New problm. RPM on chip

[TonyE]

Think this is a new problem. Have searched the other forums and cant find anything to help me.
<!--break-->
MJLJ works fine on the bench.

SAW signal is changing the advance without any problem.
The indicated RPM however is jumping around all over the place.
Made up the little hysteresis circuit but still no better.
Tried a seperate power supply from a different battery, and still no better.
I then disconnected the PIP signal and the problem is still there.

Now for the interesting bit, with a seperate power supply and only the +ve and -ve attached to the MJLJ (and P.C) I still have an erratic RPM (should have non at all) turn the engine off and the RPM goes to zero, start the car again and get erratic RPM, move the MJLJ away from the car and RPM goes to zero.

My conclusion is that the MJLJ is picking up a signal that the car has around it when the engine is running as whenever it gets near the car I start to get an erratic RPM signal.

Dont know how to get round this unless there is some kind of bag or box that the MJLJ can sit in to eliminate the interference.

HELP

[Todd Charles]

Strange. What kind of appliction do you have? Are you running solid core ignition wires? Charging system working OK? Any chance the vacume tubing is conducting some sort of interference?

Todd

[TonyE]

The vehicle is an MGB.
The H.T leads are carbon core.
The charging system is fine.
I have been proving the system with the vacuum tube still connected to the dizzy.
I removed the fuses one by one to eliminate other circuits, so the problem seems to definatly be with the ignition, or I guess it could be the fuel pump but that seems highly unlikely.
Next step is to put the dizzy back in use and totally disconnect the Edis etc and check the MJLJ with seperate power and take it from there.
One thing I did not do in the installation is put shielded wires from the Edis to the coil pack so that could be the problem, (hopefully).
Tony

[Todd Charles]

Not sure I have this correct. You are running the car on the stock ignition while testing the MJLJ. Is the "limp mode 10°" working properly? Do you have spark without the MJLJ connected? If so, are you grounding the coil leads during the testing of the MJLJ? If not, you could be creating a pretty large electro-magnetic field causing your problem. Grasping at straws here.

Todd

[brentp]

You might have a build problem. Are you saying the mjlj works fine on the bench- 100% fine: meaning proper runtime screen operation, and proper SAW signal generation? And when it's installed in the auto you're seeing this problem?

If you're convinced the MJLJ has been built correctly, I would check all of the ground connections, and also see how 'noisy' your electrical system is. i.e. put a scope on it to see how clean it is?

Brent

[hotrodfil]

Just a thought - has your MGB got a dynamo? I was about to ask the self same question as to whether anyone had any experiences with a Megajolt fitted to a vehicle running a dynamo.
Phil