740 and 760 - differences in air-conditioning systems?

[tofufi]

Originally Posted by Laird Scooby

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!

Just a quick thanks, I've re-soldered the joints on that board as they were indeed duff - although some of that could have been from when I removed the wiring plug from it...

I've now started dismantling the 740 to see if the A/C heater box will fit... fingers crossed

[Laird Scooby]

Glad the information helped! If the solder joints were good they would have easily stood the plug being removed and refitted quite happily!

[tofufi]

So, I managed to get the A/C heater box into my 740 today.

A few points to note:

1) If you're ever tempted to do this conversion, don't. It's a ballache.

2) I had to drill a few holes through the bulkhead to fit the A/C pipes. The heater ones were the same as those for the non-AC.

3) My glovebox doesn't fit. It fouls on the fan motor. Anyone know if the 740 had a different glovebox between those with AC and without?

4) If you're still tempted to do this conversion, really really don't.

5) The panel above the bulkhead is different between 740 and 760. And they don't line up. One has an oval hole for the heater box inlet, the other a round hole. The 740 a/c heater box currently on eBay has the same round hole as the 760 one I'm using...

Wiring and many other things still to be completed.

[tofufi]

Just checked on Vadis... it appears the glovebox part numbers are different pre '90, '90 and again for '91/'92... Grr.

[Laird Scooby]

Can't answer the glovebox query but will the 760 glovebox fit?

[tofufi]

Thanks for the thought, will give it a go

Managed to get a bit more done yesterday, as follows:

Plumbed in the heater controls and managed to get all the wiring in and working.

With the engine running, the heater flaps all appear to operate, with the exception of the recirculate function. When this is selected, there is a hissing from the passenger footwell. I've yet to investigate further, but I fear the bellows may be bust. Is it possible to replace these?

I did move the flap for the recirc function by hand... is it possible that this has damaged or disconnected something?

The donor car had the heater valve (vacuum operated bit that controls the flow of hot air) removed. Am I correct to think that without this the system will always run hot unless the minimum temp is selected?

I'll be purchasing a new valve soon, is there any difference in quality?

[Laird Scooby]

Sounds like the recirc solenoid has got a leak - usually the recirc flap is in the passenger footwell and with vacuum applied opens the recirc flap so the blower sucks air from the passenger compartment instead of fresh air.

You can check for a leak on the recirc solenoid by removing the vac hose on it, pushing it open by hand then plugging the stub on the solenoid where the vac hose goes with a wet finger (lick your finger or put it in your mouth to make it wet, the moisture just helps it seal on the stub) and letting go of the the flap.

If all is well it should stay where it is, if not then you either need a new/working secondhand recirc vac solenoid or just do without the recirc until you find one. It's only used for increased heating on cold days or increased cooling on hot days so it's not something that gets used much by most people.

You'll definitely need to fit a heater cut-off valve - not a problem at the moment but when the weather warms up you'll know all about it if you don't! When i picked up my 760 (with the fault on that valve block shown) the heater valve was in the default position of allowing full flow - on one of (if not) the hottest days of the year! Worse still i got stuck in traffic on the M25 and started melting!

PS - while i've been typing this, the valve in my central heating system has just failed causing a shutdown of the central heating! Could be funny for someone else but not for me!

[tofufi]

Thanks for the help

Tried it today and it instantly returned the flap, so there's clearly a large problem with the vac solenoid.

Do you know if these are easy enough to replace? I couldn't see how they remove from the heater box...

Also, is the recirc solenoid the same as the others on the unit?

Sorry to hear about your central heating valve! Hope it's as cheap and simple a fix as the new heater valve for my car

[Laird Scooby]

You're welcome - i've never had to replace one (on a car anyway) so not sure how easy they are to change and/or repair. Might be worth a call to Volvo to see if it's something cheap and easy to get hold of.

To be honest you can probably answer the question about whether it's the same as the other servos (did i call them solenoids before? Servos! Sorry about that!) on the heater unit as you can see it but i would think they're pretty similar.

Normally i'd say check the linkage between the flap and servo but given you can hear hissing when it's meant to move i'd say there's a very high chance the servo is leaky.

My central heating will be a free fix thankfully, one of the good things about renting! The guy got here, confirmed the diagnosis i'd already worked out (from the fault code on the controller) by manually operating the servo valve and by shear chance it's working again since he did that.
However they're coming a week friday to fit a new one, controller and tank 'stat so all will be well hopefully!

[tofufi]

So, a bit of an update

I faffed around some more with the 760 heater box, but it became apparent that the 760 heater box didn't fit properly behind the 740 glovebox.

I then stumbled upon an opportunity to get some bits from a 940 which was being broken, so drove to Reading and spent a fun evening in the dark stripping the heater/ air-con unit out of a 940 turbo. I also got the vent ducting which connects the heater to the face level vents in the centre of the dashboard, as this differs between 740/760 and between 740 with and without air-con.

So, here are the heater boxes - you'll be able to spot some differences.

Top to bottom are:
940 (MCC - Manual Climate Control)
760 (ACC - Automatic Climate Control)
740 (heater only)



I followed this excellent guide to convert my 940 MCC to ACC.

https://www.volvoforums.org.uk/showthread.php?t=97912
The same guide can be found here with working pics: https://volvoadventures.com/ECC%20conversion%20740.pdf

I used all of the bits from the 760 to do the conversion, including the wiring loom. The main components are the same between ACC and MCC versions of the 940, just the ACC one has a load more sensors and automation.


With baited breath, having fitted the bits to my 740, I connected the battery. Everything seemed to work electrically, so I started the engine to allow the pneumatics to work.
I turned up the fan to 5, and started to check the vents worked in each of the dial positions, when I heard a loud bang...



Oops.

A second fan was installed, and now all seems to be good.
Spent the rest of the weekend putting it together, and did 200 miles in it today.

To my mind the system seems to slightly over-cool (I've had the temp set to 24 degrees in the car today over the 200 mile journey, but I like it warm). Otherwise, all seems good - I just need to connect up the A/C pipework under bonnet (which could be a struggle as some bits don't fit together between 760 and 940) and get the system re-gassed