4,500 Tacho Issues

[Roady]

Hi Everyone,

 

I have a very unusual issue which I am hoping someone out there with more knowledge, patience and whatever else will be able to help with.  I will keep it as brief as I can.....  I cannot find any information on the site regarding this particular problem

 

Background

240z (280zx engine N42 head with F54 block)

Electronic dizzy (280zx)

GT40R coil

280zx alternator (with the adapter plug to prevent reverse current)

Tacho is the original 240z (i.e. 4 wire, positive loop set up)

 

Now I know many people say that the electronic dizzy with this tach set up will not work but it has worked for the last 5 years that I have had the car....  And I have wired it as per the diagrams on this site and it has worked perfectly thus far.

 

The Problem

Currently the tacho will only rev to 4,500rpm and it will not go past this point.  The engine however is accelerating well past this point.  (A recent problem as in only the last month)

 

Trouble Shooting

I have sent the tacho away to be inspected with no fault found, I have also installed a spare tacho which has displayed the same problem. In discussions with the guy who tested the tacho he is confident it is something on the ignition circuit (something causing high resistance as the current is increased).

 

The distributor has also been pulled out and tested - no fault found

 

The ignition coil has been swapped

 

I have tried disconnecting everything that is attached to the ignition circuit (headlights, all other gauges, thermo fans, fuel pump) but with no joy.  The headlights, electric fuel pump etc are additions I have linked off the ignition circuit during a re-wire which thus far have caused no issues

 

I have also “hot wired” the ignition, cutting out the immobiliser from the starting circuit, ignition circuit and also the main power wire which connects into ignition switch

 

In a stroke of genius I thought I would also swap the wiring in the back of the tacho to see if that would make any difference but still nothing.

 

Removed the diode from the adapter plug (replacing it with a 10A wire)... all this did was prevent the car from turning off, hence the purpose of the diode

 

Also removed all fuses except the igntion fuse

 

All of the above yielded no improvement at all

 

When I re-wired the car (over a year ago) the only things I changed were:

 

Removed the original ballast resistor (which was not actually connected to anything, due to the upgraded 280zx distributor, it was simply bolted to the car body and the 2 wires from it were connected to each other)...  Which I have since thrown out

 

Removed the capacitor / condenser or (not required as I do not have points) - I have since tried re - installing it but with no luck

 

The key thing that I must stress is that the entire set up has worked correctly it then became an intermittent problem for a day or so, then a consistent problem after that. During that time no changes had been made. Logic says to me that something (a system, not the wiring) has failed due to the nature of this problem and how it developed....

 

The Auto Electrician I left it with for a day handed it back saying he had tested the battery and it was charging fine, beyond that he didn't have any suggestions..... It wouldn't have started if that was the issue.

 

Any ideas however ridiculous they might sound are most welcome as I am at my wits end.  I am thinking of possibly the ignition switch itself but I am simply clutching at straws

 

Thank you ALL

 

Roady

[Linton]

you have probably done what I would try, only extra check I would do is put an external tacho on the coil see if you get the same result. I had different issues with my z I found the coil was wired up back to front, as in Pos to ground, it ran with resister fitted but not removed It took me a while to find that one.

good luck

[jamo240]

Hey Roady

 

You've certainly tried a lot of reasonable things to fix the problem. One thing i did notice is you are running a GT40R coil...why the R model? This is intended to be run with a ballast resistor, and as you don't need one now because of the electronic distributor, you would be over-saturating the R model at the full B+ supply...perhaps this is contributing to the problem. While I accept the problem didn't used to be there and has develped, have you got a conventional coil you could try? You may have had some form of degradation occur in the coil over time that is causing the tach to fall over at 4,500 RPM that wasn't there when the coil was new.

 

Good luck

 

Jamo

 

 

[Roady]

Linton, good idea mate....  I had been mulling that over as well.  I will hook up another tacho (externally) at the same time as the one in the car and see if it does the same or something different.  Thanks for the tip.

 

Jamo - we must have thought the same thing at the same time.  One thing I did notice when I did the rewire was that the car started much easier than previously.  So perhaps the old tired wiring was restricting the amount of current and even the voltage to the old coil hence no damage to it.  But then when it was refreshed it was getting full power and slowly wearing it away.  The coil I replaced it with was just one I had lying around which was coated in red metal but with no other markings on it.  I've ordered a higher performance coil which does not require a ballast resistor and also a ballast resistor to install on the car body only and see if that makes any difference.

 

I'll also test the voltage at the (+)ve side of the coil.  Is there an easy way to test the current on the ignition line or at the tachometer?  I tried disconnecting the "loop" wire (green/white and the black/white) and connecting them with a multimeter but I could not get the car to start to then actually test it.

[jamo240]

Hey Roady

 

The green/white wire actually feeds the coil through the ballast resistor into the black/white wire. When the key is in the run position, the power comes through the green/white wire, and hence loses a bit of voltage through the ballast such that the voltage on the black/white is dropper to ~8V for normal running. WHen the key is in the crank position, there is still battery voltage applied to the green/white, but the ignition switch also supplie battery voltage directly to the black/white wire, hence the coil is at battery voltage while cranking to aid starting.

 

With the ballast resistor no longer required (provided you are using a coil intended for full battery voltage all the time, such as a GT40, NOT GT40R, and you are using the 280ZX elec distributor that doesn't have points to wear out), then you can simply connect the green/white to the black/white where the ballast used to be, and the coil will be at battery voltage all the time....happy days.

 

Don't get too confused by the tach operation...it is simply a part of the coil primary circuit...it doesn't matter how you fire the coil (ie points, elec dizzy or crank triggered)...provided the primary side of the coil is wired into the tach as designed and the coil is working properly then the tach should work.

 

Cheers

 

Jamo

[Roady]

Hey Jamo,

 

Yeah that's exactly how I have got the set up wired.  In essence, normal driving I don't need to reach the higher power band of the car anyway. The reason it is frustrating is that the whole build has been completing it as a track car. With the triple webers and upgraded cam now all installed and I'm finally happy with the tuning of them and now I can't really utilize the main power band safely. I can guess (reasonably accurately as I am familiar with the car after 4 years of slowly building) but even so.... If I can solve this issue I will try and install a 280z tacho (3 wire), working off the negative side of the coil. I dont suppose you know how the wiring is actually connected for that set up? I have looked on the site but can't see an actual description. I believe the ignition wire still goes to the positive on the tach and the signal now comes off the neg side of the coil (which is now just 1 wire instead of the loop) with the final wire being earth??

[jamo240]

Hey Roady

 

I had a look in my 280ZX workshop manual, and it shows the positive supply to the coil also being wired in parallel with the supply to the IC pickup on the distributor, as well as a supply to a 'thermo switch' (not sure what it does, nor whether you still have it in your system). Otherwise, it all looks pretty conventional...so I would just wire to the positive side of the coil as discussed. If it's any consolation, I had crank triggered ignition on my L28, and used a mercedes ignition coil, and that drove the tach just fine.

 

CHeers

 

 

Jamo

[Roady]

Thanks for your help Jamo I will keep it all in mind. As I said I'm running out of options and ideas so if I can't track down why it stopped working I'll just have to change the set up entirely.... Stay tuned I'll report back here as I get any progress!

 

Thanks for you help it is much appreciated

 

[jamo240]

You're welcome ROady...good luck.

 

Jamo

[Roady]

Jamo and Linton - again thanks for all your suggestions relating to this tacho issue.  After swapping in numerous new parts (including a 12V coil that specifically stated that unless I was using points I did not need a ballast resistor), re-installing a new ballast resistor...... still no joy.  After installing the new coil instead of 4500rpm it would now go to 5000rpm.  I also tried hooking up another tacho in a different spot but it displayed the same thing.

 

 

Long story short I had run out of time and patience so I simply swapped the internals out of a 280z tacho into my 240z tacho housing.  This has since solved the problem.  Only tips I can add for other users doing this swap is:

 

The positive and negative posts in the tacho are reversed between the 240z and 280z tacho

 

Don't forget to install a 2.2K resistor between the (-)ve coil and the sensor post on the tacho itself (I believe this is already installed in the 280z.)

 

Finally, go slow when removing the internals from the housing.  One simply fell out when I removed the backing bolts, the other however was a bit sticky.  Gently started prying it out and part of the mounting plate snapped......  Nothing that a little glue couldn't fix but just another pain in the butt thing you then have to go and fix!  I have seen suggestions to use a hair dryer to loosen the double sided tape that is in there, which is a good idea but I can tell you now I barely used any effort and this thing broke so just be careful!

 

Enoy and thanks for all the suggestions

 

Roady

[jamo240]

Good to hear you've got it sorted out Roady, but a shame we never figured out exactly what was at fault. Never mind, it's working now so happy days.

 

Jamo

[Roady]

Exactly Jamo,

 

I don't like being beaten by the old girl but we got there in the end. I suspect one day that I the answer will reveal itself but in the mean time at least I have it working.

 

Roady