Serious Megajolt Wiring Issues
Turbo Mini Forums

Hi

I wired in a seperate fusebox, and have used this to take feeds for things like gauge wiring, electric fan, fuel pump and megajolt etc. The earth for the megajolt is on the body, aswell as the EDIS earth on the same point.

The car has not been running on the MJ yet, but testing a few electrics today, the MJ decided to melt itself when i put the ignition on :(. I cant understand why it melted. It was the 16 pin chip inside the unit (Version 4, MAP), closest to the map sensor that melted. The unit runs through a 10 amp fuse, which was still ok. I've had the ignition on before and it has been fine upto know. The only thing different since then is I've wired up the crank sensor to the EDIS.

Does anyone have any ideas as to why it melted like this please? If so, any help would be appreciated.

Thanks, James

i only run a 5amp fuse and its never blown so id swap that in the future.

the only thing you can do is double check all your wiring again.

13.95 1/4 mile on a 2.95 fd. Carnt be that hard to beat!

must of got some pins/wires mixed up on the connector :(

Edited by danrock101 on 7th Aug, 2011.

I had the molex connector made to order with the correct length wires etc, double checked it all again and cant fault it. The only thing I can think is stupidly when testing other electrical things, a 30A fuse ended up in the megajolt slot. This was in when it melted. Then swapping back to 10A the unit still powers up and starts to glow, but the fuse is still ok? Could it be my stupidity with the wrong fuse that caused it to melt?

Thanks for the replies.

The wrong fuse wont cause it to melt, it just wont protect your megajolt from stupidity
If all the wiring is good then get onto autosport labs

A photo of what exactly has happened would help.

Assuming it wasn't faulty in the first place one of the common causes, especially if you have been working on things nearby and the cover has been off, is something small and metallic having got in there un-noticed and shorted out a couple of terminals or PCB tracks. Small screw, washer, even just a strand of cable from where you have been cutting/stripping it to do the wiring.

As above, the wrong fuse won't have caused it but it allows things to get worse quicker because the fuse won't blow.

Schrödinger's cat - so which one am I ???

Fuses really protect the supply, not the faulty item.

If you measure the resistance between the supply and earth it should be a few k Ohms at least, if there is a problem it will be 0 Ohms.

You need to trace the short/low-resistance path. By disconnecting things.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

the only thing it can be if you know and can prove your wiring was fine is a faulty mj.

was it a brand new unit?

13.95 1/4 mile on a 2.95 fd. Carnt be that hard to beat!

Thank you all for the replies.
I've double and triple checked the wiring and still cant fault it.

I thought that about the fuse, but mentioned it just incase!!

Very good point about swarf etc getting inside the casing. I hadn't had it apart untill after it melted, but i did have it hanging down by the vac pipe when i drilled a couple of holes to mount it inside the car. Its a possibility something could have got inside then?

I cant get a pic unfortunately, as I sent it off to someone who used to build MJ units a couple of years ago, who thinks he might be able to ressurect it using some spare parts he has left. If someone happens to have a pic of their MJ pcb i could easily explain where has melted?

As said, its sent off at the minute, so unable to do resistance checks.

I bought it used but new if that makes sence. The guy who had it bought it new, but never got it all installed.

Thanks again.

Also forgot to mention, i will be considering buying a new MJ unit in the next few days, especially if mine is beyond repair. So if anyone has one kicking around, please drop me a PM. Thanks.

Spot on Amp, I've had an email back from Martin. I'll copy and paste exactly what was wrong when I get on the laptop, but to cut a long story short it was built wrong, and I'm told it would never have worked properly since it was built!

Right, here is an explination from Martin of exactly what was wrong with the MJLJ:

"{One of the diodes (D1 at the top of the board by the regulator) was the wrong way round and it caused most of the power supply to go directly to the 2 main chips - the processor and the chip that deals with communicating with the computer - without being reduced to 5 volts first . Both chips are dead, one more obviously than the other.

They may have both died even before your first 7.5A fuse blew - but the processor chip obviously died with an internal near-short circuit and putting a 30A fuse in just meant that the near-short circuit got hotter and hotter, without the fuse blowing, until it blew the chip carrier apart! It's certainly a house of horrors story inside.

It was obviously never even tested - or if it was tested it was guaranteed to have blown up straight away, so maybe it was dead when you got it. }"

I had a call from Martin tonight, and his last tests show the map sensor has also blown, so that is being replaced too. His services are second to none. He received the MJ at 9am this morning, he's found the cause, and the faulty parts, fitted new parts, tested it all and it is ready to be sent back in the morning. Brilliant.

Just thought I'd update this incase anyone comes accross similar problems

Thanks everyone for your help.