I noticed on a run yesterday that the cabin was getting cooler over time and realised that the left side vents were blowing colder air, and the right side warmer air. Today, I took it for a spin and initially through the warm up, it got properly hot on both sides for a while, then changed to pushing out different temps on each side.

Its got me a little stumped at the moment, and i'n open to suggestions / ideas / thoughts.

A while back, I fitted one of the newer style control panels that has the heated sets integrated into the temp dials. I thought this may be the source of my problem, and swapped it back out with the old original panel. I get the same issues with both panels.

I'm using the IIDtool to look at live values. On the face of it, the AC side of things appears to work ... the evaporator sits down at 2-3 degC or so, clutch is engaged, and the outside temp and cabin temp appear to read correctly too.

To try and illustrate, here are a few pictures of the values. Can had been out and about, and was unto temp, and then sat idling on the drive for these:
1st row - fence duct temp sensors
2st row - foot duct temp sensors
3rd row - temp blend motors measured position. higher value seems to equate to higher temp position
4th row - distribution motor measured. low value seems to be towards face ducts, higher value to floor ?

(I chose the measured position readings as the desired value and measured values always seemed to catch up with each other at normal rates.)

Firstly - set to LO on the dials.

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I had very similar on my SDV8 and it was the diverter valve on the FBH, check the temp of the 2 water hoses going upto the EGR from the FBH, they should both be the same temp, I had one hot and the other cold and it caused differing temps on either side of the cabin. New diverter valve and normality resumed. L405 SV Autobiography LWB SDV8, finished in Aston Martin China Grey by ETO division.

You might be missing some teeth on the blend door actuator. They are plastic tooth and drive so do commonly wear out.

The other thing worth looking at is your system pressure with the compressor running to ensure that is in spec with the H1234yf pressure/temperature data. You can download charts online for this information.

I glanced at the pressure values last night and they seemed ok, plus the cold air is getting fully cold evenly so I’m less inclined to think this is the issue at this time, but I’ll dig out the charts and compared just in case.

4RRS

I had initially discounted the FBH valve because of it feeding a single coolant supply and because I was getting side to side differnence that didn’t make sense. However , with a bit more thought you could be right .

From cold, both sides heated up and got properly toasty, and then it changed to being different. In hindsight, this could realistically match the time that fuel burning heater was running. So if the valve were stuck, the fbh did all the heating and got both sides warm enough. When it goes off, the stuck valve the prevents enough flow.

This would also match how neither sides get properly warm even when set to HI on the dials.

I’ll do a bit more investigating… and source a diverter valve by the looks of it. V8 or else ...

Last edited by fisha on 29th Mar 2025 10:59am. Edited 1 time in total

I had just the same, hot both sides initially and then heat dropping, more on passenger side but drivers side still wasn't hot even on high setting. It also affected the rear climate, which kind of makes sense because the valve is isolating the interior from the coolant loop to aid cold starting.

It puzzled me for a while, flushed the matrix and all sorts until I hit on the valve, took it out and tested it and it seemed fine in operation but still didn't work. Took it out again and it seems that the rubber o ring that the valve plunger closes against had swelled up and was restricting flow, fitted a new genuine part and all was good.

Cheap fix for once!! L405 SV Autobiography LWB SDV8, finished in Aston Martin China Grey by ETO division.

I think in my mind it could be either the diverter valve, or the heater matrix, or possibly both.

If everything was up to temperature, the diverter valve was open, and the temperature set to HI, I would expect vent temperatures more like 60 degrees. So maybe yes, the diverter valve is not allowing the coolant to flow from the engine circuit correctly.

I think with the heater matrix, if the coolant flow was slow, I still wouldn't expect to see such a big disparity between left and right sides.

Could you enable the timed climate, let the FBH burn away, and see what the temperatures are for each side? If they are higher, then that might be a sign that the diverter valve is not functioning correctly. If they are even on both sides, then that might be a sign that there is no problem with the heater matrix. If there is a difference between each side, then maybe you also have a heater matrix blockage.

Last edited by hmallett on 30th Mar 2025 11:54am. Edited 1 time in total

All valid points and similar to my thoughts, but the more I think about it, the valve makes the most sense.

Low flow through the valve will reduce the temp in general … as you say I should really be getting 50+ degC but I’m not.

As for temp difference across left to right on the matrix, if the flow is so low I think that’s very possible for it to happen. The matrix is described as a dual pass matrix and the flow into and out of the matrix is on the right. So coolant passes right to left then back again. Hot coolant in on the right gets hit immediately by the 3degC cooled air that’s just passed through the aircon evaporator. That cold air will wick a big chunk of the temp out before it gets a chance to flow across to the left hand side where it continues to get chilled further by more air. It really would explain the left-right difference.

Even in a house central heating system, rads are designed to drop 15-20deg across them at full flow in a room that’s already warm and the air drawn across it is by convection alone. In the car, you’re force blowing markedly chilled cold air across it with a low flow so it is easily possible I guess. V8 or else ...

Totally this, plus if the matrix was blocked why would you get even heat initially. I was pleasantly surprised it was such an easy fix, expected to be pulling the dash to change the matrix at one point! L405 SV Autobiography LWB SDV8, finished in Aston Martin China Grey by ETO division.

So drove home from work just there and logged it … pretty conclusive to what we’ve talked about. Monitored vent temps, ambient and evaporator temps as well as whether flame detected on the fbh.

Had the temps set to HI on both sides, fan about 3/4 and after I could see the difference appearing I put the fan to max and windows down to put as much load onto the heater matrix as I could.

So the left and right side are equal temperatures while the FBH runs, so to me that says that the heater matrix is fine, and the diverter valve is the most likely culprit.

Brilliant data driven/verified diagnosis, and shows the lovely graphing feature of the GAP tool.

dupe post. V8 or else ...

Last edited by fisha on 3rd Apr 2025 9:51pm. Edited 1 time in total

So to close out.

Changeover valve fitted. (A clone one with the ribbed hose connections - comments about that below.)

Not the worst of jobs, and took a couple of hours taking it easy. Raymies Garage on youtube is good for following how to do things in general. I found that you dont need to remove the front wheel. In off road height, you can pull rear half of the arch liner out the way enough to get to the bolts of the FBH exhaust. At the lower end of the exhaust box, you only need to undo the nut underneath the box part, and not the side part. ( Shown on raymies video ). Also beware the big electrical connector is live ... whilst moving about the unit with it disconnected, the bracket went into the connector and sparked. So just make sure you tuck it well out the way when disconnected. I think that potentially, whilst a lot of places list multiple compatible part numbers, there may have been a revision of hole locations between part numbers, which on the earlier L405 makes a difference.

Net result is fixed heating. Graph below doesn't show warm up using the FBH, but shows it mid-run with engine upto temp.

First section, set at at 27degC AUTO, so for tMidhis auto setting the foot vents blow more than the face vents, but even still the face vents are properly hot.
Second section, set at 21degC AUTO, and you can see the face vents regulate down to lower temp. There is a difference, but I think that is because the fans were blowing so low speed, so difference could build up.
Third section, set back up at 27degC, face vents only, 3/4 fan speed. Shows properly hot and equal temps.
Forth section, set back at 20degC , face vents only, 3/4 fan speed. Again equal temps both sides.

Put it in Auto and set to required temperature --- simples  2018 SE SDV8 4.4 Byron Blue

The point here is that it can't maintain the required temperatures.

I do that.

My wife however... cranks it to LO, or HI, but turns the fan off cause she doesnt like the air blasting in her face .

Me: "How the do you expect it to change temp if it doesn't blow any air?" is met with shrugged shoulders !

I've given up. Its a pity that with the memory seats it doesn't also remember air-con settings too !

 V8 or else ...