Coolant shortage circuit probs

[RogerT]

Hi folks.
Not the usual problem of a flashing red light giving a false indication of low coolant, but no flashing red light, even when the coolant shortage sender is unplugged. There is no short on the wire that goes from the sender to the fusebox, and the earth wire is ok. I've replaced the control unit on the relay board.

There is power to the control unit.

When I have been testing the circuit, I haven't started the engine, as it's usually in the evening. So its just been by turning the ignition on.

The two control units I have are different electrically - but i guess I don't know if either works. On one of them, (the replacement) there is continuity between the "functional" terminals (as opposed to the supply and earth terminals), on the other (the original - which might indeed be wholly original to the van) there is not that continuity. The replacement I bought is this one - https://www.vwheritage.com/shop/1919193 ... corrado-j/" onclick="window.open(this.href);return false;

I don't know what to do next. Any ideas?

Thanks in advance...

[ZsZ]

Check and clean the earth point at the engine bay and under the fuse box too. In the engine bay the point is under the air filter on diesels, Left side of the bay, about half way from the black junction box to the D pillar hollow (same spot on petrol, but not sure what is around it, maybe the coil)

[RogerT]

Thanks for the suggestion, but I made that earth connection in the engine bay just a wee while ago as it was completely missing. You mention the one by the fuse box - do you mean the crowns on the A pillar on the passenger side? They are clean and remade too. And a fresh heavy cable from the starter mounting bolt to the body, and thence to the neg post. I now have a well earthed van!

Is there a way to test the function of the coolant shortage control unit? If I put a current across the supply and earth terminals should I be able to read the blips it send to the temp gauge?

[Smiffo]

Does the temp gauge work?
If so, might be faulty component that encorperate temp gauge?

It reads like you don't get the flashing light even when you turn ignition on without turning the engine over and starting it up.

Try this link below:

http://archive.club8090.co.uk/viewtopic ... 79be8f913b" onclick="window.open(this.href);return false;

( Goto 4th comment for a good diagram. )

[RogerT]

Hi, thanks, I guess I wasn't clear enough. I forgot to say temp guage works, and the LED flashes before startup as the system presumably self tests. I have seen the LED flash on overheating about 3 yrs ago. But that was also due to low coolant, and it didn't give me a warning even then that the coolant was low.

I'll read the link later , when I'm at work, early start today...

[RogerT]

Ok, I've read the thread.none possibility that occurs to me is that moisture on he coolant level sender plug is keeping the resistance low enough to fool the LSCU into thinking there is coolant present. (it got wet as the old sender leaked, and I changed it a week or two ago). So I could dry it (with someone's hairdryer when they aren't looking...) And also test by unplugging at the 7pin connector in the engine bay. That said, I can also test at K5 surely, by disconnecting the K plug at the fusebox. Only K5 (coolant level sender lead) and K 4 - aahh, no, I need K4 to take the output from the LSCU. leave K plugged in.

Anyone know how often the LSCU sends pulses? I read somewhere it was "every few seconds"... Not sure what that is. And the coolant level sender circuit should work when ignition is on but engine off?

[marlinowner]

Yes circuit works when ignition is on and engine not running, it takes a minute or two to start the light flashing if coolant level warning is triggered. If you ground the coolant temperature sender wire then the gauge will go to max, does the light flash then? If so ground the output of "relay" 42, should do the same thing.
A bit of moisture on level sensor plug won't keep the light off when it's unplugged.

[RogerT]

Thanks marlinowner, I'll try that and report back. Not tonight tho, going to see La La Land with wife and daughters! I appreciate the suggestion tho and it'll be good to "do" something rather than just hold bits of wire with a quizzical face...

[RogerT]

Update-Grounding the temp gauge sender wire produces the desired results on the temp gauge - up to the top and red flashing light. Ditto with grounding the 42 output.

So we know the temp gauge end of the circuit is pretty good.

Confirmed there is power to the 42 LSCU, and it is earthed.

Confirmed there is continuity from the coolant shortage sender to the 42LSCU and the level shortage sender earth goes to ground.

So does mean that the only possibility that the 42 LSCU is banjaxed? Both the new one and the old one? The new one came from VWH, BW don't stock one

Or is there a way to check the output of the 42 LSCU (well - maybe I should just see if it is measurable, but if I get no reading that could still be because it isn't measurable with the electrical tester I have.)

[marlinowner]

It would be nice to try a known good 42 in your van. If you like you could post your two down to me, to test in my van, assuming there is no-one local to you way up north - pm me if interested.

[RogerT]

That's a kind offer - if they work then I know they aren't the problem. There's no one I know locally, there was one chap nearby but he sold his and don't think he got another yet. I'll pm you.

[Smiffo]

Are the terminals clean where the relay/control unit sits?
if you are sure the wiring between the coolant level sensor is OK, and gauge is OK, then it could be poor connection when plugging control unit in?

Kind of trying to think of all possibilities here to help...

Also, are you sure the wire from control unit to coolant level sensor doesn't have an earth fault on it?
( That would mask a low coolant level state, I think. )

[RogerT]

I think you're right, it would mask a low coolant level. I've checked and there's no continuity between either end of the wire and earth. I'll dig out a skinny file and clean the terminals on the relay board. Thanks again for twisting your head around it.

[RogerT]

UPDATE- Marlinowner tested both my coolant level control units - both faulty! Contacting VWH in the morning to get for another new one. Many thanks to Marlinowner, and the rest of this community. The good news is that for the first time in the four years I've owned this van, I seem to have no coolant leaks!!! 100 miles brim full now! Getting cramp from driving with my fingers crossed...

[Smiffo]

Well done that Marlin man..!!