Rev counter problems

gelmonkey · Post by **gelmonkey** » Thu Jul 16, 2009 9:17 pm

Hi all
I have a problem with my rev counter not working with my MSD ignition system.
The tacho is for a four banger engine but with a divide by two gizmo put in line it should read correct with the 8 pot.
Many of the other V8 Mantas have used exactly the same gizmo and it has worked perfectly.
I have power to the back of the tacho(5.7 volts) and it did read correctly for about 5 seconds and then nothing!
All of my connections are sound and in the right place and now Im completely baffled.
Can anyone shed any light on this for me as after a twelve year build(long story)all I want to do is get out and drive the tyres of it but I want everything working properly after so much work and this is a royal pain now.
Thanks in advance
Paul

ChrisJC · Post by **ChrisJC** » Fri Jul 17, 2009 7:21 am

If you remove the gizmo, does it work (but read 2x)?

Chris.

gelmonkey · Post by **gelmonkey** » Sat Jul 18, 2009 9:45 am

Hi Chris
Yes that is exactly what happens.
The original tacho on all the old Mantas and Cavaliers run a little erratic at times.
I have heard that by changing a small component in the back of the tacho it can be made to read correctly and without fluctuation but I do not know how to do this myself.
If I can get htis sorted then I do not need to run the gizmo in line.
Do you know of a way that I can test the unit with it out and on my work bench before any in line components are added?
I ask this because if the tacho is iffy in the first place I'm on a hiding to nothing before I've started.I have tried another unit in the car and had the same original results but that does'nt mean to say that both units might be at fault.Any help with this would be good.
Thank you for coming back to me
Cheers
Paul

ChrisJC · Post by **ChrisJC** » Sat Jul 18, 2009 12:45 pm

Testing tachos:
The easiest way if you have no electronic test kit is to wire up a battery, tacho, distributor and coil. Then stick a power drill on the distributor and see if the tacho moves.
This is very crude, but because it is using all factory components, it's pretty foolproof.
Moving on a bit, one can substitute the coil / distributor with a signal generator. These produce (amongst other things) a square wave of a known frequency (that you can adjust). With these you can check that the calibration is somewhere near.
Then you are in a position to tinker with the electronics in the back, and adjust the 4 cyl to an 8 cyl version 'natively', i.e. without gizmo.
I am an electronics engineer, so this is exactly what I did with the tacho in my van. It used to be a Montego Turbo rev counter, now it's a V8 rev counter!
Now there are all sorts of unknowns with mismatched components in an ignition system. Some rev counters work by current sensing, some work by picking up the flyback pulse (so the signal generator technique doesn't work), some work by measuring 12V square wave. If you are mixing / matching stuff like that, then there's all sorts of scope for issues!
Now I don't know diddly about any of the kit you've got, so I can't think of anything more helpful to add to be honest!!
If it were me, I would take it to work and fiddle with it on the test bench until I'd worked out how it all worked.
Maybe you know a tame electronics bloke?

Chris.

gelmonkey · Post by **gelmonkey** » Sat Jul 18, 2009 1:14 pm

Hi Chris
Thanks for coming back to me.I do know a tame electronics engineer and think that he may be able to help with this.As you have seen with my car it's the detail that matters as much as anything else so not having the tacho working is a no no.
I have looked at Auto Meter Tachos on the web which can be fitted in-dash and work straight off the MSD but they are not cheap and will not match the rest of the instruments.
Ah the joys of mixing and matching eh.
Thank you again for your time.
Will keep you updated on the progress if you are interested?
Cheers
Paul

ChrisJC · Post by **ChrisJC** » Sat Jul 18, 2009 3:08 pm

Yes, definitely I'd like to know the outcome.

Chris.

Post by **katanaman** » Sat Jul 18, 2009 6:31 pm

A cheap constant signal source for calibrating a tacho from is a door bell transformer. They put out 50hz since they aren't rectified and you can fiddle until you have half your original reading in your case.

gelmonkey · Post by **gelmonkey** » Sun Jul 19, 2009 11:11 am

Hi Guys
As a Fibreglass laminator and grp repairman I struggle with the more complex side of electrics.Clearly not my field of expertise!
Whilst you guys might think that this is Really basic stuff it confuses a poor old soul like me.After spending so long building the car I thought that once the MSD acme back from repair it would be a simple case of re connecting a few wires and then the dash cuold go back in and then about ten hours work and I could be ready for the MOT.
I have e mailed my tame electrics bloke but he has'nt come back to me yet.
He may be on holiday or something.I will keep you posted on how things go down here and hopefully very soon I can be back on the road after twelve looooooonnnnnnggggggg years.
Thanks for your help
Regards
Paul

gelmonkey · Post by **gelmonkey** » Sun Jul 19, 2009 4:10 pm

Hi Guys
A mate of mine has let me try his 8 cyl tacho this afternoon and it works fine so it has to be the little gizmo that is causing the problem.
Anyone got any idea as to how much a tacho recalibration might cost?
Cheers
P

gelmonkey · Post by **gelmonkey** » Tue Jul 28, 2009 10:04 pm

Hi Guys
My electrics bloke came over this evening and checked everything out.
Gizmo knackered.
Suggests that I have tacho recalibrated for V8.Will find out prices tomorrow and then find suitable time to tell my wife how much.Oh, she will be pleased that I am going to spend even more money on the car.I bought her a bag of chips once so this should go in my favour!!!
Will keep you posted on how this all goes and if push comes to shove I can always set up a camp bed in the garage.
Not a bad idea.
Cheers
P

Muscle-Manta · Post by **Muscle-Manta** » Sat Aug 01, 2009 4:23 pm

Hi Paul still enjoying yourself trying to sort your tac? Did you get a price for a recaliberation?

Paul H