Bizarre Rev Counter Behaviour
Discussion
Anyone any ideas on this????????
My rev counter is behaving oddly. I couldn't quite establish what it was until just now. When you put your foot right down, the rev counter reading stays exactly where it was until you lift off to part throttle again. This equates to a wholly pointless rev counter! When you drive "enthusiastically", you don't know how hard you're revving the engine until you A) hit the limiter or B) Feel like you're screaming its nuts off.
Has anyone had experience of this fault? Do I have a wiring problem or could the rev counter be F
d?
As always, any help gratefully received.
My rev counter is behaving oddly. I couldn't quite establish what it was until just now. When you put your foot right down, the rev counter reading stays exactly where it was until you lift off to part throttle again. This equates to a wholly pointless rev counter! When you drive "enthusiastically", you don't know how hard you're revving the engine until you A) hit the limiter or B) Feel like you're screaming its nuts off.
Has anyone had experience of this fault? Do I have a wiring problem or could the rev counter be F
d? As always, any help gratefully received.
Nick P said:
have you changed the HT leads recently?
Unlikely to be the problem as it sounds like the car's revving properly, the tachometer's just not showing it.
Sounds more like the lead that connects the rev counter. I'm sure someone will be along shortly who knows how to clean/repair/replace...
Nuggs said:
Nick P said:
have you changed the HT leads recently?
Unlikely to be the problem as it sounds like the car's revving properly, the tachometer's just not showing it.
Sounds more like the lead that connects the rev counter. I'm sure someone will be along shortly who knows how to clean/repair/replace...
no.no.no....i asked if he's had new leads as it's a known fault that magnecors can affect the tacho and make it erratic.....mine does it...or did until i re-routed them
Nick P said:
Nuggs said:
Nick P said:
have you changed the HT leads recently?
Unlikely to be the problem as it sounds like the car's revving properly, the tachometer's just not showing it.
Sounds more like the lead that connects the rev counter. I'm sure someone will be along shortly who knows how to clean/repair/replace...
no.no.no....i asked if he's had new leads as it's a known fault that magnecors can affect the tacho and make it erratic.....mine does it...or did until i re-routed them
Ahhhh - I see. As you were then
me said:
Unlikely to be the problem as it sounds like the car's revving properly
that daddy said:
faulty leads can affect the tacho magnacors or otherwise,this go,s for most cars with electronic triggered tach,s.just because the car appears to run fine,dont rule out faulty lead/or leads.of course may be a tacho fault of its own,good luck.
Nick P said:
...it's a known fault that magnecors can affect the tacho and make it erratic.....mine does it...or did until i re-routed them
miketvr said:
I've just fitted a new set of plug leads and it's resolved the erratic behaviour of the tacho that I was having.
I don't think I've ever been more wrong...
Nuggs, i wouldent worry,we dont know our suggestions about ht leads have cured the fault yet,it could also be a ignition module/faulty coil/wiring defect causing the tach,s weird behavoir even though it cant be felt,seems strange it plays up uder load,sounds ignition related to me,we are all like doctors with out the patient present.
Try resistive plugs if you have not got them already such as the NGK BPR6ES. Although it goes against the grain loosing some EHT energy in the plugs, its unlikley to realistically bother that lazy old V8 as long as the rest of the HT is in good nick, and it does reduce the level of spikes to the tacho. Otherwise it needs electronic skullduggery with chokes and capacitors to make a little filter to go inline with the tach feed. Sorry I can't give you values as Ive not had to make one for a TVR yet.
torquemada said:
Nick P - I have Magnecors and erratic rev counter. How did you reroute to correct this?
I just tried various different routes away from the coil until i found one that worked....i think i'm correct in stating that the tacho takes its feed from the coil? hence the HT leads near coil upsetting the tacho readings.
under the piping on the rocker cover seems to work.
The technical bit: When the coil dicharges its magnetic field collapses and cuts the primary windings, and this inturn produces a high voltage in the primary windings, typically a 300-600 volt spike on the 12 volt rail. Its not a single spike, but a series of pulses getting smaller as they go as magnetic fields oscillate in the coil core. This is called ringing, and all magnetic coils do it. With low resistance HT leads the alternating spikes are a much higher voltage as the coil discarges very quickly, although each one a is shorter duration, it can be mistaken by the rev counter for additional coil pulses from the ECU, so you may get sudden high readings, or none at all as the rev counter electronics trys to sort what is a real pulse and what is not. One reason it happens at different throttle openings is the voltage needed to fire the mixture goes up as you put in more fuel and air, so the point the spark occurs may go from 15kv to say 20kv in the secondary before the coil discharges through the plug, and a 20kv discharge makes for bigger spikes in the primary, hence further confusion for the poor old tach.
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