P38A Switches

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Grrrrrr

Grrrrrr

Technician, Bodgit & Scarper Ltd
Posts
19,842
Location
Buckinghamshire, UK. ('95 DT)
Do any of the centre switches (e.g. fog-lamps) actually work as just switches or do they all signal the BECM and then the BECM switch on whatever it has to?

Was thinking of buying a spare switch and using it for something other than its intended purpose but not sure if it will work.

Cheers.
 
They are switches....some of them latch and some are momentary....

Check the switching voltage and current first as in the P38 they only send a signal voltage to the BeCM and may not be 'man' enough to handle switching high currents.
 
Front fogs are "momentary". You could use a relay to latch it though......until the ignition is turned off. I'm sure Maplin will sell a kit to build a latching power switch to connect the momentary fog lamp switch up to.


Screen Shot 2016-03-13 at 22.05.04.png
 
The tracks on the boards (when I've had the switches apart to do LED conversions) don't look like they are capable of a very high current, so would definitely use them as a trigger for an external circuit like they do with the BECM.

The front caps are removable aswell, so if you wanted the icon from say front fogs on a latching switch you could fit it to one of the ones that wammers mentions. Also on manual vehicles I believe the Transfer Hi/Lo switch is latching aswell.
 
... and ****ing advert keeps killing keyboard. 5 ****ing minutes to try and get this reply. Grrrrrr.

Thanks for that. So pin 6 for it to light up when working and pin 2 to illuminate at night?
 
I think so. You probably have the plug for the back of the switch already there in the loom, so the night time illumination will (probably) work anyway.
 
MrGorsky said:
I think so. You probably have the plug for the back of the switch already there in the loom, so the night time illumination will (probably) work anyway.
Click to expand...

That plug will be connected to the BECM though, won't it? What happens if I splice a power cable in and the BECM isn't expecting it? I think I might play safe and splice it into another red & white wire and leave the switch itself isolated from the BECM.
 
Yes it will, but for the sake of a neat(er) install, I would (assuming the plug is on the loom) fit the switch to the dash and plug it in. You can just connect it all up, switch on cruise control with your Nanocom, and then take power for your parking sensors from Pin 18 at connector C255 on the BECM. This is the output from the BECM that switches on the operation lamp on the Cruise Control switch, and sends the signal to the other bits of the cruise control system to switch on. There are a few unknowns. You don't have the other cruise control bits fitted, so it might throw up a fault. I doubt it though, the circuit just shows the BECM providing a "switch" service to the rest of the cruise control system. You'd probably need to use a relay to provide power to your parking sensors (you should be doing this anyway) because the BECM won't be able to provide enough power. But the advantage is a dead neat install, you can do all your electrical connections by the BECM where you have room to work, and you'll get an operational tell tail on the switch when the parking sensors are on!

EDIT: I have editied this from a longer answer that would not have worked so well.
 
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I would avoid the BECM altogether if you're wiring in something aftermarket other than for the illumination of the backlight.

If it comes with the plug, the I would splice in the ground wire from pins 1,3,4 to the grounds from the rest of the dash switches. I would also splice the red/white illumination wire into the loom where it goes to the other switches. The lamps are only 1.2W or something low like that, so not going to draw much extra current.

I would then wire in the relay for whatever is being powered by the switch, so that one side of the relay coil connects to +ve voltage (possibly though a diode for protection and probably ignition switched) and then the other side of the relay coil to pin 5 of the switch. so when the switch is pressed/latches, if provides the ground path for the relay.

If you want to use the telltale lamp on the switch, then this would just need to be fed with +12V when the external unit is powered. If the relay is actually switching the 12V across the contacts, to power something external then you could just take a feed off of that to then illuminate the lamp when the relay contacts are closed and supplying power.

With this, you would only need to run 2 additional wires into the dash - one for the switch contact, and the other to power the telltale for whenever the external unit is powered or active.
That would be my way of doing it, without then having to mess about with the BECM, settings, or any of that.
 
Think I feel a bit more comfortable using Marty's way. Power is taken from the radio ACC (red wire, IIRC) so should only be live when ignition on.

I did think of using a relay but then thought I'd get away with it as the sensors draw so little current?

If I connect pin 3 to 6 won't that make it light up when switched on? Or all 3 of those earths linked so it would also light up with the dash lights are on?
 
I've just had a look in the wiring diagrams, and it appears that the pins are different between early/late models - but only on pins 4/6 as to which one is ground and which one is the positive for the telltale lamp.

If you wanted the lamp to come on as soon as the switch is pressed to power the sensor module and power the sensor module through the switch, then you would need to cut one of the ground wires from either the telltale or the switch contacts to make it work properly.

The way the switches work for the BECM is that they supply a ground feed (either latched or momentary depending on the switch) which pulls an input circuit to ground in the BECM, which detects the change in state and it then figures out what needs to happen, eg turns the cruise control on.

If you just want to use is as a switch, then you either need to use the switch to be the ground connection to whatever device you are using (as the 3 pins link to ground anyway - hence the relay suggestion) or you could re-wire the switch slightly to switch the positive feed to the sensor module and then yes, you could make it turn the telltale lamp on aswell.

I will draw up a couple of quick wiring diagrams for you in a while and post them up, so you can then choose which option will work best for your application.

Marty
 
This is exactly why I'd do it my way! All this rewiring switches and stuff. Just splice in to Pin 18 and you're away. The circuit diagram suggests it's just a power feed from the BECM, and if you're worried, you could test it with a multimeter first.

I think you should use a relay though. You don't know what the current draw of the sensors is, and we don't know the rating of the switch is, it might only switch 5v in normal use. It's designed to put a feed to ground, not to switch power.
 
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I did look in Maplins at relays but the smallest they had was 10A which I assumed needed a fair tug to pull it in. May have another look.

Switch is here already! Here's piccy of the back. I expected all ground to be on 1 side but apparently they're not. The bottom right one that is obscured here is also black and presumably ground.

Looks like a judicious snip would sort the tell-tale issue? Just need to work out which number is which.

20160316_114114.jpg
 
Grrrrrr said:
I did look in Maplins at relays but the smallest they had was 10A which I assumed needed a fair tug to pull it in.
Looks like a judicious snip would sort the tell-tale issue? Just need to work out which number is which.
Click to expand...

10A will be the switching capacity, not the required current to pull it, that'll be almost nothing. I'd just buy an automotive relay. It'll be waterproof, and have a nice hole so you can screw it down somewhere safe.

Screen Shot 2016-03-16 at 12.50.32.png


1,3 and 4 Black wires are the earths.

2 is Red/White and the feed for night time illumination.

5 is White/Yellow and the "switch" positive side.

6 is Ornage/White and the telltail positive side.

If you're going to use it like a switch, you need to cut the wires from Pin 3 and use them as your negative side of the switch.

And then join 1 back up to 4 so there is an earth for the telltail and nighttime illumination.
 
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