Mitsubishi Delica Owners Club UK™

Andy W · Joined: 03 May 2015 Posts: 560 Location: Brighton

Hi,

an electrical problem for a change; the 100amp alternator fuse keeps blowing. I replaced it once in case it was a random thing but it blew again the next day.

I've replaced the alternator today as I had a spare 100 amp one and had been meaning to replace it for a while but already I can see the fuse discolouring and getting warm.

Where should I start to look for the problem, will the fuse only blow due to an issue between it and the alternator or could the issue be anywhere?

Also, as I've installed a larger alternator should I increase the size of the fuse?

Thanks, Andy.
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1994 LWB Super Exceed L400, Black & Silver

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Lewis · Posted: Sun Jul 16, 2023 8:37 Post subject:

Check for something shorting against an earth.

I had the same problem, with them blowing as soon as I put them in, then turned on the ignition. It turned out to be a bolt that had dropped down the back of the dash and lodged against the steering column.

HTH
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Andy W · Joined: 03 May 2015 Posts: 560 Location: Brighton

Thanks Lewis, this is a slow blow taking over 10 minutes as that's how long I had the engine running yesterday before I saw that the fuse plastic was turning brown with the heat, I stopped the engine before the fuse blew though.
What is the best way to search for something like this problem?
Cheers, Andy
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1994 LWB Super Exceed L400, Black & Silver

Lewis · Posted: Mon Jul 17, 2023 9:25 Post subject:

I reckon the best way is to use an autoelectrician.

I am not one, so it took me a few hours, a multimeter and several fuses to work out what had caused my problem.
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skodarapid · Joined: 10 Jul 2018 Posts: 49 Location: Northamptonshire

Having read some warnings about cheapo ebay 100A slow blow fuses I bought two, one to keep in the glove box. It's discoloured but hasn't blown and it's been in about 15000 miles / 4 years.

Lewis · Posted: Mon Jul 17, 2023 15:48 Post subject:

Take a trip round a local scrappers, if they let you in to wander, and checkout the German made Japanese models. Wink

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Do not blindly go where the path may lead you. Go where there is no path and leave a trail that others may follow!
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Andy W · Joined: 03 May 2015 Posts: 560 Location: Brighton

So far I have managed to turn the engine off before the fuse blows but what I have found is that it only gets hot if the engine is running, with just the ignition on the fuse is fine.
When I disconnect the alternator from the fuse, 90% of the electrics shut down with or without the engine running, is this right because it seems a bit of to me?
Cheers, Andy
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1994 LWB Super Exceed L400, Black & Silver

andyman · Joined: 08 Dec 2012 Posts: 5602 Location: Penrith

This is where you need the electrical knowledge of Deker or Ian (Fostlongstrider). Is there a possibility that BOTH alternators are faulty, - possibly the voltage regulator(s) is not reducing the output of the alternator as the battery reaches full charge? What is your voltmeter reading (a) immediately the engine starts (b) a minute or so after, when the glow plugs have shut off and (c) after a few minutes of driving, when the battery should be fully recharged after starting, and should be reading 13 - 13.2V. Ignition on, but engine off, it should read around 12.8V, soon after switching off, hot. Nearer 12V during preheat, before starting the engine from cold.
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Andy C L300 Super Exceed - over 100 smiles per gallon!

Andy W · Joined: 03 May 2015 Posts: 560 Location: Brighton

Well, I'm still struggling Crying or Very sad

If the ignition is on, everything works as it should and the fuse does not blow.

If I start the engine, the fuse blows.

So to me, the fault has to be with something that only operates/is live when the engine runs.

Does that make sense?

Cheers, Andy
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1994 LWB Super Exceed L400, Black & Silver

andyman · Joined: 08 Dec 2012 Posts: 5602 Location: Penrith

What is the maximum rated output of your new, higher powered alternator? Also, is the engine only running at idle/low revs when the fuse blows? Derek (Deker) Manly has asked me to ask you if your battery is well depleted by the time the engine starts (in my experience, a diesel will struggle to start on a depleted battery as the heater plugs require sufficient power to reach temperature. The surge of current, he proposes, (if I understand him) from the (high output) alternator to the depleted battery, may be enough to blow the fuse. An in-tune diesel, with good plugs and battery, and a few seconds heating, should start instantly the starter has turned the engine once, especially in the summer, and not deplete the battery(s) significantly.
The other thing Deker suggests, is that the glow plugs are not powering down, possibly due to a faulty glow plug relay, so the heater plugs are drawing high current from the alternator, through the battery, for an extended period, causing the slow-blow fuse to overheat. Your dashboard voltmeter and the clicks/clunks from the relays should reflect the draw on the battery and charging system.
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Andy C L300 Super Exceed - over 100 smiles per gallon!

Andy W · Joined: 03 May 2015 Posts: 560 Location: Brighton

Thanks for that Andy, I will check this but as I left the ignition on for 10 or 15 minutes earlier, I think the glow plugs would have stayed on during that time so if they were the problem, they would have blown the fuse, I will check this weekend though just in case.

This issue was happening with the old standard alternator as well as the new 120 amp one so I don't think that's the issue.

I have twin batteries which are in good condition and the engine starts on the button.

Cheers, Andy
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1994 LWB Super Exceed L400, Black & Silver

Lewis · Posted: Sat Jul 29, 2023 11:50 Post subject:

I replaced my alternator with a 125Amp unit, from a 3.2L Diesel Shogun sport, in the process of being scrapped.
The shogun had the same size fuse as my old one, so I doubt that would be an issue with your 120Amp unit.
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andyman · Joined: 08 Dec 2012 Posts: 5602 Location: Penrith

Andy W, if you leave the ignition on for 15minutes, the glow plugs should reduce power or shut off completely after a much shorter time: if they don't, they will burn themselves out. They are only intended to stay on for a couple of minutes even from a stone-cold engine. Also, the 100 amp fuse is in the charging circuit, not the glow plug circuit, so simply leaving the ignition/glow plugs on would only pull power from the battery, so would not blow the fuse. Leaving the ignition (glow plugs) on, but without the engine running, you should hear the relay(s) open and shut as the glow plugs cycle; heating then shutting off. I don't know if the L400 has the two stage glow plug system, where they reduce to half power for a few minutes while the engine warms up, after starting and running for less than a minute on full power.
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Andy C L300 Super Exceed - over 100 smiles per gallon!

Lewis · Posted: Tue Aug 01, 2023 9:29 Post subject:

Andyman
The L400 glow system keeps the glowplugs on until the engine reaches normal operating temperature. By that I mean they turn on and are at their hottest when the solenoid clicks, which is the indicator to start the engine. They solenoid then switches power into them, as required, until the engine temp is stable. In cold weather it is also possible for the glowplugs to switch in again, as the coolant temp drops.

This is the system diagram:-

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andyman · Joined: 08 Dec 2012 Posts: 5602 Location: Penrith

Lewis - hence why I started my comment to indicate that the glow plugs should power down after a short time, or even shut off - If they stayed on full power for 15 minutes, they'd be definitely burnt out! I don't know of any vehicles where the glow plugs can remain on full power for long periods.
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Andy C L300 Super Exceed - over 100 smiles per gallon!

sir naggedalot · Joined: 07 Sep 2010 Posts: 206 Location: Southampton

Late reply, but for future reference, If you check the output for ac rather than dc and it shows ac current then this is often a good indicator that the rectifing diodes have failed. usally you'll get an egg smell long before the 100amp fuse blows if the output is too high or flipped to ac so I would suggest that there's an earth fault or high resistance somewhere.