Anorak time re LSCU's

[ghost123uk]

Firstly, what does "LSCU" stand for, I cannot find out !

It is the cooling sensor circuit trigger device that looks like a relay in your relay box, numbered 42 or 43 (they are not relays but quite complex chips etc).

A pair of probes in the coolant header tank monitor the level and one of the functions of "42 & "43" are to trigger the flashing oil warning light on the dash, at the same time moving the temp gauze to hot so you can see the warning led ( I think).

It does more that that but for now I am referring to this function.


Angelo, in another thread, suggests that there are 2 types of chipset in these cooling sensor circuit trigger devices.

Poss 1 = a "number 42" on your relay panel and a similar but different chip type = "43"

Angelo suggests the unit sends a pulse to the probes and if it gets a bad return, it triggers the warning LEd (and poss moves the gauge to hot so you can see the needle)

btw Angelo has put up a really neat digram re the cooling electrics on this thread here =

https://club8090.co.uk/forum/viewtopic.p ... U#p7327975" onclick="window.open(this.href);return false;

Like this



(Ooops = if that picture link is not appreciated and I will strike it)

I was hoping it was a simple 741 type comparator chip monitoring the static resistance between the probes, and it sent a DC to the binnacle circuitry...

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The reason = I wanted to set up a low water warning buzzer.

If I had been right about the way the LSCU worked, this would have been easy with just a transistor, a couple of resistors and a cap to feed a 12V buzzer.

Any thoughts ?

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Off to bed now, so thoughts on a postcard please to...

[AngeloEvs]

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 activates the coolant LED for a few seconds (the Self Test), thereafter it waits. This circuit will be triggered and operate the LED for a few seconds whenever the resistance across the gauge falls below 50 ohms (the coolant senders resistance will fall below 50 ohms 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).

If the pulse waveform alters (increases in amplitude) due to loss of coolant 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 and fool it into thinking the resistance has dropped below 50 ohms. Once the LSCU is triggered its stays triggered until the coolant is replaced and the LSCU reset by switching the ignition off.

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) and to protect the gauge.

In the event of overheating the LED stops when coolant temperature returns to normal (sender resistance returns to a normal value greater than 50 ohms) unlike the LSCU which remains latched once coolant is lost. The reasoning for this is that whilst coolant temperature can return to normal coolant cannot replace itself. 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.

[ghost123uk]

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).

Ah good

I have a Scirocco steering wheel on mine which obscures the warning LED and gauge somewhat.

I also had a narrow overheating escape ( on my recently acquired VW recon motor ) when a front to back pipe let go. Luckily I was in traffic right near home at the time, so no overheating occurred.

So I want this low water buzzer.

If the LSCU's sole purpose is to monitor the level in the way Angelo says, then I can ditch the PCB and in the same No. 43 box, fit a simple 741 based comparator to trigger a little 80Ma buzzer. (I can live with a few milliamps of DC across the probes).

Off to the drawing board...

Will report back, as I reckon a buzzer would have been a better warning option in the first place.
We know how these engines react badly to low coolant levels and the resulting airlocks and / or overheating !!

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[AngeloEvs]

If you want a buzzer or an additional LED that you can place somewhere that is easily seen (i.e in the expansion slots under the LED display window) and keep the original system intact then let me know and I will post a solution but I don't want to spoil your fun at the drawinbg board....sounds like you are keen to crack this one yourself........for some folks another buzzer of doom would be an absolute nightmare........

[ghost123uk]

If you want a buzzer or an additional LED that you can place somewhere that is easily seen (i.e in the expansion slots under the LED display window) and keep the original system intact then let me know and I will post a solution but I don't want to spoil your fun at the drawinbg board

Hey post away (re a buzzer), it might be simpler / better than my solution

for some folks another buzzer of doom would be an absolute nightmare........

Not so much of a nightmare as losing all your coolant on a hot day on the M6 and not knowing it until your engine seizes up !!!!!

.

[AngeloEvs]

Ok, first, having discovered that the temp gauges were no longer available and, like you, didn't fancy driving without a coolant level warning system came up with this...........based on the humble 555 timer in monostable mode. The LED is a 12v flashing type but you can substitute a 12V Piezo in its place. Also included a pic of it connected across the gauge with the VW system still in place and functioning as normal ....its an add - on. I seemed to remember it worked fine...worth a try and won't cost you more than a couple of quid!

Gauge + connects to Gauge Voltage regulator in side

Gauge - connects to sender side of the gauge

0v speaks for itself.......the earth nut of the gauge

Let us know how you get on, if I have time next week can make you a PCB and post it.......might even have the one in the pic floating about ready to fit.........if I find it you can have it..........

[ghost123uk]

Hey thanks Angelo

I take it then the gauge has electronics inside it then re the trigger for the LED to flash after it gets the "go" pulse from the LSCU.

I am a bit of a "breadboard" type and that looks so simple I would be inclined to build it in an 16pin I.C. holder, test it, if all OK, I would then set it in epoxy !!

Off now to dig through my components drawers, though a trip to Maplins may be required !!

Will report back.

[AngeloEvs]

Spot on, gauge has an internal monostable circuit (probably VAG coded too- what a suprise!) that is triggered by a low resistance (or negative pulse) on the sender side. Once triggered, it activates the LED (flashing type) for pre-set number of seconds. If the resistance across the gauge remains low, or it continues to receive pulses, it simply retriggers, etc, etc, etc. bliddy over-complicated system to be honest, bit like the DOPS............VAG could have stuck a float switch in the header tank and saved us a load of hassle!


E D I T: hang on a mo! thought you said (in the other thread) that your coolant level warning circuit wasn't working properly? The circuit I have posted relies on your LSCU doing its job otherwise it won't work either.........oops!.....forgot to mention that......

[ghost123uk]

E D I T: hang on a mo! thought you said (in the other thread) that your coolant level warning circuit wasn't working properly? The circuit I have posted relies on your LSCU doing its job otherwise it won't work either.........oops!.....forgot to mention that......

Aye - I intend to look at that this weekend.
I recently fitted a very good 2nd hand ex VW recon motor because the previous one leaked loads of oil into the water.
It has taken ages and dozens of flushes to get the residual oil mess out of the water.
I think the probes may be coated in oil and as we know, oil does not conduct electricity !!
So, cleaning them might fix it.

This weekend is, I hope, the time I will consider it clean enough to this time re-fill with the 50/50 mix of G12 and water too

I agree re the float switch, would have been much simpler !!

[AngeloEvs]

One other thing, when you test the LSCU by removing the plug at the header tank there is a delay of a few seconds before its triggers the coolant LED....so don't expect it to start flashing immediatley.