Club 80-90 Forums

Following on from a topic this evening on coolant tanks, this occurred to me =

How many of us have pulled the plug off the top of the main coolant header tank (engine running) to check that the low coolant warning light on the fuel gauge works ?

I ask because on two occasions I have had a narrow escape from engine disaster (until I figured out the reason) when coolant pipes burst or blew off (note, this was years ago).

Since then I have always made sure this system is working.

I have had 3 T25's and it may have been just bad luck but I had to fix this function on ALL of them

1 was a faulty control unit (number 43 on the relay panel, note, it is NOT a relay but a quite complex circuit) - 1 was oil on the coolant sensor probes and 1 was a simple wiring break (the earth wire).

Test em folks, it only takes 5 minutes

can you give us a step by step?

engine running - remove coolant sensor from top of coolant header tank -
low coolant warning light on the fuel gauge should read?

is there any way of checking relay 43?

where would the earth wire go to/come from?

for us wannabee mechanics

edoh wrote:can you give us a step by step?

Yes

edoh wrote:engine running - remove coolant sensor from top of coolant header tank -
low coolant warning light on the fuel gauge should read?

Yes

(detailed info = the temp needle should move to the right (as it would if the engine is too hot) E D I T = That last sentence has proved incorrect and should be ignored... and the little red led should flash like to does when you first turn the ignition on. Note that you need to wait a few seconds for the thing to "trigger" after you pull the plug off the header tank. Also it remains in "triggered" (warning) mode until you turn off the ignition, when it then re-sets itself.

edoh wrote:is there any way of checking relay 43?

No, because it is not a relay, it is a complex circuit that sends short pulses to one of the probes. If the other probe receives these pulses (via the coolant conducting them across the 1" gap) then the circuit in "box" 43 is happy and does nothing. If the pulses stop getting back to "box 43" it triggers the gauge to move and the light to flash.

edoh wrote:where would the earth wire go to/come from?

It was an earth connection from the "box" 43 on one that I had to fix.

Note = I have a known working spare "box" 43 to use to test vans as this part is a common failure.
The same part is fitted to many 80's injected Golf's Scirocco's and Passats (and probably some Audis too) so the breakers is your friend here, plus the guy will think it is a simple relay and only charge you £3

AngeloEvs on here knows a lot about this, and designed a warning buzzer too (I never did get one, but would still like to)

I suppose if removing the sensor connector results in the red flashing coming on, then that has tested 47? Also does it not flash for a few seconds when ignition first on I thought that was the "47 working test"?

thanks for the info chaps -

Thanks,very useful insight/information, if I ever got an engine with water in it "running"

Cheers
Andy

ghost123uk wrote: Yes

(detailed info = the temp needle should move to the right (as it would if the engine is too hot) and the little red led should flash like to does when you first turn the ignition on. Note that you need to wait a few seconds for the thing to "trigger" after you pull the plug off the header tank. Also it remains in "triggered" (warning) mode until you turn off the ignition, when it then re-sets itself.

The temperature guage needle should not move, the level sensor is just that and nothing to do with the guage, just the LED. The temperature guage sender is on the thermostat housing ... single 6mm spade (red/yellow wire) on early type system and on the front face of the t/stat housing on the late type. Put the early spade to earth and the guage should go to the top, bridge out the later type plug and again the needle goes to the top.

Ian

Module 43 (or 42) sends a continuous sine wave voltage to the level sender probes in the tank. You might be able to measure this with a multimeter though an oscillosope is better if you have one. It's the amplitude of the waveform that changes depending on wether the probes are sitting in coolant or fresh air (The reason an ac voltage waveform is used is to prevent elctrolysis eating away one of the probes and it disappearing altogether).

The module 43 senses the amplitude (how big the wave form is) and if it is larger than it should be because there is no water it triggers the module 43 to activate the LED. The temperature guage also contains a microchip that controls the LED flashing sequence. This circuit in the tempertaure gauge is triggered by module 43 - in the event of loss of coolant module 43 sends a series of pulses to the tempertaure gauge microchip which, when triggered, starts the LED flashing - and will continue to flash untill ignition off, any coolant problem sorted and ignition back on. The initial self test is also controlled by the microchip in the gauge.

The main problem with the coolant flashing red light warning system is that the entire coolant level warning system depends on 'resistance'. In a perfect system the only resistance is the presence or absence of water around the probes. However, a poor earth connection will fool the sytem into thinking you have lost coolant. There are three earths that can cause erroneous flashing LED problems -

a) the coolant level probe earth path (usually terminates under the ignition coil via various conX)

b) the module 43 earth (this terminates at the crown earth cluster next to the door pillar)

c) the centre nut of the temperature gage (this earths the microchip in the gauge)

finally, any contamination of the probes or excessive amount of antifreeze will cause problems as these will affect the resistance and the amplitude of the ac waveform and trigger module 43 into action.

As Ghost points out, simply removing the connector at the probes will, after a short delay, start the LED flashing.

Ian Hulley wrote:

ghost123uk wrote: Yes

(detailed info = the temp needle should move to the right (as it would if the engine is too hot) and the little red led should flash

The temperature guage needle should not move,

Ian

Ian is of course correct, the gauge does NOT move. I got that bit from reading somewhere that is is supposed to so the needle is not obscuring the LED from view. I have edited my above post to correct this fact.

Hacksawbob wrote:I suppose if removing the sensor connector results in the red flashing coming on, then that has tested 47?

Correct

Hacksawbob wrote:Also does it not flash for a few seconds when ignition first on I thought that was the "47 working test"?

No, the LED flashing is done from another (very small) circuit on the dash PCB. If you remove "box" 43 (is it 47 on yours ? iirc there is an early and late type, interchangeable) the led will still do the self test thing (though what it is testing god knows !)

P.S = Just noticed, I knew Angelo would be along with the definitive explanation

Another job on the never ending list for tomorrow lol

I meant relay 43 not 47

Nij wrote:Another job on the never ending list for tomorrow lol

Weekend is getting shorter, with the things to do........ bookmarked and printed for future reference

Re tested the one on our new van yesterday (to check the needle thing mentioned above) and would just like to remind folks it takes a good few seconds after pulling the sensor plug off the top of the coolant tank before the LED starts to flash = normal.

I take it you don't need to run the van up to temp before removing the sensor?