Thread: What would cause the 150 Mega fuse to blow?

There's no sense replacing a 150A fuse with an 80A otherwise LR would have only fitted an 80A in the first place. If the fuse is on the alternator B+ cable then it has to be caused by the alternator or a short in the cable itself. I doubt that any supply from the battery would be protected by a 150A fuse.

Edit: Alternators provide spike protection for the vehicle which involves shutting itself down. Some alternators shut down by shorting their output back through the windings which will can drain more current than the alternator's rated output. A fuse on the alternator output cable should be a little higher than the alternator's rated output, eg 140A alternator then at least 150A fuse.

The mega fuse had to be ordered in so the 80a fuse was in place temporarily to get the D2 back home. Just trying to pin point what the cause could be. The fact it doesn't seem to blow on idle makes me think it could be alternator related - i.e. once revved.

If it drives with the fuse blown then that fuse is only protecting the alternator. Either the charge rate is higher than your 80 amp fuse (highly likely), the 150 amp fuse was faulty or your alternator has an intermittent internal short circuit.

I guess one of those shunts would be the best way to test the alternator?

As for the fuse, it could be that it was going/gone. I could just put one in tomorrow and hope all works as it should be. Just don't want to be stranded in the middle of nowhere due to a short. Also, it doesn't really drive with the fuse blown, as soon as it goes the lights come on the dash and it dies - basically the battery just drains.

I wouldn't drive it anywhere except to get it fixed. Check the battery temninals and earth point to make sure they are tight. If available, try another battery in case the battery has an intermittant internal break that would cause the alternator to momentarily output at very high voltage which in turn causes the alternator to shut down. If the alternator uses zener power diodes (D3's do) then a failing zener diode will short circuit if its trigger level has been compromised. Zener diodes are designed to short circuit once their trigger level has been reached which for an alternator is around 24-30V IIRC but if failing could be much lower. D3 alternators fail with dead shorts but D3s effectively don't have a fuse so destroy the battery.

Originally Posted by Graeme

I wouldn't drive it anywhere except to get it fixed. Check the battery temninals and earth point to make sure they are tight. If available, try another battery in case the battery has an intermittant internal break that would cause the alternator to momentarily output at very high voltage which in turn causes the alternator to shut down. If the alternator uses zener power diodes (D3's do) then a failing zener diode will short circuit if its trigger level has been compromised. Zener diodes are designed to short circuit once their trigger level has been reached which for an alternator is around 24-30V IIRC but if failing could be much lower. D3 alternators fail with dead shorts but D3s effectively don't have a fuse so destroy the battery.

I don't have a multimeter or spare battery with me so I recon my best bet is to get it to an auto electrician. I found a company in the UK who can send over a new regulator/brushes for £55 - postage. Need to figure out what the problem is though. Home - RepairkitsUK | Starter Motor Repair Kits | Don't Buy a New Start Motor - Repair It for anyone interested.

Was checking the connection to the alternator which sparked against the engine when I touched the rubber tubing with a screw driver. Wasn't my intention but I think the cable was a little worn. Have re-insulated it and also added new connections at the battery and alternator end.

The alternator is apparently running a little hotter than usual, but everything is working as it should for the moment. I think I'll go ahead and order a new regulator and brushes from the UK and keep them on hand in case it does go at some point. At least it has new bearings.

Hi,

I followed the link to the SHUNT.........

I understand the volt/amp meter part but what is the SHUNT for?

Is it like a FUSE?.

The reason I ask. Is I'm having Alternator issues..And just doing general research.

I have set up a multimeter on long cables into the cab so I can monitor battery state and alternator output.

I'm getting 12.35v to 12.75 volts in the battery before I hit the key.

Then 14.55 volts on cold start up.(Alternator generating)

Then I get a range of figures but generally it goes down point one of a volt every forty to 60 seconds. Until it gets to 12.18 volts then bounces around the 13.5v for the last few kms to work.


I travel 20 kms to work.

It all started with the M and S limp mode a few weeks ago on a trip in the country. An RACT visit to jump start us and we made it home.

The Auto Electrician from work was nice enough to check fuses etc found a broken green and white wire on the plug that goes into the alternator. He said that the alternator needed a signal/power to tell it to charge.

I fixed the wire with a short extension so it would'nt be so tight/short. I rang him and he said if it was charging 14 volts or more it is fine.

Now he was good enough not to charge for the two 20 mins sessions after work to find the fault so hesitant to ring him about this. Also he has said many times he does'nt like working on cars he only likes to work on commercial gear. And of course ribs me about owing a Disco

My question is.........

Could the alternator READ/Compute the charge needed for the battery and be regulating accordingly? Ie starts on 14.4V and then regulates down to 12.18v then bounces around the low 13 's.

Or should it ALWAYS be charging 14.4 to 13.6 Volts regrdless of the battery state of charge?

If it is the later then would it be brushes or diodes? or both etc..

Also COULD it be the battery?

Cheers people any suggestions will be appreciated.