Xantia HDi Water Coolant Heater

[Rhothgar]

Afternoon,

Progressing steadily through fault codes on the Xantia and one I've left until last is the wonderful Water Coolant Heater block which tried to set on fire last year.

So...

the question today is how does it actually work?

I cannot see a timer on the wiring diagram at all.

The wiring diagrams has wiring from the battery to the ignition and via some maxi fuses and relays to the heater block.

Wires 1706 and 1707 I assume are switched earths (from memory) at the ECU which basically go to Pins 58 and 85 (again from memory at the ECU).

So assuming everything else is in order with the wiring, the ECU must contain the instructions to switch the earths back to open (and hence disconnect the control circuit of both relays) after a set period of time.

Does anyone know or has anyone ever tested how long they should be on for or have ANY useful information please?

I really don't want the coolant heater to try and set the hoses on fire again.

I would doubt that a short in a glow plug within the heater block would could cause them to stay live.

Thanks in advance.

Really want to have the function working though I have read horror stories.

[xantia_v6]

Their purpose is as Imperial states to heat the water entering the heater matrix, they are controlled by the engine ECU via two green relays located either side of the pink headlamp washer relay within the fuse/relay box behind the battery.

The unit comes into operation a couple of minutes after starting the engine and remains on until the coolant temperature reaches about 80°c.

Removing the unit is a complete pain in the you know what as it is secured by two totally inaccessible Torx screws to a metal bracket held onto the bulkhead by two 13mm headed bolts which are located under about six metal hydraulic pipe's.

How do I know all this? Well the unit on my HDi is leaking in the joint between the two halves of assembly so I've managed to remove one from a HDi in a breakers yard which was a right old palaver.

You could potentially fit one to a petrol car but I'd wonder why you'd want to, apart from the additional wiring and water pipe adaptations you'll also need to upgrade the alternator to the 120 amp one fitted to HDi's fitted with this unit.

Having now owned a later HDi Exclusive with the Eberspacher diesel burning heater and now one with the glowplug heater, comparing their heating performance, the glowplug one gets going quicker but once underway (it takes the best part of 5 minutes) the Eberspacher one produces more heat to get the engine temperature up to operating temperature, ultimately swings and roundabouts.

[Rhothgar]

Their purpose is as Imperial states to heat the water entering the heater matrix, they are controlled by the engine ECU via two green relays located either side of the pink headlamp washer relay within the fuse/relay box behind the battery.

The unit comes into operation a couple of minutes after starting the engine and remains on until the coolant temperature reaches about 80°c.

Removing the unit is a complete pain in the you know what as it is secured by two totally inaccessible Torx screws to a metal bracket held onto the bulkhead by two 13mm headed bolts which are located under about six metal hydraulic pipe's.

How do I know all this? Well the unit on my HDi is leaking in the joint between the two halves of assembly so I've managed to remove one from a HDi in a breakers yard which was a right old palaver.

You could potentially fit one to a petrol car but I'd wonder why you'd want to, apart from the additional wiring and water pipe adaptations you'll also need to upgrade the alternator to the 120 amp one fitted to HDi's fitted with this unit.

Having now owned a later HDi Exclusive with the Eberspacher diesel burning heater and now one with the glowplug heater, comparing their heating performance, the glowplug one gets going quicker but once underway (it takes the best part of 5 minutes) the Eberspacher one produces more heat to get the engine temperature up to operating temperature, ultimately swings and roundabouts.

Many thanks for this.

So taking this ones step further, does it rely on the outside temperature being below a certain temperature as measured by the external temp sensor in the nearside mirror?

I ask because I've checked the relays and they are fine after being cleaned up.

I learned yesterday that if the slider switch is set to demist (I now understand the reasoning behind this) that the AC con is switched on albeit no lamp lights on the switch which in turn switches the radiator fans on. Until it was explained to me yesterday, it just seemed bonkers that you would immediately be cooling the engine and obviously have increased fuel consumption. I guess best practice in cold weather is to get the screen clear and then turn demist off!!! BONKERS!

I'll now crawl back under my rock and cogitate on AC issues that I have.

[xantia_v6]

I believe that the auxiliary heating is controlled by the engine ECU, based on engine temperature.

[Rhothgar]

I believe that the auxiliary heating is controlled by the engine ECU, based on engine temperature.

That would make sense.

Be nice if there was info somewhere that detailed it.

In reality, it should come on at any temperature below 80 really to get temperature up to required temp. Well certainly anything below 50 degC to give it a burst.

Having said that, primary reason is screen clearing I guess and not fuel efficiency.

[Rhothgar]

I believe that the auxiliary heating is controlled by the engine ECU, based on engine temperature.

Right! I’m finally back on with this.

The earthed switched wires from the ECU have good integrity. They come from Pin 56 Green and Pin 85 Pink at the 88 pin ECU connector.

Applying 12V to the disconnected connector pins the relays click loudly. Good. That rules out the switch wires.

I might look in the ECU later to see what the components look like that control the switching but first I want to glean as much information as possible.

According to the internet, these should come on below 50degC. DickieG states there is a 2 minute delay from engine on. That seems weird to me.

Now! My fans come on as soon as engine starts which is daft and likely the fault I have with the air con system. I will move the switch away from demist, pull the fuse for the fans and the air con and see if the coolant heater kicks in. Maybe the engine fans circuit is switching off the coolant heater as they are polar opposites. The ECU maybe thinking that the engine is too hot and NOT allowing the coolant heater circuit to energise. Just a thought.

[ekjdm14]

I would assume the 2 minute delay would be to allow the alternator to "catch up" and put some juice back in the battery prior to loading it up with the heater. Especially in extremes of cold it'd be desirable to have the battery topped off asap after starting. Perhaps also due to the engine's own GP's running for a couple of minutes in post-heat function, that would be even more load.

The fans running full time is worth investigating as possibly the ECU is thinking the engine is hot all the time and not activating the coolant heater, but wasn't the original fault too much heat (relay contacts welding/driver shorting in ECU)?

Was it this car that I read about the ECU having corrosion on a couple of traces that had to be repaired? My memory's rubbish now lol EDIT- YES, yes it was the one. I'm not going (more) loopy after all! Given the history of internal corrosion I'd be tempted to go into the ECU again and trace back from the pins related to AC/fans/temperature sensors and coolant heater. Not sure if there's many electrolytics in these units but I'd imagine if there are that they'd be of the corrosive fishy type given the age. Also on re-reading that post you mentioned at the time there was an iffy looking semi on the board. Did that ever get replaced?

[xantia_v6]

A minor (current) fault code in the ECU could be triggerring the abnormal fan operation and the lack of auxiliary heating.

[Rhothgar]

A minor (current) fault code in the ECU could be triggerring the abnormal fan operation and the lack of auxiliary heating.

This is one of my issues. It was scanned 6 months after the issue and had P1403, P1404 and P1408.

If those permanent faults, cause the ECU to not switch the circuit then it could be as simple as testing the glow plugs and replacing any that are shorted.

For it to smoke like it did from the coolant block, there must be a shorted glow plug I suspect (drawing battery amps perhaps). It might be as simple as that.

I've got the ECU indoors now and it wouldn't take much for me to crack it open and have a look around as there was a few areas of corrosion last time I did a repair. That's probably easier than taking one of the glow plugs out in fairness.

[Rhothgar]

Perhaps also due to the engine's own GP's running for a couple of minutes in post-heat function, that would be even more load.

Now that is a very valid observation!

The fans running full time is worth investigating as possibly the ECU is thinking the engine is hot all the time and not activating the coolant heater, but wasn't the original fault too much heat (relay contacts welding/driver shorting in ECU)?

Switching off demister (windscreen logo) turns them off so I am going to discount them for now.

Was it this car that I read about the ECU having corrosion on a couple of traces that had to be repaired? My memory's rubbish now lol EDIT- YES, yes it was the one. I'm not going (more) loopy after all! Given the history of internal corrosion I'd be tempted to go into the ECU again and trace back from the pins related to AC/fans/temperature sensors and coolant heater. Not sure if there's many electrolytics in these units but I'd imagine if there are that they'd be of the corrosive fishy type given the age. Also on re-reading that post you mentioned at the time there was an iffy looking semi on the board. Did that ever get replaced?

You are right and no I've done no further component level work on it.

Yep. This diode is looking ugly. I’ve not traced it but it stands good chance of being a candidate.

[RichardW]

When you put it on demist the AC is switched on, which is why the fans run at low speed

[Rhothgar]

When you put it on demist the AC is switched on, which is why the fans run at low speed

Cheers for acknowledging that. The AC isn’t engaging but there is a corroded terminal within IC02 on the nearside arch so I guess the power for the AC is getting that far. I can double check that when the time is right.

I reckon that one glow plug on each relay is shorted to earth at the heater block. Hopefully the ECU cuts off the switched earth to the relays so that it is protected. I’ll power the relay independently tomorrow and check for voltage on the earth side of the heater block. If there is any voltage then there’s a short to earth.

At that point, I have a go at trying to remove one glow plug at a time.

I fear that easier said than done. A combination of bimetallic corrosion and the heat produced when it went awry may mean they will snap. I’ll PlusGas them to help them along.

[Rhothgar]

I must say that was an interesting exercise!

With ECU disconnected, I decided to power the relay output ie pin 5 wire which goes to the coolant heater block whilst the glow plugs were still connected.

The female spade connector in the plastic holder glowed bright orange within no time. I believe this suggests a short to earth on that wire somewhere.

On the other relay no such response despite them each set of two glow plugs at the block being strapped together and earthed via the body (and a separate earth wire).

Removed all glow plugs, one showed open circuit the other three were 1 Ohm measured between the threads where they screw into the block and the face where the nut come for the wire screws onto.

Well get this!!! I tested each one and every single one glowed. I noticed that the actual thread and nut area on the one that shows OL get really very warm as compared to the others suggesting some kind of internal intermittent fault perhaps.

Next. The markings on the GP are AET 11 721 068. Can anyone provide me please with an equivalent reference number please? They only have a 29.5 mm. long heating element. I can find references to other AET numbering online - just not this one.

[Rhothgar]

Just an update to this specific post as I think I probably posted the update on my main blog sometime around 7th Dec 2025 onwards.

Basically, it turned out to be a loose nut at the terminal connecting the thick white wire at the heater block on the bulkhead.