'Coolant shortage indicator' (low coolant level circuit)

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DevonAid
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'Coolant shortage indicator' (low coolant level circuit)

Post by DevonAid »

Morning all, hope we've been enjoying the Bank Holiday sunshine?

I have a slight weep on the above sensor (through the terminals) in the pressurised coolant tank. Whilst waiting for Brickwerks to restock the sensor, I've had a checkthrough the rest of the circuit to ensure it works, as I'm always on high alert when it comes to all things cooling system. It turns out it doesn't work.

My understanding is the red LED over the coolant temperature guage should flash if there is a low coolant level (and doubles up as an overheating warning as well). The sensor is nice and simple (two prongs that sit in the coolant presumably making the circuit unless the level drops), I've check continuity of the supply and earths at the sensor connection, back to the large engine bay connection box, and gave the local earths a good clean up anyway, as you need to feel you're doing something helpful.

Voltage on the supply seemed to be 120mV, but thinking this through I'm not expecting 12V as this would end up with a short-circuit as the prong sits in the coolant, which flows around the (earthed) engine. Same voltage was found for the same cable at the fuse box.
Leaving the sensor connection disconnected, linked out or earthed doesn't seem to produce any flashing LED warning.
Red LED flashing pre-start test when you first switch the ignition on seems to work OK, and it works OK if I ground the temperature sensor.

The Haynes circuit diagram doesn't make things any clearer at all to me (sensor is F66) as it doesn't show a connection to the fuse box, but I've found the same cable colour and voltage at the fusebox.
Any experience or wisdom as to why the coolant level circuit can't be forced to show a warning?
Many thanks in advance.

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Robsey
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Re: 'Coolant shortage indicator' (low coolant level circuit)

Post by Robsey »

The only thing that springs to mind is the coolant shortage indicator unit.

The thing that looks like a relay with either 42 or 43 printed on it. In relay position No3.

Maybe this is faulty.
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AngeloEvs
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Re: 'Coolant shortage indicator' (low coolant level circuit)

Post by AngeloEvs »

It is an AC voltage across the level sensor with one side earthed and the other to the output of module 43. The AC voltage across the sensor varies according to wether the sensor probes are submersed in fluid or not. With no coolant across the probes the AC voltage increases and this triggers module 43 into sending pulses to the circuit inside the temperature gauge and that circuit flashes the LED.

With loss of coolant, the triggering of the Coolant LED is not instantaneous and there is usually a delay of around 10 to 20 seconds. To test the coolant level sender circuit you have to disconnect the connection to the sensor (not short circuit the connections) and wait for a minute or so, disconnecting emulates loss of coolant. If the LED fails to start flashing after a delay then module 43 may need replacing.
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Re: 'Coolant shortage indicator' (low coolant level circuit)

Post by Mocki »

Depending on which style plug you have on you low coolant sender loom I have a stock of new senders if you are desperate for a replacement before BW has stock . 
 
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Re: 'Coolant shortage indicator' (low coolant level circuit)

Post by ZsZ »

I have found a diagram of the #42 control unit "relay" on some russian forums recently: (J120 on your circuit diagram)
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if the unit losts its gorund through the in line (blue/green wire from level sender to K5 connector on the late fuse box) then it sends signal to the temp gauge circuit (yellow/red wire from K4 to D29)

I think most of the misbehaviour of these units are from failing capacitors.
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DevonAid
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Re: 'Coolant shortage indicator' (low coolant level circuit)

Post by DevonAid »

Thanks to all that replied - excellent level of detail there, much appreciated. 
Aside from a leaky sensor, checks show that the relay is also likely to be at fault - unfortunately also currently not in stock at BW or other usual stockists

Mocki, thanks for your sensor stock info: I have the push on (not clip on) type
1985 T25 Devon conversion 1.9DF

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