I've been reading the service manual (rubbish) and searching around, but I can't find an answer. The heater core may have been replaced before I bought this vehicle, so I'm not sure if things are right.

My vent selection flaps do not appear to work properly. For some time, flow was directed to the windscreen vents under heavy acceleration. It never seemed right, but now it seems utterly confused.

Every now and again, I hear a hissing from that area, but I've not found a proper leak anywhere yet.

There is, however, a possible issue.

To the right of the driver-side foot-well, there is a silicone vacuum manifold. There are a number of vacuum lines attached to this manifold, and an enclosed diagram illustrates their configuration.

I can provide an extended diagram if possible, but for now this is sufficient to determine if the problem is obvious.

One of the ports receives no vacuum line, and I cannot find a line that may have been disconnected. The port remains open, as a result.

Now, I don't know if this port is supposed to be open. Perhaps it's got a purpose. Perhaps, however, a line is missing.

That occasional hissing, coupled with the lack of directional flap function, seems to indicate a vacuum issue down there. When tested, that empty port showed no vacuum at all.

Some of my other issues may be related, but I can't diagnose until I rule things out, and this is thing one.

 

What kind of car, exactly ? :-)

Cheers
DD

 

It's a 1996 XJS. Sorry about that.

 

I finally got around to messing with it.

I believe I've determined that the hissing only occurs when the air is directed toward the windshield.

Regardless, there is a hiss behind the dash, and it doesn't appear to come from any readily accessible location. I've had a vacuum tester all around the area, but without knowing what I'm dealing with, it's a tough question to answer.

The only place it failed to hold vacuum was on the passenger side, black line running toward the driver side.

Now, it's connected to the above manifold that seems to hold vacuum just fine, so I suspect this is an issue that only presents itself when the variables are just right.

This leads me to suspect a vacuum solenoid. How likely is this? Does anyone know just how deep I'll have to delve into the dash to get to them?

I bought the darn workshop manual disc, but I've lost the thing. Very frustrating. A vacuum diagram for the system would be very nice right now, not that the disc is easy to navigate.

If someone just happens to have one at the ready for a 1996 4.0, I'd appreciate it.

 

Hi Kennith

This is the closest I have. I am not sure if the later cars had a different system. I know the later cars have extra buttons on the heater control panel, compared to the earlier cars, but I'm not sure if that means only their electronics are different or if the whole vacuum system is different. The Delinaire III system was fitted to XJSes from roughly 1987 onwards..... I have a 92 V12 and this diagram helped me diagnose that I had a bad solenoid



There is a mistake in this diagram (of course). The green vacuum line doesn't operate any side vents on the XJS, it operates the demist/windscreen vents.

You said you were looking in the drivers side footwell...... there should be 2 vacuum solenoids there and another 2 on the passenger side. To get to them you need to take off the trim pieces on the side of the transmission tunnel. These are held in by 2 screws that go through the airvent. You then need to take off this airvent. This is a real pain without taking the whole centre console out but it can be done. Once you have those vents out of the way you will be able to see the solenoids. They sit on a metal plate which you can unbolt and swing them both out.

You need to apply vacuum to the demist vent to close it first, then apply vacuum to the centre vent to open it. My problem was my centre vent wasn't opening, but this was actually due to the demist vent not closing first (because the solenoid had failed) To test the solenoid, check the resistance.... around 30ohms is good. My failed one was open circuit.

Let me know if any of the above helps.... there's a series of tests you can do to work out which flaps are working and which are not. For instance, when you select full cold, the demist vent should close, and the centre vent should open. When you select max defrost, the demist vents should open, the centre vent should close, and there should be no heat coming from the foot vents.

 

Well, that's a darn sight more than I had before.

I shall use that information as best I can. Hopefully the weather will be good tomorrow. If not, I'll get to it as soon as possible. I will definitely keep this updated frequently. It may help a future user.

This has been troublesome to diagnose, as I replaced the driver side blower. Shortly after, the passenger side blower died...

At that point, I ignored it all for a while. Heck with it all, right? Well, it's bugging me now, and I'm going to fix it for good. This is a phenomenal car, but it sat in a collection for a long time, and apparently it didn't like that at all.

Lord knows I've been at my wit's end sorting the little things. I can tell you this though: The 4.0 is a heck of an engine, surviving running with all those failed sensors this long.

I can only hope the same holds true for that air blender and it's vacuum bits. All that stuff is incredibly well-built, though. I've never seen so much silicone and steel behind a dash, let alone in a climate control system.

 

This should help you get the vacuum system sorted out. You have a Delanair Mark IV system, the other diagram is for a Mark III system, similar but not quite the same.

If you get the vacuum system right and still have trouble, there may be a sticking blend air flap motor, also called a mode motor/gearbox; they are pointed out on the third page. Sometimes a small rap with a screwdriver will free them up so they will work for a while.

Good luck!

 

@Steve

Thanks for supplying the exact info. I didn't know they changed away from Del III.

My replacement solenoid came off a 94 car and it fitted and worked fine, so is it just a case of same components rearranged a bit for the Del IV ?

 

I've now dropped the car off to completely sort the climate control system, so I'll report the results soon.

 

Thanks to all for this useful thread.
The AC in my 92 works, but the centre vent flap kicks in when it feels like it. Here was me thinking it was somehow related to the vacuum created on acceleration/deceleration thinking back to my old 1957 Meteor.
Argh. :-(


Gerry T.