I got my car running a few days ago which is what I was focused on before proceeding with any other work on the car.
However, as soon as I turn the key to the 'ON' position, the blower motors turn on.
Being a Delanair Mark 3 system which I am unfamiliar with, I am just throwing this out there to see if anyone with the Mark 3 system has seen this before.
I received some Mark 3 info from Doug which I am going to review before I start disconnecting anything.
What I do know so far is that the control unit was just hanging under the dash with 1 of the 4 plugs disconnected, and it is apparent that the PO was working under the dash behind the glovebox so I do not know what he did.
It is also blowing only HOT air. ( Could be the Heater valve or Vacuum source to Valve)
I am more concerned with the electrical portion of the system and want to resolve that first.
It does go through all of the speeds and responds to the DEF mode as it should.
Any ideas of where to start diagnosing this system?
Thanks.

control module and PO yellow wire nut connection. I will see where it comes from and where it goes

At the top of the picture you can see the disconnected plug behind the red and blue wires.

 

From what I understand, the electrics in the fan fail to ON... From memory, there is a capacitor and I think a resistor that need to be replaced.... See this link...

 

Thank you Jay. I will start looking at the system in the next few days. I will post my findings as I go along.
I still have to study the Mark 3 system before I get to the blower motors.
NB: I mentioned in the post that both blower motors work and the speed changes as I select 'low', 'auto', 'high' and def.
I will start at the fuse panel and work my way in.

 

I'm sorry. I was confused...

If you're saying what I think you're saying,,,, is that when you turn the key to position ON,,, you get a short blast of air from the fans/blowers - depending on how long and to what degree - I think that is normal. When I turn the key on in my car the fans kick on for a split second, then to OFF. No blowers, no air from vents.

Only blowing hot air might have something to do with either a MAIN vac line that delivers vacuum to the whole system and I'm pretty sure (please correct me guys) that the default, when the hot water tap at the rear center of the engine bay is getting ZERO vac, I think it defaults to wide open... That MAIN vac supply is supplied by a vac hose off of the BACK of RH Intake. Look around there and see if you can find it. If you use your mouth and apply vac to it, if it holds vac, you might get lucky and watch the arm on your heater valve move to the closed position.

I think you have ZERO vac in the HVAC vac system itself.

Let see what others have to say.

I wouldn't mess around with that HVAC ECU until you are sure you need to. They are kind of expensive and hard to find.

 

If the blower motors on the D III come on with the key, even though the aircon is switched to "off"; then that is a classic symptom of the Darlington transistor in the electronics actually on each fan unit having failed.
If your search Darlington transistor on here and the XJ saloon classic forum, you will find info about the fix.

 

If you are getting nothing but hot air, the vacuum system fails to a heater mode to assist in pulling heat from the block as a failsafe.
I have a 1988 as well (HESS) and I can confirm that the hard plastic vacuum lines are VERY brittle. I had several broken on mine - appears to have happened when the P.O. installed a new radio that was just a little deeper than the original. Of course, what a better place to install brittle vacuum lines than right behind the radio?
Attached is the vacuum schematic of the DIII system.
Also, you mentioned H & E in your heading - is yours a HESS? if so, you have an extra board on your AC amplifier. the only thing it does is turn on your AC when the car comes on (Compressor) for fuel cooling. It is actually fitted on the amp like an "add-on card".
Hope this info helps. for this portion of your issues.
Your other issue that your fan comes on (in the cabin) look at the capacitor that everyone is talking about!

If you find a break, you don't have to replace the whole line, just use a small length if rubber vacuum line that fits tightly over the hard line to fix it.

 

To the responders;
Key 'on', fan switch 'off' = blower motors working.
The blowers respond to the different speed options.
@ Jay: I have no plans of messing with the HVAC ECU. I just want to know where the disconnected plug goes.
When I go into the car I will take a pic of the HVAC ECU connections. When I got the car 3 were connected to the ECU and 1 was just hanging there. I want to know where the disconnected one goes before I use the trial and error method.
@ azbrye: very helpful schematic. Thanks.
If you can take a pic of the connections to your ECU, I would be grateful.
@ Greg: At age 72, I can no longer contort my body to do the work on the blower motors. The one behind the glovebox is OK.
Driver side is a different story, unless I pull the steering column and the whole dash. What say you?

 

Me neither at 73! You will have to remove the blowers, and removing the column makes it much easier. Getting the ducting back on is VERY hard too, there is a thread about this on the classic XJ saloon section.
Actually removing the blowers is also a royal pain. They are held in by two metal bits that come down each side from above. 1/2 a/f undoes the bolts on the fan, which thankfully have captive nuts. then you MUST wedge open the bottom door of the fan unit with a piece of wood etc. This releases the top door and if not done the motor will NOT come out.
Then you pull and pull rock the unit with all your strength, all the while lying on your back in the footwell, and you will swear the fan is still bolted in somewhere, but it is not and eventually it will come out!
You will have to search how to replace the Darlington transistor as I know nothing about that at all, my car thankfully having the DII system. I am ASSUMING you have to remove the blowers to do this fix, but check in case you do not!

 

@ Greg: Fortunately, I have the seats out of the car and I have to do some work on the steering column as it does not lock in place.
Right now I am just gathering all of the necessary info on the Mark 3 system before I go tearing into the system blindly.
I am still searching for a photo or schematic of the connections to the Mark 3 HVAC ECU.
There are 3 open slots on the ECU where the disconnected plug can go but I don't want to chance connecting it to the wrong slot and destroy the ECU in the process.
I do not know for sure but it is possible that the disconnected plug can be causing the system to go into the default 'FAIL' mode.

 

These may help