740 and 760 - differences in air-conditioning systems?

[tofufi]

Hi folks,

Does anyone know if there are many differences in the HVAC systems between a 740 and 760 of the same age?

I know the dashboards are different, but are the heater/air con associated bits under the dashboard (blower motor, controls and all of the pipework etc) different too?

Cheers,
Jim.

[Laird Scooby]

There are some differences but it depends what age you're looking at! They seemed to change some things on a yearly basis, for example the 740 had manual control but the 760 was automatic ECC (Electronic Climate Control) one year, other years they were both semi automatic, it's a minefield trying to work out what's what and the evaporators (the "radiator" that makes the air cold) changed at will from what i've found as well!

[tofufi]

Thanks for the reply. I've possibly got an option to get hold of all the bits from a late 760 diesel, which has the ACC (the later fully automatic one). Wondering if it'd be possible to get the bits to fit my 740 diesel, which currently has heating only

I'll keep posting with progress if it goes ahead...

[Laird Scooby]

Good luck with that! There will be a lot of wiring and other shenanigans - the headlining will have to come out for a start as the temperature sensor for the ACC lives above the interior light. It is vacuum fed to pull air over it from the car to get a true reading.

In all honesty, if you want air-con, you might be better off buying the late 760 and sorting out its other problems rather than trying to retro-fit from one to t'other.

If you do go ahead and then change your mind after buying the 760, i might be interested in the evaporator for mine if it is just a spares car.

[tofufi]

The 760 is a saloon and not been on the road for 5 years, I'm told.

My 740 is an estate, has a great interior, cruise control and is very clean. I prefer the 740 controls to the 760 also.

Not seen it yet, will assess tomorrow and go from there. If it'll all fit, I'll happily do the conversion

[tofufi]

So, I've been playing with the 760 air-con donor today...

The blower fan works but noisy. Control panel illuminates and seems to function as expected - BUT - the system doesn't seem to change the flow of air regardless of position of the switch to control the airflow.

With the fan running, you just get air coming softly from the footwell, face and windscreen vents.

When you rotate the control for air direction, you can hear a single click noise from under the dashboard as it reaches each switch position.

Does it require manifold vacuum (i.e. engine running) for the system to control the air direction? Getting the engine running on the 760 will be a challenge as the flywheel is missing several teeth so the starter won't engage (and it's an auto, so can't bump start).

Thanks

[Laird Scooby]

First point - you never, ever get inlet manifold vacuum on a diseasel engine. The vacuum source for the heater controls will come from a vacuum pump, either an electric one mounted somewhere for the purpose of providing a vacuum for the brake servo, heater controls and cruise control or sometimes a vacuum pump mounted on the back of the alternator. If you can find this pump and work out whether it's electrically or mechanically driven you can then set about testing your 760 system.

If it's mechanically driven and there's little or no hope of starting this engine, start your 740 and take a vacuum hose from that and temprarily connect it to the 760 main vacuum hose, wherever it may have been hidden. This should give you enough vacuum to test the system

However, you've pointed out you can hear a solenoid clicking as you change the position of the air-flow direction switch. This is good news!

See the square, silvery box on the right?



Here's a close up :



That box contains the vacuum solenoids that control the vacuum servo-motors that alter the air direction and temperature. Inside is a circuit board with 8 (from memory) vacuum solenoids. I think only 6 are used but :



And another close up :



You can see the dry joints that have failed - this was why the previous owner of mine decided the evaporator was blocked and had it removed!

Half-hour with the soldering iron would have saved him a big bill and the air-con evaporator in mine! If you are hearing a click for each position, this solenoid block is still working but while you're swapping the bits over, check those joints!

The big black "4 balls in a sock" plastic thing is a vacuum reservoir so if you have a hand vacuum pump you can pump down on the hose for this and prove the system but i would think it works from everything you've said.

If memory serves correctly, some of the ECC components had elbow vacuum connectors, when you fit it, don't push these too far on as it will shut the vacuum off in the elbow and cause you all kinds of headaches trying to find the fault!

[tofufi]

Originally Posted by Laird Scooby

First point - you never, ever get inlet manifold vacuum on a diseasel engine.

That's a good point - I'd forgotten that

Thanks for the comprehensive and informative reply - I'll take a look at it when I can and report back

Just looked in more detail at VADIS, and it appears that all of the under-dash components are completely different between 760 and 740. So I may not actually be able to use much of what I have here anyway, but have to get them from a (1990-on) 740/940 instead...

[Laird Scooby]

If you're going to have most of the dash out anyway to swap the heater/air-con boxes etc, it might just be easier to fit the stuff from the 760 that you've got.

As far as i know, the 1990-on 740s are the facelift versions so most of it will be different to the older 740 - what year is the 760 diesel? If it's 88-on, chances are most of it will look the same so shouldn't look out of place.

If all else fails and you can't use that evaporator i would still be interested if it's the right one.

[tofufi]

Originally Posted by Laird Scooby

If you're going to have most of the dash out anyway to swap the heater/air-con boxes etc, it might just be easier to fit the stuff from the 760 that you've got.

As far as i know, the 1990-on 740s are the facelift versions so most of it will be different to the older 740 - what year is the 760 diesel? If it's 88-on, chances are most of it will look the same so shouldn't look out of place.

If all else fails and you can't use that evaporator i would still be interested if it's the right one.

Looking on VADIS, the heater systems are completely different and connect to the dashboard piping differently.

Here's 740 without A/C:



740 with A/C:



760:



The trunking under the dash appears to be different, and the bit that connects the heater box to the 'tree' (7a in the first pic, 17 in the last pic) where it goes to the vents is at an angle to the car on the 760, but aligned across the car on the 740. Or at least appears to be - I'll take a look at the actual vehicles in the next few days