Thread: Super annoying low coolant alarm going off until car warms up (earth issue??)

Originally Posted by Young Angus

The alarm actually goes silly right when I start driving and stays like that until the engine warms up....So when it warms up it turns off. It only carries on while the engine is cold.

Even after letting if cool down just a little it will fire back up again as I drive off until the engine warms up properly again.

OK.
I am pretty certain then that your cooling system needs to be bled to remove ALL the air from it, This is very easy to do

Going from this description

Originally Posted by Young Angus

.... the low coolant alarm keeps coming back on softly...Then loudly...Then it goes up and down in volume with the indicator or it gets lower if I put my foot on the brake or turn on the lights then it gets louder again...It's driving us bonkers and it doesn't go away until I've been driving for a while, the kind of while that leads me to think it's at operating temp.....

... and the section where Ben describes it started or got worse with the installation of the lights and UHF ...

Originally Posted by trout1105

....
What you are describing is consistent with there being air at the top of the coolant system when the engine heats up and this lack of coolant is setting the alarm off.
....

Originally Posted by trout1105

OK.
I am pretty certain then that your cooling system needs to be bled to remove ALL the air from it ....

Seems improbable.

The mechanic added an earth between rad and front of body, and it appeared to settle for a bit then started again with the install of the lights and UHF.

While it'd make sense that a bleed may be a cause, it's less likely to be the case in this instance due to the new electrical work done.

I dunno how these devices work myself, but the advice to pull out the sensor and give it a clean is probably the best start point.

Originally Posted by AK83

While it'd make sense that a bleed may be a cause, it's less likely to be the case in this instance due to the new electrical work done.

I dunno how these devices work myself, but the advice to pull out the sensor and give it a clean is probably the best start point.

These low coolant alarms work by detecting the fluid in the cooling system NO fluid and the alarm goes off, Simples
They are also designed to work in a corrosive environment so unless this particular alarm is several years old the sensor should be good to go.
If young angus removes the sensor when the engine is cold before startup and looks to see if there is indeed enough coolant where the sensor is fitted he can see if it is the sensor or a low coolant issue.
Anything else is just guesswork.

Originally Posted by trout1105

....
If young angus removes the sensor when the engine is cold before startup and looks to see if there is indeed enough coolant where the sensor is fitted he can see if it is the sensor or a low coolant issue.
Anything else is just guesswork.

The way the Tdi cooling system works is that if there isn't enough coolant in the radiator at the bung, the obvious telltale sign is a totally empty coolant reservoir(unless there is a major blockage of some type).

The coolant reservoir empties into the radiator via a small bleed sized hose, off the threeway junction, that Ben recently had renewed.
Could be a faulty threeway junction .. but what's the chances of that?

So in an ideal setup(ie. no blockages, etc) if you remove the radiator bung, coolant will continue to overflow out of the access port until the level of the reservoir drops down to the that of the radiator bung port.

If the radiator bung is removed to check coolant level or clean the sensor, there is a 99.9% likeyhood that air will get back up into the thermostat housing and the cooling system WILL need a bleed.
Bleed sequence is easy to do if you use blknight's method.

My understanding of some of these coolant level things is that they run a very small current, and the contact with the coolant completes the earth path(as water/coolant is conductive).
Considering the recent electrical items fitted, I'm more inclined to think it's an electrical issue.

eg. if the coolant alarm runs off one particular power feed, and it's sensitive to how much current/resistance is required for it to complete this power path, then if new electrical items have been tapped into the same power feed(like a UHF and or driving lights) then theres a good chance that those new items could be causing false(lower) current/resistance readings.

Electrical issues do not disappear when a motor warms up. When the motor warms up, the thermostat opens. I'm inclined to think there is air in the system and it needs bleeding (as others have said). I have a low coolant alarm and it goes off on mine now and again due to a weeping welch plug (in the process of being replaced now).

Tom.

I probably know the least amount about things out of everybody here, but it's great reading that's for sure

They all sound like great points but the overwhelming consensus seems to be bleed the silly cooling system and at least rule that out. Btw it's an Engine Saver little black module under the steering wheel where those fuses are if that makes any difference.

Now how to bleed a cooling system on a 300Tdi...hmmmm

I'd suggest you look carefully for a tiny leak. Perhaps a coolant hose has a pinprick. I had one in my bottom hose, under the jubilee clip, so it took months to find it. I only noticed it because I saw a crusty build-up coming through the slots in the jubilee clip band. I had exactly the same symptoms as you, and same engine, and same alarm.
i figured that the hole is not big enough to allow a great amount of coolant out, but it does allow air to be drawn in as the engine cools. The bleeder hose eventually bleeds that small amount of air out as the engine warms up and the alarm goes off. Since I've changed my bottom hose, the alarm is functioning normally.
Don't forget to check heater hoses and heater as well.
Don.

Originally Posted by trout1105

If your coolant alarm is constantly going off I suspect that you have a problem with the cooling system and Not the alarm itself.
You say that it self test on startup, this is Good.
The alarm doesn't activate until the engine warms up, This is telling me that the alarm is working properly.
What you are describing is consistent with there being air at the top of the coolant system when the engine heats up and this lack of coolant is setting the alarm off.
Does the level in your header tank fluctuate? Have you tried bleeding the air out of the coolant system? What New work have you had done on the engine?

Thought I should clarify, part of the new work I had done was the complete removal and flushing of the cooling system, radiator got shipped off to a radiator place and tested and cleaned out then brought back and re-fitted, all new radiator hoses (most of them I think) and of course new coolant top up...so part of this would have been to bleed the system so I guess that's a possibility.

It's just odd that the problem went away after they got the car back and did something to the earthing...then it came back again after more work had been done...none of which makes sense when we're talking about the much more sensible possibility that there may be a little air in the system until the thermostat opens up.