megajolt woes. RPM does not register

[Nick B.]

Hi,

I am having yet another go at my megajolt install. It is all wire up. I can load the map on the pc from the module. When I crank the engive the vacum registers from the manifold. However no action on the rpm gauge, despite the green diode flashing on the megajolt module.

I am running a 'dry' install, where it is all wired in, bit the sparkplugs are not connected, and the old distributor is still in place.

Should I not be getting any readings when the green diode is flashing? Should I just give up and get a 123 distributor?

 

Best wishes

Nick

 

[Nick Jones]

Hi Nick, great to hear from you. 

Not so great that you are having problems….. and don’t give up and get a 123 - this is better once it’s working.

So…. When you say it doesn’t register on the rpm gauge, is this on the computer screen or on the cars tachometer?

Have you checked to see if the ECU is actually switching the coil earths? 
Presumably you can still run the engine using the the original distributor - if you do this, and select the timing map in the software, can you see the ECU moving around the map.

In fact, if the EDIS part is working you should have spark anyway with timing fixed at 10deg BTDC….. have you got that?

[Nick B.]

Hello Nick

Thanks for helping. It is very frustrating. 

I did not actually start the car, it is in a closed garage, so I just cranked it until battery went flat.  The smith mechanical rpm gauge of the old dizzy dit fluctuate as i cranked the engine, and I would suspect at least some movement on the megajolt screen. There was absolutely no action on the megajolt screen apart  from the load gauge. And this surprises me as the green led on the megajolt box was flashing when I cranked the engine.

I suspect the connection between megajolt and computer is good, as the MAP sensor is showing vacum fluctuatin on the load gauge.

I have set the ignition advance to 12.

But my theory was, that I could get a reading, and work with that with the original ignition in place, and when I have confirmed all works, I could switch.

And if by swithching coil earths you mean the four green output dots in the lower right screen, they are all inactive. No lights.

Best wishes

Nick

 

Edited December 20, 2025 by Nick B.

[Nick Jones]

Ok….. problem for me is I never worked with MJL itself, only MS. However, my MS installs use EDIS for the crank position sensing and ignition in the same way. 

You should be able get the engine to run even without the MJL connected. The EDiS just defaults to 10deg BTDC “limp home” if it doesn’t get a timing signal. 
In your position I would be going to “analogue testing” - that is checking whether you get a reliable spark when cranking. Don’t forget it’s wasted spark so the companion spark plug needs to be in place when you do this. 

If you are getting a spark but the engine isn’t firing at all then either you have the plug leads on in the wrong order (I have this T- shirt!) or the primary channels switched, which has the same effect (I this this T-shirt too).

Alternatively you have got the relationship between trigger wheel and sensor wrong.

If no spark at all then possible problems include

-trigger wheel to sensor gap (0.5 to 2mm iirc and it needs to be roughly the same all the way around)

- VR sensor polarity

- power and earths on the coil pack and EDIS. 
- Do you have the coil pack capacitor installed? Some say it doesn’t matter…. I found it does.

I suggest concentrating on making it run in limp home mode first. I reckon once you get that rest will follow easily.

May the force be with you!

[mpbarrett]

not quite sure of your setup but I would check that you are getting a good signal from the timing wheel. \\you can do this with the MS setup not sure about the Megajolt. 
I did have problems with the starter motor on my herald not turning the engine over fast enough to generate a good timing signal, the sensor signal amplitude depends on the speed of the teeth passing the sensor.

mike

[Nick B.]

10 hours ago, Nick Jones said:

Alternatively you have got the relationship between trigger wheel and sensor wrong.

This one might be the culpruit. Thing is - embarrasingly - it is appr. 14 years ago I put the triggerwheel on the crankshaft, and I cant remember the logic, whether I used the 12 oclock top dead venter as suggested sensorplacement. However, as life turned out, sensor was laced at appr. 8 oclock. I figured, I could adjust this in the setting after setting up.  I better get a new timinggun to confirm what is up and down. That one also got lost in moves.

 

I will hack on!

 

Best wishes 

Nick

[Nick Jones]

Do you know for sure if you are getting a spark? In my experience, if you are, even if the timing is miles out you get some popping and banging, sometimes at a scary level 

 

[Nick B.]

Hi, and thanks for the input.  I got slightly sidetracked by familyobligations

On 12/21/2025 at 9:16 AM, mpbarrett said:

not quite sure of your setup but I would check that you are getting a good signal from the timing wheel.

The setup is a spitfire mk3. I am running on a somewhat statndard dizzy setup. The megajolt unit and all bits and bobs are wired in, and should in theory be good to start the car. However, I would like to get some kind of confirmation that all sensors were well before I changed from dizzy to megajolt, hence I tried to get a good reading from spinning the startermotor.

On 12/21/2025 at 10:41 AM, Nick Jones said:

Do you know for sure if you are getting a spark? In my experience, if you are, even if the timing is miles out you get some popping and banging, sometimes at a scary level 

I have not tested for spark - I did not pass the first test - see the signal from the crankshaftsensor. 

On 12/21/2025 at 9:16 AM, mpbarrett said:

I did have problems with the starter motor on my herald not turning the engine over fast enough to generate a good timing signal, the sensor signal amplitude depends on the speed of the teeth passing the sensor

Aaaah, I hope this might explain the absent signal. When I get back from christmass I will change over abandoning the dizzy setup and check for spark and hopefully have the spark and sensorsignal.

Fingers crossed. I have 8 months to get the car soreted prior to the 10CR.

Hope you all have a nive holiday.

Best wishes

Nick

[Nick Jones]

The timing of the wheel won’t affect getting an rpm signal showing but would affect the ability to run. 
Sensor to wheel gap and sensor polarity could affect getting an rpm signal. 

[ahebron]

I have an Isuzu starter on my Vitesse 6 which I intend to MJ at some time after I get the car back on the road.
When I first spun it up on the new starter I thought I had lost compression as it is that fast compared to the 60 year old Lucas one.
I even pulled the rocker cover to make sure all valves were opening and closing as they should.

 

Adrian

[egret]

It's been a while since I moved away from the megajolt, but I'm inclined to agree with Nicks approach.  Set the system up so it is capable of generating spark and check this before getting too worried.

Other suggestions are:  check sensor to trigger wheel gap to give best chance of generating a strong signal.  Then check that the sensor wire is kept away from the coilpack and plug leads, and that the shielding is connected up nicely.  This will give you the best chance of the signal having minimal interference noise.

Other musings are that I managed to get my electronic rev counter (smiths mk4 spitfire) to work directly, however it took a few seconds and often a blip to over 2.5k rpm for it to start working.  It still does that with the new ECU setup so I think this is probably an issue with the rev counter or other wiring rather than the ignition system signal.

[RedRooster]

I ssem to remember having to shim the sensor closer to the trigger wheel when I did the Sprite. 

[Nick Jones]

On 1/2/2026 at 12:21 PM, egret said:

Other musings are that I managed to get my electronic rev counter (smiths mk4 spitfire) to work directly, however it took a few seconds and often a blip to over 2.5k rpm for it to start working.  It still does that with the new ECU setup so I think this is probably an issue with the rev counter or other wiring rather than the ignition system signal.

These things (Smiths RVC tachos) are very variable and a law into themselves. I had to make a “coil emulator” using a relay coil to produce a sufficiently spicy output to reliably trigger the 6 pot one in the Vitesse. The RVI version is a bit easier. Just connect the coil pack supply through it and it works.

Should probably add that the EDIS6 does not have a dedicated tacho-out pin which I believe the EDIS4 does have. So I use the so-called diagnostic-out which is possibly even lower voltage than the dedicated tacho out. Possibly the MJL box has a tacho out function as well?

[RedRooster]

That reminds me, I changed the type of tacho as well in the end.