Rev Counter Fluctuating
Discussion
Hi and good morning folks!
I know there are some very knowledgeable chaps on this section.
On my MK1 Scirocco (1.6 KJet injected).
Rev counter fluctuates wildly, not always, but often, normally between change up between 3,4,5 gear.
I’ve ignored it for years, but it’s becoming one of those little niggles I’d like to address.
Any ideas?
Thanks Jim.
I know there are some very knowledgeable chaps on this section.
On my MK1 Scirocco (1.6 KJet injected).
Rev counter fluctuates wildly, not always, but often, normally between change up between 3,4,5 gear.
I’ve ignored it for years, but it’s becoming one of those little niggles I’d like to address.
Any ideas?
Thanks Jim.
I am not 100% sure, but with the age of the car I would be at least 1/2 expecting a mechanical rev counter.
That being the case if you remove the instrument binacle the rev counter will have a bowden cable like the speedo and there will likely be one end of it not fully tightened up, or possibly the square end of the inner cable worn.
A simple enough fix, but likely rather fiddly to get at.
Either that or if it is electronic, very closely inspect the rev counter area of the instruments for a possibly dodgy connection.
That being the case if you remove the instrument binacle the rev counter will have a bowden cable like the speedo and there will likely be one end of it not fully tightened up, or possibly the square end of the inner cable worn.
A simple enough fix, but likely rather fiddly to get at.
Either that or if it is electronic, very closely inspect the rev counter area of the instruments for a possibly dodgy connection.
Rev counters can be driven either from the ECU or from the ignition system. It sounds like yours is driven from the ignition system. This used to be universal but got less common as ECUs and instruments got more sophisticated.
With this type of system the rev counter is connected to a part of the ignition system which produces a big voltage spike each time the ignition fires. This is a very noisy signal so the rev counter has some filtering in it to try to exclude the noise while leaving the main spike. If the signal is stronger than the rev counter expects, it may still pick up some of this noise as extra spikes causing a high reading. If the signal is weaker than the rev counter expects it may exclude some of the valid spikes causing a low reading. Just components in the ignition system that are out of spec might make the difference.
Since the problem has been happening for a long time it's unlikely it's caused by components in the ignition system and more likely it's just the ignition system not giving a clean enough signal. This problem comes up from time to time with aftermarket engine management systems. There are a few techniques you can try to clean up the system. The simplest one is to pass the signal through a small inductor. The relay circuit of an ordinary relay often does the job ie terminals 85/86 of a standard automative relay. You can also try connecting a condensor between coil negative and ground to smooth out some of the noise. A condensor is a small capacitor that was connected across the points of an old fashioned mechanically switched ignition system. They aren't used on modern cars but you can still buy them.
With this type of system the rev counter is connected to a part of the ignition system which produces a big voltage spike each time the ignition fires. This is a very noisy signal so the rev counter has some filtering in it to try to exclude the noise while leaving the main spike. If the signal is stronger than the rev counter expects, it may still pick up some of this noise as extra spikes causing a high reading. If the signal is weaker than the rev counter expects it may exclude some of the valid spikes causing a low reading. Just components in the ignition system that are out of spec might make the difference.
Since the problem has been happening for a long time it's unlikely it's caused by components in the ignition system and more likely it's just the ignition system not giving a clean enough signal. This problem comes up from time to time with aftermarket engine management systems. There are a few techniques you can try to clean up the system. The simplest one is to pass the signal through a small inductor. The relay circuit of an ordinary relay often does the job ie terminals 85/86 of a standard automative relay. You can also try connecting a condensor between coil negative and ground to smooth out some of the noise. A condensor is a small capacitor that was connected across the points of an old fashioned mechanically switched ignition system. They aren't used on modern cars but you can still buy them.
GreenV8S said:
E-bmw said:
the rev counter will have a bowden cable like the speedo
Gosh, I've never seen those.Isn't the signal taken from the coil negative on that age of VW? Been a long time since I messed with one of such age so might be wrong but I'm 99% sure it's not mechanically driven.
The Road Crew said:
GreenV8S said:
E-bmw said:
the rev counter will have a bowden cable like the speedo
Gosh, I've never seen those.Isn't the signal taken from the coil negative on that age of VW? Been a long time since I messed with one of such age so might be wrong but I'm 99% sure it's not mechanically driven.
GreenV8S said:
E-bmw said:
the rev counter will have a bowden cable like the speedo
Gosh, I've never seen those.https://montysclassicmotorcyclesshop.co.uk/clocks-...
Often as an unsightly sticky-out lump on the timing cover with the cable going up to the head unit on top of the forks.
Jim H said:
Rev counter fluctuates wildly, not always, but often, normally between change up between 3,4,5 gear.
Sounds like clutch slip to me.The classic test for clutch slip is to drive fairly slowly in 3rd gear, floor the throttle and see what happens. This applies max' engine torque to the clutch at a time when the car is hard to move. Like a cyclist standing on the pedals. Any clutch weakness and it will slip.
GreenV8S said:
Panamax said:
Sounds like clutch slip to me.
I don't think the OP is suggesting the engine revs are fluctuating, just that the gauge is misreading.It’s an electronic type counter, not off a Bowden cable.
Panamax said:
It's not obvious why, if this is an electrical glitch, it occurs during gear shifts and not all the time. Something doesn't add up.
It might only happen within specific rpm bands and/or throttle (load) positions.I've seen that sort of thing with analog rev counters where they will behave reasonably well part of the time and just not quite cope under different conditions.Have sent you an email
Take a close look at Red/Black from coil to fuel pump relay to shift light control unit to tachometer
Early VW instrument clusters do fail, there could be a pcb fault or tacho fault, wiring could also be a problem, water/corrosion in the fusebox could be interfering with the tacho signal
Anyway, you could disconnect the shift light control unit and leave it out of the circuit, drive the car to check if anything has altered
Disconnecting the Red/Black cable from the coil, tacho and fuel pump, running a new cable and testing may also help prove what's failing
The fact that the engine runs fine when the tacho fails tells us that the amplifier to coil negative pulse that also drives the tacho is good and a problem somewhere along the Red/Black from coil to tacho would often cause the engine to miss a beat but not always
Take a close look at Red/Black from coil to fuel pump relay to shift light control unit to tachometer
Early VW instrument clusters do fail, there could be a pcb fault or tacho fault, wiring could also be a problem, water/corrosion in the fusebox could be interfering with the tacho signal
Anyway, you could disconnect the shift light control unit and leave it out of the circuit, drive the car to check if anything has altered
Disconnecting the Red/Black cable from the coil, tacho and fuel pump, running a new cable and testing may also help prove what's failing
The fact that the engine runs fine when the tacho fails tells us that the amplifier to coil negative pulse that also drives the tacho is good and a problem somewhere along the Red/Black from coil to tacho would often cause the engine to miss a beat but not always
Edited by Polly Grigora on Thursday 21st July 07:44
From ex Auto sparks....now retired ...with the Bosch "K & KE" systems a Fuel pump relay is used, the coil negative pulse re - rev counter is used also at pump relay...in case this is interfering with rev counter [ posible ]...you can do this test...unplug the relay..in order to start now the test...bridge out 2 terminals in the relay plug..pins 30 & 87...this will enable engine to start...has the fault gone..all ok...then new Pump relay needed, remember to unplug the bridge relay after switching off engine...again pin 87 & 30 to bridge...DONT get he numbers wrong. Peter
Morning Peter, and thank you!
As an interesting aside, I’ve always had problems with fuel pump relays with this vehicle in my 28 years of ownership.
In short it eats them, I must have gone through more than 10 - and now they are no longer available.
I’m currently working on a work around but it’s been on the back-burner. I was unaware for many years that VW did a bit of a recall and re-routed the fuel pump relay off the fuse board as it always used to burn out one of the plugs/sockets.
My relay is now routed off the board, but it burned out recently (mind it hadn’t consumed one in a number of years). It was only when I came to replace it I found out they are NLA. I had to buy a second hand one.
Perhaps the relay issue and Rev counter are related?
As an interesting aside, I’ve always had problems with fuel pump relays with this vehicle in my 28 years of ownership.
In short it eats them, I must have gone through more than 10 - and now they are no longer available.
I’m currently working on a work around but it’s been on the back-burner. I was unaware for many years that VW did a bit of a recall and re-routed the fuel pump relay off the fuse board as it always used to burn out one of the plugs/sockets.
My relay is now routed off the board, but it burned out recently (mind it hadn’t consumed one in a number of years). It was only when I came to replace it I found out they are NLA. I had to buy a second hand one.
Perhaps the relay issue and Rev counter are related?
Hi Jim....yes back in the day have seen fuse boxes damaged etc etc...The fuel pump they use as you know with the "K " system runs all the time 85/90psi, it takes 9 amps .... so make sure yours has not increased in Amps due to wear...they do wear and amps go up...this burns fuses / fuse box terminals / wiring..your need a amp meter to check and done at the pump.......60k miles they get noisy etc...what i used to do for customers is to fit the pump mod [ Best ]...high rate relay in boot to take all the loads...this relay now suppled by the original relay...now ony seeing "point 4" of an amp....30 pin of the new relay in boot haveing a new 17/20amp cable run directly to battery via a 30amp fuse...if you get stuck getting original pump relay...you can use the Ford RS Turbo relay ..numbers on terminals may differ...i would need to know your terminal numbers ....then i can tell you how to wire the Ford one...but the Mod ive told you is the very best thing to do...to end all heat on wiring / relays /fuses / terminals. Peter
Thank you again Peter,
The pump is 10 years old, but the car has only covered about 3-4 thousand miles in that time - it only ever goes out in nice weather.
I will check the amps though. It’s had a new filter in that time too.
Yes I’m going for a 2 relay set up, I’ve bought them just not got round to it.
PIN numbers on relay are: 30/87/15/31/31B.
The pump is 10 years old, but the car has only covered about 3-4 thousand miles in that time - it only ever goes out in nice weather.
I will check the amps though. It’s had a new filter in that time too.
Yes I’m going for a 2 relay set up, I’ve bought them just not got round to it.
PIN numbers on relay are: 30/87/15/31/31B.
Those numbers i reconise.....but i dont see the " 1 " ....which would be on the relay......looking into this more with all my books i see there is no coil pulse used...they use 31b to get a signal from the metering unit flap....so Ford relay cant be used....as it looks now your system cant interfear with your rev- counter...sorry its many years now....im 79...but as soon as i saw 31b it clicked my memory......got all my books out. Just to say yes you could fit a Ford relay...i would advise...but we getting away from your original fault...31b is the safety circuit in the event of an accident. Peter
Edited by Peter-jforf on Thursday 28th July 17:14
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