Rev Counter

[Guest bloke]

i have instruments from an 87 efi

 

from memory the rev counter wasnt working! any ideas why?

 

[Guest timswait]

If you dismantle the instrument binacle you will find 3 connections to the rev counter, an earth a +12V, and a conection which pulses each time the ignition fires. The earth and the power supply are easy to check with a multimeter (just make sure the ignition's on). If there's no fault there then try the third connection. On the EFi this comes straight from the management loom, but to test it try connecting it to the low tension side of the coil. If it doesn't work now then give up and get a new one, they're not much from the scrappy anyway.

From my experience with an E Reg EFi the most likely cause is that some monkey has fitted an alarm badly, on mine there was a very confusing array of wires under the bonnet which stopped certain electrics (although not the rev counter) from working.

Tim

[Guest bloke]

cheers timswait that sounds good but woooooooooh! i think ill need to learn a bit about the electrics to do this it sounds a bit complicated but ill let you know the outcome

[Guest Alasdair]

(My first post)

 

Did you get anywhere with this problem?

 

I bought a 2B recently and the rev counter doesn't work (never has). It has had 2 engines and been tried with points and electronic ignition. I even wired it directly to the coil and still no joy. I have a spare instument cluster which doesn't work either so I suspect both of them may be faulty.

 

I wil get a tester on it shortly but meanwhile, has anyone else experienced this as well?

 

Thanks

 

Alasdair

[Guest Fred2b]

On my 2B the rev counter works intermittantly, always has, one day it works the next it does not.

I believe the green wire is the one that receives the pulse from the distributor so I have tried connecting that direct but no go. There must be a problem inside the cluster so a replacement is probably needed. Yours sounds like it may have the same problem.

Peter

[Peter Bell]

Have you tried connecting it to the other side of the coil? I had a similar problem recently and by accident connected it to the other side and it works fine perhaps it got a cleaner signal.

Peter

[Guest Alasdair]

This can't be right (casts mind back to elecrical theory).

 

One side of the coil is at 12v since it is connected to the battery via the ignition switch. The other side goes to the points the other side of which is connected to ground. The first side of the coil will therefore always be at 12v, the other side will be at earth (when the points are closed) or will rise to 12v (via the primary winding of the coil) when the points are open. That is the side that must be connected to the rev counter or it can never work at all.

 

Connecting the rev counter to the other side of the coil will do nothing since it is always at 12v.

 

Mind you, I could be wrong

 

Alasdair

[Guest Bernie]

Alasdair,

 

Don't forget that some systems are fitted with a balast resistor which is in circuit with the coil positive for normal running but shorted out during starting. With a balast resistor in circuit there would be some signal at the positive terminal which might be sufficient to drive some tachos. It also follows that with the resistor in place the signal on the switched side of the coil will not attain the full 12 volt switch from ground for any appreciable time during a contact cycle.

[Guest Alasdair]

Ah yes, forgot about that. I think they ran at about 9v or something during normal running and got a full 12v blast (or whatever the battery could deliver) during starting. As you say there would then be some signal on the +ve terminal but probably varying only by about 3v.

 

I think this also had something to do with the reason you were not supposed to leave the ignition switched on for long periods of time without starting the engine but I can't think why now.

[IanS]

Sierras were never fitted with balast resistors. You only should have a balast with points ignition.

With points type ignition you could cook the coil if the ignition was left on the points were closed and the engine stopped.

 

You will get some signal from both ends of the coil because the current rise time is fairly fast when the plug sparks. For the technical there is inductance due to the length of the wires from the battery throught the ignition switch to the coil so the high frequency part of the signal will get through from the battery side of the coil, but there will be a bigger signal for the tacho from the switching side of the coil.

[Guest Alasdair]

With points type ignition you could cook the coil if the ignition was left on the points were closed and the engine stopped.

 

Ah yes, I knew there was a reason for it.

 

You will get some signal from both ends of the coil because the current rise time is fairly fast when the plug sparks.

 

Agreed (it's all coming back to me now). I think I must have two bust rev counters.

[Guest Daz]

If you do a voltage check onthe tacho signal line you will probably find (I did) that it sits at 12V normally. I was looking for a spare 12V feed for something once upon a time and thought I could take a feed from the light green wire - that was until I dug in the wiring diagram to find out what the light green wire was!

 

If you cannot get a signal direct from the coil, then I would suspect the insrument. Don't they have internal regulators to steady the input voltage for all the instruments?

 

Daz

[Peter Bell]

Sorry I didn't want to cause problems, the reason I put my posting on was that my tacho had worked for about 5 years without problem, it then stopped and I assumed that instrument had failed but when I tried another it still didn’t work. That was when I reconnected it on the ‘proper ‘ side of the coil and it worked fine. I suppose something in the system has degraded causing a poorer signal. Btw. I have a ballast resistor and a hall effect dizzy!

Peter.