odd rev counter (Stewart Warner I think) which runs from the +ve coil and should work ok. But I have this problem when I hit 4500rpm. Very odd.
I checked my wiring and the rev counter is definitely running from the 12v feed to the coil. I've also checked the wiring diagram for the 350i and they are wired to the -ve coil connection.
I think the reason is that the 400SE has a different dashboard layout. The speedo and rev counter are in their own section and therefore could be different to the ones used in the 350i.
My instruments are stewart warner. I suspect TVR have bought in old stock for the 400SE style of dashboard and wired them accordingly. It's the only thing that makes any sense.
That still doesn't solve why my rev counter stops working at 4500 rpm. I think it needs replacing. I noticed an ad in this months Sprint for clocks4classics which might solve the issue.
I think the reason is that the 400SE has a different dashboard layout. The speedo and rev counter are in their own section and therefore could be different to the ones used in the 350i.
My instruments are stewart warner. I suspect TVR have bought in old stock for the 400SE style of dashboard and wired them accordingly. It's the only thing that makes any sense.
That still doesn't solve why my rev counter stops working at 4500 rpm. I think it needs replacing. I noticed an ad in this months Sprint for clocks4classics which might solve the issue.
I'm guessing here, but if the tacho works by detecting the current drawn by the coil positive, rather than looking at the trigger pulses on the negative which is more usual, it could be that it avoids the high voltage back-emf present on the negative.
Stewart-Warner made aircraft instruments as well, so maybe it's even something to do with their design... as I say, guesswork. Modern electronics make it easy to sample the negative pulses (as in the VDOs on a lot of wedges).
Stewart-Warner made aircraft instruments as well, so maybe it's even something to do with their design... as I say, guesswork. Modern electronics make it easy to sample the negative pulses (as in the VDOs on a lot of wedges).
Jolly good 
Hope you're managing to get out and drive it, I think the last time I had mine out was last year when I was walking a leg of the Cleveland Way, parked it at Robin Hood's Bay and then got the bus back at the end of the stage.
Haven't taxed it yet this year, seems a bit pointless with the weather as it is - the MOT is out next month so barring an Indian summer it'll probably just stay mothballed for another winter.
Had two trips away on the bike in the last few weeks; a week in Belgium that was a bit moist and then 4 days in Scotland that were positively damp
Hope you're managing to get out and drive it, I think the last time I had mine out was last year when I was walking a leg of the Cleveland Way, parked it at Robin Hood's Bay and then got the bus back at the end of the stage.
Haven't taxed it yet this year, seems a bit pointless with the weather as it is - the MOT is out next month so barring an Indian summer it'll probably just stay mothballed for another winter.
Had two trips away on the bike in the last few weeks; a week in Belgium that was a bit moist and then 4 days in Scotland that were positively damp
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