Brake fluid reservoir low switch replacement / repair

Hello all,

I have a 300TDi 90 with a couple of problems on the handbrake warning and brake fluid warning circuits.

Firstly, the handbrake warning light doesn't come on when the handbrake is on. I have checked the connections and switch on the handbrake, traced the cable back as far as I can into the loom and it all looks okay. I can only assume a problem with the multi-plug on the bulkhead which will have to wait until I service the air filter rubber mounts before investigating further. There is no power to the cable at the handbrake.

Secondly, the brake fluid warning light doesn't appear to work either. I removed the filler cap to which the switch is attached and let the fluid drain out. Nothing happens, no light on the dash whatsoever. I then used a short length of spade ended cable to connect between the two connectors together to complete the circuit and in this instance the dash warning light came on.

So I'm wondering whether there is anyway to service the filler cap switch or if there is a replacement part available (I've searched without success), without having to replace the entire reservoir?

Also, I understand that both circuits (handbrake and low fluid level) join at the multi-plug. Would the defective filler cap switch affect the handbrake warning light circuit also? Or is it more likely to be a problem at the multi-plug?

Thanks in advance.

Well, as I typed the above I wondered whether the previous owner(s) had got the two connectors the wrong way round on the brake fluid reservoir switch. So, I simply swapped them around and the switch now functions correctly. However, the handbrake switch cable still has no power to it so any tips on that would be appreciated.

Any thoughts people as to why there's no power to the Handbrake warning switch?

Ah can't really help as ah don't have one o' they fancy switches. Dunno if might help running a new earth or checkin' if the white and yellow wire has a break somewheres.

It's numbers 50 and 60 on here



If that helps

Ah, that's a slightly different circuit diagram from the one I have, may have to pop the old light unit out of the dash again and have another peek. My diagram shows the handbrake teeing into the brake fluid warning light circuit (I assume at the multi-plug), but it may well be wrong.

It's not too vital as it's fairly obvious when the handbrake is off/on but I'm just trying to get everything working as it originally was when it left the factory.

I appreciate your help and input though. Cheers BB.

mick miller said:

Ah, that's a slightly different circuit diagram from the one I have, may have to pop the old light unit out of the dash again and have another peek. My diagram shows the handbrake teeing into the brake fluid warning light circuit (I assume at the multi-plug), but it may well be wrong.

It's not too vital as it's fairly obvious when the handbrake is off/on but I'm just trying to get everything working as it originally was when it left the factory.

I appreciate your help and input though. Cheers BB.

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Might sound like a really silly suggestion... but ye have checked to see if the bulb works?

Not a silly suggestion, but yes I checked. One of the first things I tried.

mick miller said:

Not a silly suggestion, but yes I checked. One of the first things I tried.

Click to expand...

HAHAHAHA! Sometimes it's easily overlooked

Haven't really got much of a clue on this one, but kinda wondering why the wires are linked to the brake res? It's not as if the handbrake uses any fluid as it's straight off the back of the T-box

I'm thinking it's because the light is dual purpose on the dash. If the handbrake is on the light is lit or if the brake fluid is low the light is lit. Seems odd to me to but that's what the diagram suggests. I'll try and get it scanned and posted so you can see what I see.

I traced that wire back through the seatbox, it passes under and into the loom (where it ends up who knows). Allegedly, I'm unreliably informed it tees in at the multi-plug on the bulkhead to the brake fluid warning circuit.

This is the circuit diagram I'm working to. There is power to the bulb in the dashboard console, power to the brake fluid warning sensor (55) but no power to the handbrake switch (56). The diagram shows the W/Y cable teeing into the B/W cable (circled). I suspect that this is the problem. However, I haven't a clue where that join takes place. I suspect the multi-plug on the engine bulkhead (beneath the air filter). But it'd be nice to know for sure before stripping the air filter out.

Have ye tried running a seperate earth from the switch yet?

The handbrake switch? Yes. I've stuck the multimeter on it and I assume there should be a constant current, the switch earths that making the circuit. I stuck a double spade ended length of wire into the female spade off the handbrake and touched that to a bolt head inside the cab (which worked as an earth when I wired in the 12v sockets to replace the fag lighters), nowt, nish, feck all 'appens.

I've been under the vehicle and can trace the wire back into the loom, it appears okay. No breaks obvious. After that where it goes is my issue. If I knew its route I could investigate further or just replace that wire (I may end up doing just that anyhow if I cannot find out where it ends up).