Disco 1 Electrical gremlins

normally (on normal vehicles) the door switches ground the circuit when the door is open. should be easy to check each door switch.

a duff splice is a good call - I used to do some work on a certain tractor models series - ( not naming names !) - and the electrical problems were either a duff earth, or a duff splice - which, frankly were such a piece of cr*p from the factory, I never understood how they had ever worked - stripped back, cleaned up, soldered and HS wrapped = no more problems !

Glad you're still on the case - Good luck with it

I have checked, checked & triple checked the door switches so i'm 99.9% certain it ain't them Could someone tell me roughly where I could find the splice(s) are please???? Think looking at the wiring diagram they are S261 & S2013.

The drivers door contact feed is IN to the MFU, the remaining four door contacts are linked together and feed IN to MFU.
The MFU has a feed OUT to the security ECU (alarm) as stated in my post #37.

It's all shown in the wiring diagrams, so where is a splice coming from? have you done any circuit testing on the door switch wiring yet?

The splices are in the diagram shown below in the red circles

I'm going off the Key in the wiring fault finding book for the Discovery and S means 'Splice' H means junction connector, so I'm going from what I'm reading in the manual. BUT it doesn't say where the 'Splices' are.

that shows all the switches going to ground. ergo there should be a voltage on the other side of the switch = but it might only be 5v.

Remember that the MFU doesn't switch the wiper motor directly, the relay does that so you may have an issue with the relay or the rear washer/wiper switch, unless u or someone has been interfering with the wiring splices alone some are in the wiring under the dash, and one behind the passenger kick panel near the rear of the door switch but there is no specific ID for them.
You do need to focus on one item and test....

Sent from my iPad

Getting really frustrated with this now
Right, pulled the Purple/blue wire from the MFU, now all the lights go out - interior & ignition light. So I know they go off!
Tested the Driver door switch - when its out (door open) the key in bongs!
When I test the other doors what should I expect to see on the multi-meter??? should I be testing them from the door switch or the purple & white pin 5????

Tested all 4 remaining doors and they all state .074V, I then had a brainwave I plugged in my Hawkeye (never thought about using it before as there aren't many electronics on the old D1), went into the settings for the alarm & it stated drivers door SHUT, passenger doors "OPEN". even though every door, bonnet & boot were shut!

I think now, possibly a useless electrician like me might, just might be on to something.

From pins 5 on the MFU I'm getting current, 1 & 16 on the alarm unit I'm getting current, but nowhere else it seems. I've looked at all the wiring I can see / get to, on the door pins but can't see any chaffing so far. I'm gonna have to look further back I think into the door pillars and chase back to the main looms

Right! After a lot of faffing about today I have found out which door was at fault - It was the tailgate Now here is where I need HELP pleeeeeeease!!! I found the tailgate to be at fault by process of elimination by 1st checking all the door switches AGAIN!!! couldn't find any chaffs or other fault. I then looked at the wiring diagram post 45, and started to look at the wires from the alarm and MFU leading out to the doors.

I knew the drivers door to be ok as Hawkeye showed it opening & closing fine, so I checked each of the passenger WP & PW wires in turn Pins 1 & 16 in the alarm & pin 5 in the MFU, no changes.

The one which goes from Z277 to S261 I disconnected and to my amazement Hawkeye said "Passenger doors closed" PHEW I thought!!!! I wasn't sure which door this wire controlled so I went round all of them and n/s & both rear passenger doors showed opening & closing again on the diagnostic, apart from the tailgate which when opened still showed "closed" so I knew it was that.

Excitedly I reconnected the interior light & ignition light and still on!!!! and the only way of them going off is pressing & holding the HRW button.

I thought I'd cracked it, seems not, apart from having all 4 doors opening and closing correctly now. Is there any way that wire PW from Z277 to S261 could play a part in the interior/ignition lighting problem???

is it possible to check the diode at all - or just replace it ?

Wouldn't know where to start in checking it (as I said I'm pretty crap at sparkin) and how would the diode affect the interior lights & holding down the HRW button turn em off???
Can you get new diodes??? Just looked and ????????? dunno what part no. I'd be looking at?

Sorry for all the Q's but I'm determined to get to the sorry bottom of this but electrics is in no way my strong point, mechanics / welding (to a degree) ok but wiring

The MFU controls the interior light depending on the signal it gets from the door switches and or central locking / alarm.
The MFU switches the ground / earth for the interior lights.

Check the earth points particularly the one under the RH Side trim kick panel above the relays.

Jasonmichael said:

Wouldn't know where to start in checking it (as I said I'm pretty crap at sparkin) and how would the diode affect the interior lights & holding down the HRW button turn em off???
Can you get new diodes??? Just looked and ????????? dunno what part no. I'd be looking at?

Sorry for all the Q's but I'm determined to get to the sorry bottom of this but electrics is in no way my strong point, mechanics / welding (to a degree) ok but wiring

Click to expand...

Let me explain as basically as I can..
As u can see in the electrical diagram the diode is in series with the front LH door switch and the tailgate door switch and with the cathode of the diode towards the tailgate door.

So with you meter leads test the wiring, so place the + lead on the front switch and the - on the rear switch you will get a reading, reverse you're leads and u won't get a reading confirming the diode is functional and allowing the four door switches to be in parallel for the operation of the interior lighting.

The diode is in the link to the security ecu from the tailgate switch, unfortunately there's no information on what involvement the tailgate door switch has with the ecu and why the need a blocking diode.

Thanks for that Discool, sorry for sounding like a complete wally, but I REALLY am crap with lectricks! (I bet you're banging your head against a wall right now).

I think, think?!?!? I / we may be on to something here, just done some research on the Lucas 10-AS alarm. The input for the interior lights is Pin 1 and is the WP wire exiting from the tailgate diode. However IF the diode fails, it usually buggers up the alarm ECU as well!!! All I can do is test it in the morning (if we're not 2 feet in snow). The diode itself looks like its insulated in some kind of rubber / silicone stuff. Depending on the result (and I'm praying its this at fault) how would I know which one to buy to replace it??? Are there different sizes etc???

If it's a bog standard blocking diode then a rectifier or power diode is a standard diode something like a1N4001 diode there's a few mor the look the same, black with the silver cathode marking.

Loads of info on the web on diodes., my very quick look finds this https://learn.sparkfun.com/tutorials/diodes

Thank you!!!

I'll have a look tomorrow, do the tests & hopefully have an answer.

discool said:

So with you meter leads test the wiring, so place the + lead on the front switch and the - on the rear switch you will get a reading, reverse you're leads and u won't get a reading confirming the diode is functional and allowing the four door switches to be in parallel for the operation of the interior lighting.

Click to expand...

+1.

discool said:

The diode is in the link to the security ecu from the tailgate switch, unfortunately there's no information on what involvement the tailgate door switch has with the ecu and why the need a blocking diode.

Click to expand...

The diode will be there to stop the ECU or the door switches being connected together in the wrong way - without it pins 1 and 16 of the alarm could go to the same potential at the same time - which is possibly why when it fails, the alarm goes kaput... as you mention here....

Jasonmichael said:

we may be on to something here, just done some research on the Lucas 10-AS alarm. The input for the interior lights is Pin 1 and is the WP wire exiting from the tailgate diode. However IF the diode fails, it usually buggers up the alarm ECU as well!!! All I can do is test it in the morning (if we're not 2 feet in snow).

Click to expand...

And if you haven't got enough of the white s**t, let me know - plenty here already
So that leaves this bit:-

Jasonmichael said:

The diode itself looks like its insulated in some kind of rubber / silicone stuff. Depending on the result (and I'm praying its this at fault) how would I know which one to buy to replace it??? Are there different sizes etc???

Click to expand...

The diode will have a part number on it, which will hopefully still exist under the heat shrink tubing it's "buried" in - failing that, I would suggest one rated to more than 12V, and of a similar physical size from somewhere like maplin ( if you have one, or the bay of fleas). Diodes are rated in terms of how much voltage they can "block" or "refuse" - it is not likely to be more than 12V on a car (!), and then in wattage for how much power they can dissipate in blocking said voltage. 5W 12V diodes (1N5394B), are about £3 for four on the bay of fleas, and 600W (!!) 12V diodes (P6KE12A), are, strangely, about £3 for four from the same place !! ( those were just the first two I found, to give a rough idea )

Someone on here may even know the correct part number

Good luck with it

Cheers guys, on another note is there a replacement part for the alarm sounder at all that would be compatible with the D1??? Mine self combust ages ago and although its been nice and quiet trying to trace this fault down, if possible I'd like to get things working properly again. I've seen a few original AMR 4974's but they are gonna be as old as mine were and have the same problems.