LSCU advice....Angelo??

[ermie571]

Ok, I have the little red flashy light. Comes on and will stay on until I turn the ignition off. Turn it straight back on and it does it check then goes off. Been reading and found this by Angelo.

"If yours is a post 86 van, remove the LSCU (looks like a relay, in the fuse box, position three and marked 42 or 43). If the red light stops flashing with the LSCU removed then the fault is in the coolant level sender circuit and you can concentrate on that."


ok, pulled the LSCU when I got to work this morning with my flashing light. The temp guage is fine, by the way, and reads just below the LED.
The little flasshy light stopped flashing....and didn't restart when I put it back, still with the engine running.

so, the fault is in the sender circuit.....where do we start...or is it the balck box that is at fault?

Em
x

[Grun]

Em,
This is what AngeloEvs wrote in an earlier post on the subject.

Re: Anorak time re LSCU's
by AngeloEvs » Thu May 07, 2009 1:17 am

Almost right, firstly the chips in the Level Sender Control Unit (LSCU)are VAG coded so no idea of what they are. The gauge has its own internal circuit that flashes the LED for a few seconds (the Self Test). This circuit will be triggered whenever the resistance across the gauge falls below 50 ohms and this is the coolant senders resistance when the coolant exceeds normal operating temperature. The sole function of the LSCU is to monitor fluid level and to trigger the gauges internal circuit in the event of coolant loss. The LSCU sends a low frequency AC pulse to the probes in the header tank (probably to prevent electrolysis and degrading the probes hence the AC). If the pulse waveform alters due to no coolant being present then the LSCU sends a series of short duration low frequency negative pulses to the temperature gauge. These pulses are seen by the gauge as temporary short circuits across the gauge and the internal gauge circuit responds by activating the LED. The reason for sending short duration pulses 'once every few seconds' to the gauge is to prevent it from deviating from the current temperature position (otherwise it would swing hard over to the right). In the event of overheating, the internal circuit of the gauge will switch the LED off when coolant temperature returns to normal (sender resistance returns to a normal value greater than 50 ohms) but if the LSCU is activated it has to be reset by switching the ignition off. The reasoning for this is that whilst coolant temperature can return to normal coolant cannot replace itself and hence the continual flashing in the event of coolant loss and temporary flashing in the event of temporarily overheating. The problem with the LSCU is that it is easily triggered by sudden fluctuations in Voltage, poor earths and has a very narrow margin of error regarding the resistance of the coolant fluid - according to VAG service bulletin - antifreeze mixtures over 60% will cause flashing red light syndrome.

Just thought it might help.
Mike

[AngeloEvs]

Removing the LSCU stopped the Led flashing so the fault is in the coolant level circuit, these are the usual suspects.

!. Coolant level Low? you prabably checked this Em and found its ok.

2. The LSCU Earth connection at the earth crown next to the fuse box.

3. The earth under the ignition coil.

4. Poor connection at the coolant level socket.

5. The LSCU itself

6. Loose nuts behind the temperature gauge.

I would check these first Em and see how you get on.

[ermie571]

Thanks Angelo and Grun, both very useful, particularly the bit about flashing stopping if its temp related, but stays on if low fluid detected. At keast i can be confident that the guage is correct when its reporting no overheating

I drove the van home after having had the lscu out, and no flashy light. Will drive it to work tomorrow and see what transpires. Will start with the easy bits first if flashy light comes back.


Thanks again....and I know a little bit more about the van!

Em
x

[ghost123uk]

Reviving an old thread because it is relevant and the info on here (largely thanks to AngeloEvs might be of use to others in the future).

My LED flashes as it should on turning the key.
My Temp gauge works fine.

My low coolant level sensor does not work, if I remove the plug on top of the header tank, engine running, no warning appears on the LED etc.
It is unlikely to be an open circuit in the wires going to box 43 as this would simulate a low coolant situation and cause the LED to flash.

I changed box 43 for another, though this spare one was untested so may be faulty (they are delicate electronic circuits not the relay's they look like).

It occurs to me that if I knew which pin on box 43 was the negative pulse that is produced when there is an open circuit at the sensor, then I could check with my meter to check that the pulse is present, if it is, then the fault may be between box 43 and the electronics near the LED

I REALLY want a low coolant warning that works !

At present looking to borrow a "known good" box 43 (or 42 as it s the same thing) to test if it this, (but I don't get to the shows these days though so that ain't quite so easy).

[Aidan]

John if you are passing I have several of these relays on the shelf

[ghost123uk]

John if you are passing I have several of these relays on the shelf

Thanks Aiden
I do drive past the end of your road around once a fortnight on my way to a customer @ Birchwood, so may well take you up on the offer

[Aidan]

I'm away from tomorrow till Monday evening

[AngeloEvs]

First, there is a delay before the LSCU start to transmit pulses in the event of coolant loss. So, when removing the level sender plug don't expect the LED to start flashing straight away. Disconnect the plug and wait (with the ignition on). I can't remember the exact delay time but within 30 seconds if I remember correctly. I use an oscilloscope for testing but a meter may work.

It sounds as though your LSCU is not working, not connecting to the Temp gauge or getting a 12V supply but here is some info to help you understand how the system works if you replace the LSCU and still get the same fault.

The terminal marked 'Level sender' transmits a low voltage Sine wave to one of the level sender probes in the tank (the other level sender probe connects to chassis and why a poor Erath connection causes the AC waveform to change amplitude and one of several a causes of continual flashing syndrome). Setting a meter to AC Volts should show a reading, set it to read around 12V though the sine wave is actually a lot less than this, but it varies in amplitude, this how the LSCU determines wether there is water across the probes by monitoring any change in amplitude.

The terminal marked 'temp sender' feeds to the temp guage and links into the same wire that goes from the Temp. gauge to the Coolant temperature sender in the thermostat housing. This connection from the LSCU sends a negative pulse to the temp gauge output connection., It's low frequency and a standard multimeter set to DC 12V should be seen to 'kick' the existing reading that is normally there. Normally you will see a steady DC voltage (the exact value depends on the temp of the engine). Once the LSCU is triggered this steady voltage will appear to kick once every few seconds - this is what triggers the circuit in the temp gauge to wake up and start the LED flashing.

Don't expect a big kick in terms of how far the needle moves or (if using a digital display meter) the digiatl readout suddenly changes for a very brief time , its a damned fast pulse that just repeats every second or so!

Note that the LSCU (module 43) has an earth connection. This is another real trouble maker and terminates at the earth cluster crown on the door pillar next to the fuse box. If this is anything else but perfect you will get random triggering and the Gauge LED starts to flash.




The best way to test the LSCU is with it removed and setting up a dummy circuit by connecting it to a 12V supply and connecting a Level sender dunked in a cup of water and seeing if you get low frequency negative pulses when the sender probes are removed from the water.

If you could get a probe behind this socket with the LSCU in place, or maybe some very thin wire in the socket then fit the LSCU so that you have fly wires to which you can connect a meter , then I suppose it's possible to take measurements with the LSCU in situ.

[ghost123uk]

Hi Angelo, and all

Thanks for the detailed info Angelo

Yup I tested it in situ and found good earths, good continuity from sensor, pulses at sensor, but no output to dash. I then simulated negative pulse with my analogue meter set to 1 Ohm and leads the appropriate way round and got flashing LED !

So looks like both the "43's" I have must be faulty.
This gives me the confidence to buy another and assume that will sort it out

Whilst I was doing a few jobs under the lid I decided that as I was fitting a new dalek cap to replace the one that was not holding pressure, I would change 1 hose I knew looked dodgy, the short one below the alternator. What a pig to get at