1999 300 TDI 90 CSW what does the 5 amp fuse in slot 20 do?

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ryan_1993

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So had a bit of an “incident” on Monday with my 300 TDI when out in Peak District.

Driving along and the little red light on the dash for the immobiliser started blinking which it has never done before, as if it was activated, but I was actually driving along.

When coming to a stop and turning off the engine it would no longer start. The immobiliser wasn’t behaving how it normally would. Normally you get a red flashing engine light with an X through it if you don’t disarm the immobiliser, but it wasn’t even doing that.

I tried swapping relays about, but to no avail, being in the absolute middle of nowhere we had no phone coverage and had to call for help from the visitors centre.

AA man discovered it was the last fuse, number 20 which had blown, just a 5amp one. He put another one in and it started right up, but after a minute or two it blew again, engine still was all running fine. I made it home the 60 miles no issues.

Obviously though back on drive it wouldn’t start after switching off, put a new fuse in and it started right up before blowing again. I then put in a 10 amp in and that hasn’t blown yet and started about 5 times in a row, at this point the immobiliser also started behaving more like it should have, by showing the red flashing engine light on the dash when it was armed, pressing the key fob disarmed it and it started right up.

Both me and a mate assume it’s likely spider control box for the immobiliser or possibly it’s associated wiring? I have heard these units can be a bit problematic?

just wondering if anyone else has had anything similar happen to them or what fuse number 20 actually has on its circuit to try and narrow down fault finding?

All the fuse diagram shows for slot number 20 is a picture of what I assume is the Defenders handbook

 
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So had a bit of an “incident” on Monday with my 300 TDI when out in Peak District.

Driving along and the little red light on the dash for the immobiliser started blinking which it has never done before, as if it was activated, but I was actually driving along.

When coming to a stop and turning off the engine it would no longer start. The immobiliser wasn’t behaving how it normally would. Normally you get a red flashing engine light with an X through it if you don’t disarm the immobiliser, but it wasn’t even doing that.

I tried swapping relays about, but to no avail, being in the absolute middle of nowhere we had no phone coverage and had to call for help from the visitors centre.

AA man discovered it was the last fuse, number 20 which had blown, just a 5amp one. He put another one in and it started right up, but after a minute or two it blew again, engine still was all running fine. I made it home the 60 miles no issues.

Obviously though back on drive it wouldn’t start after switching off, put a new fuse in and it started right up before blowing again. I then put in a 10 amp in and that hasn’t blown yet and started about 5 times in a row, at this point the immobiliser also started behaving more like it should have, by showing the red flashing engine light on the dash when it was armed, pressing the key fob disarmed it and it started right up.

Both me and a mate assume it’s likely spider control box for the immobiliser or possibly it’s associated wiring? I have heard these units can be a bit problematic?

just wondering if anyone else has had anything similar happen to them or what fuse number 20 actually has on its circuit to try and narrow down fault finding?

All the fuse diagram shows for slot number 20 is a picture of what I assume is the Defenders handbook


I think Fuse 20 is the alarm, but not sure, maybe an earth fault ...

Later models have the alarm unit behind the glovebox area ...
 
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I think Fuse 20 is the alarm, but not sure, maybe an earth fault ...

Later models have the alarm unit behind the glovebox area ...


Is this that green box? I can see it when I remove the speaker from the drivers side and look inside the opening.

I know I have a spider in a black tamper proof box in the battery bay which is to do with the immobiliser.
 

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Is this that green box? I can see it when I remove the speaker from the drivers side and look inside the opening

Yes, I've not worked on one, and a grown-up will be along soon ...

But it's usually caused by a bad solder joint in the box ...

By the way if your alarm module is a Z163 it will use a transistor instead of a relay... but can still be jumpered ...
 
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As always, the God of LZ lectrickey has arrived ... :D

thanks for this, so the alarm diagram seems to show fuse 20 controls the engine immobilisation warning lamp and something on the spider C863-1?

Am I missing anything else? Taking it to a mates later today who’s a qualified auto electrician, but anything to make the job easier.

looking like it might be a grounding issue?
 
I have this exact same problem. Where was the fault and how was it fixed? Many thanks.

sorry it was the main alarm control unit in the dash behind the binnacle, they are often a main failure point can suffer corrosion and poor quality from manufacture.

mate of mine is a qualified auto electrician and sorted it though, alarm back now all working.

was just a dodgy alarm box.
 
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