MJLJ tach out died

Yvan · Post by **Yvan** » Fri Feb 08, 2008 8:30 pm

I have installed and successfully used for a couple of weeks my MJLJ

(and I will post photos when I finish zinc coating and painting all the metal parts).

I also decided to keep the original ignition system until I finish ignition map for use with LPG.

In attempt to record ignition curve of my original ignition system I managed to blow MJLJ tach out.

I connected tach out to the rpm gauge in my dash (original one - BMW E30). It worked as it should. But when I disconnected +12V to MJLJ and used the original ignition, tach_out from MJLJ was left connected to the tach, as was tach out from the BMW ignition system.

I than disconnected the BMW ignition system, reconnected MJLJ, and it works, but tach shows 0 rpm - needle is not moving.

Looking here

http://www.autosportlabs.net/asl_dist/m ... v3_sch.png

it seems that tach_out is connected to the 2N2222.

Perhaps it blew out? I can get 2N2222A at a local electronics part store. Can I use that one? Or have I done bigger damage?

Post by **brentp** » Sat Feb 09, 2008 9:41 pm

Interesting one.

First thing to check is if the MJLJ is operating correctly other than the TACH_OUT. Does it communicate with the software? Can you verify it's adjusting ignition advance when you check it with a timing light?

When you had it connected that way you were feeding 300-500v spikes into the collector of the transistor. If your TACH_OUT died, but the MJLJ is othewise working OK, then it could very well be the transistor. Try replacing it and see if that works for you.

Please let us know what you find out! I'm curious.

Regards,

Yvan · Post by **Yvan** » Sun Feb 10, 2008 8:26 am

I do not have a laptop, I have to borrow one from my friend, so I can not check right now if it does communicate with the software. Also few days before this happened my timing light died. I have another, old timing light, without the pickup (the one that connects in-line with the HT lead), and it shows that ignition is changing - but I can only verify that when I fix my newer timing light (or get another one).

But 300-500V spikes?

I measured tach out voltage before I connected to my tach, it was IIRC ~25V AC. And roughly the same was the tach out from the original ignition system (measured at diagnostic connector at the engine bay).

I went to the local electronic shop, they do not have it in the shop, but they promised to get it for me by the end of the next week. I hope it is only the transistor, not something I can not replace locally...

Post by **brentp** » Sun Feb 10, 2008 9:24 am

Is the BMW's original tach wire connected to the (-) terminal of the BMW's ignition coil? If so, this terminal sees a high voltage spike every time the coil discharges to fire the spark plugs.

How did you disconnect power- did you just remove +12v, but still grounded, or did you disconnect MJLJ +12V power and ground?

Yvan · Post by **Yvan** » Sun Feb 10, 2008 2:16 pm

Yes, tach is connected to the (-) terminal of the BMW's ignition coil, but not directly. Wire goes from the ignition coil (-) terminal, to the service indicator electronic module inside instrument cluster (and there is a bunch of IC-s inside - I looked). So if there is a high voltage spike it should toast that module too. Unless it has some sort of "spike filter" in it...

And I just disconnected +12V, not the ground.

Post by **brentp** » Tue Feb 12, 2008 9:05 am

Hi Yvan,

If the BMW electronics has a connection to the (-) side of the distributor then it was designed to accept this type of signal.

Please, let us know what you find out from your testing.

Yvan · Post by **Yvan** » Tue Feb 12, 2008 12:14 pm

I've just done some testing, and good news is that MJLJ can communicate with software.

I did not repair my timing light yet, but I tried to change advance from 0 to 45 deg, and nothing changed.

RPM stayed at 850 with only slight fluctuation (but it also fluctuates light that when I do not adjust advance).

Looking at the MJLJ schematics. it looks that tach out is connected to the SAW port of the 68HC908KX8 (via 2N2222), and the EDIS module. So it might be only 2N2222 or I might have fried 68HC908KX8 or EDIS module, or any of the combination.

Any Thoughts until I repair timing light or get 2N2222?

Post by **brentp** » Tue Feb 12, 2008 5:41 pm

Good that it's communicating with the software. it shows the processor is at least 'alive'.

So to be clear, the RPMs do indeed change in the software when you rev the engine? This means the processor is receiving the PIP signal.

On the SAW line, try to measure voltage when the engine is running.

If you don't measure anything, try removing the 2n2222 and test advance again. If the transistor was damaged it could be pulling the signal up or down. However, there is a 1K resistor between the transistor and processor- not sure what level of protection that offers. I did not test injecting the MJLJ with high voltage so we are exploring new territory here.

The other consideration is the high voltage was also being fed to the power supply section. Since the unit is still alive it would seem that it wasn't damaged.

Yvan · Post by **Yvan** » Tue Feb 12, 2008 7:04 pm

I just found 2N2222 (actually 2N2222A, does it make any difference?), pulled old one out, fitted new one in, and now rev counter works again.

But I returned laptop back to my friend, so I can not test right now if changing advance makes any difference.

Is it OK to use the old timing gun without inductive pickup? I have to ask everything now, besides blowing this 2N2222 I also managed about a month ago to blow BMW original ignition coil by leaving it connected while I was testing EDIS setup

So can i just connect timing gun like this: ignition coil HT terminal for 1-st cyl. -> timing gun -> HT lead -> spark plug and not damage anything?

Post by **brentp** » Tue Feb 12, 2008 9:00 pm

That's good news. If your rev counter is working again then that means the processor is emitting the SAW pulse. You should be able to observe your ignition timing changing with the timing light.

I have only used the clamp style inductive pickup timing light, so I cannot comment on the other types.

Good job fixing your unit!

Yvan · Post by **Yvan** » Wed Feb 13, 2008 3:25 pm

Final update.

I can now confirm that my MJLJ works as it should, and perhaps it worked before I changed 2N2222. I checked with a laptop earlier today, and it seemed that again nothing changed when I change advance to 0 deg. But than I remembered that I have to click the Write Ignition Configuration icon. RPM dropped immediately. So, back to the tuning.