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Eleck-Trickery Question re Immobiliser


Well-Known Member
Jan 20, 2021
OK. This is going to be a bit of a vague "How long is a piece of string"? type question. But here goes:

Having endured nothig but rain for about the past fortnight here, I went out to give the engine a turn over today as, the battery was pretty flat last time I started it.

Anyway, the batttery was dead, so I started the truck off my jumper pack and drove it up and down the lane for a bit. Just to get the juices flowing. When I pulled up and stopped and blipped the key to lock the doors, nothing happened. I then noticed that the immobiliser light wasn't flashing on the dash either. So I put in the key and turned on the ignition.... Nada. Not even a single light on the dashboard. Completely dead electrically.

So, thinking the battery had really died this time, I went off to fetch the jump pack again. When I got back to the truck, I noticed the immobiliser light flashing again as normal and, when I tried my key fob, it locked/unlocked the doors, also as normal. I got in and turned on the ignition and got all my lights back on the dash. Even started the engine, though it was a bit reluctant, due to flatt battery.

When I switched off again, the same thing happened. No immobiliser, no lights at all on the dash and no keyfob locking. I went off to put the jump pack away again and, when I got back to the truck, the immobiliser light was back on and everything was working properly again. It was like the car was completely electrically dead but, if I left it a few minutes, everything would come back to life again.

So, my question, not knowing much about immobilisers and their mysterios ways; does that sound like something any of you have experienced with a flat battery / immobiliser scenario?

I've taken the battery off and put it on my smart charger, which suggests it was on about 12.2 volts and somewhere around 50% charge. So it was fairly flat. But I wouldn't have thought the immobiliser circuitry would need more than a trickle voltage to operate. And it was also weird that there was totally nothing electrical showing at all. Not even a light on the dash, when the ignition was on.

Most peculiar!

BTW -- the immobiliser isn't an OEM one. But was fitted by a previous owner. So that adds the potential for random spaghetti wiring into the mix.
Does seem a bit of a strange one but running the car up and down the lane won't charge a battery (unless your lane is 100 miles long) but I would say it needs a good 30-40 mins of driving to to get any thing near a charge.
But what I would do is as you have said put it on charge with the smart charger and get it up to 100% then once charged disconnect the battery then reconnect and see what happens then
I've had a few small gremlins and initially changed the alternator and then upgraded the battery from 75Ah to 100Ah which has helped compensate small losses.

Had odd occasions when the alarm randomly beeps although the engine is running and even beeping when I've stopped the engine and opened the door but not 'set' the alarm or locked the doors. Just putting these down to technical/device oddities, also have a foldout 200w solar kit that I put out if not expecting to drive the truck for a while to keep the battery topped up.

Have found the odd wire here and there as you note from previous owners but tracing is difficult sometimes.