Everything on the 1998 S90 Automatic Climate Control seems to work fine, except that the air-flow management is not working. Eeven though little flow actually comes into the cabin, there's enough sound and air leakage to know that the blower is fine, and that the A/C and heater work. All indications are that the control head is fine--for example, when the middle (air-flow) **** is turned to the windshield, the recirculate light goes out, indicating that the signals from the switch are performing as they should.

I've gone through the vacuum lines under the hood, replacing any that are perished, but I've not (yet) found a vacuum problem.

How should I take the next diegnostic step? What are the likely causes of failure, and where should I dig to find the relevant components? I have recently repowered the car with a junkyard engine which, with new timing gear, is working great, but to get full use from the car I need the HVAC system working right.

Any suggestions are very welcome.

 

You should check if all blower speeds are working. Go thru them one by one. If air-flow increases as you turn the **** up, then you know you don't have a blower or blower resistor problem. This controls the speed of air-flow.

Secondly, check if air flow changes direction when you accelerate. If it does, you have vacuum problems.

If not, read on. There are three common causes with HVAC in these cars.

First, solder points on the Control Panel (the one with *****) are known to weaken with time, and system will work erratically.

Secondly, the same defect rears its ugly head at the Vacuum Controller (which is behind the glove compartment). Solder joints on this unit weaken too.

Both issues can be fixed by re-flowing solder. Cheap and easy if you can solder.

If these steps are performed I would pull the HVAC codes. Do you know what to do that?

Lastly, there is an inherent system weakness. Some people think (and I tend to agree with them) that the vacuum reservoir is not sized properly in these cars. Some people have fixed simply with installing a bigger reservoir.

But first, you should go thru the earlier steps.

 

Henry:
Thanks for your ideas. It's also great that you have an S90 since I need some guidance specific to that model. Here's my what I have on your suggestions:

1. All blower speeds work progressively faster 1-5.
2 There is no change to vent flow direction with acceleration. The problem is that there isn't any flow direction as such. The air flows by leaking through all vents at a just-detectable flow. Selecting fresh air or recirculation makes no difference. Eventually the heater warms the car nicely (in 50 degree weather), cooling is really just an absence of heat after a while, and I'm keeping the AC turned off because I don't want to freeze the evaporator due to no flow through it.
3. The solder points seem a promising place to go, and here's where I run into trouble: I have the glove compartment out but can't identify the vacuum controller (is it the unit to the left with the multi-colored lines coming out the top?). With the control unit, I haven't figured out how to remove the fascia. I've removed the two screws under the ash-tray, but the only other fasteners I see are double rivets on each side high in the "radio" cavity. Don't want to break this. Also I want those two screws back so would like to know how to remove the trim around the gear selector.
4. No I don't know how to get HVAC codes, but don't have any HVAC error signals at this time. Something I should check?
5. Also curious where the vacuum reservoir is? I thought I would see it behind the glove compartment, but nothing obvious, to me anyway.

I much appreciate your response and welcome any further guidance and ideas.

 

You can download my Repair Manual and documents here.


Before you get into electronics, you should do one vacuum test -- check if there is vacuum at the "Vacuum Source" hose on the Vacuum Controller (see diagram, part of the files linked above). It should be 18-20 inHg, the same as manifold vacuum. You can use a Vacuum Gauge like this Fuel Pump and Vacuum Tester.

If vacuum is at that hose, then you know you have a problem with electronics. Of course, check the fuses (see wiring diagram).

You can remove the Climate Control Unit by following these instructions (Rspi: thanks) Instrument panel removal instructions on a Volvo 960.


The how-to is in one of the documents of the Repair Manual I linked above


That's shown there too.

 

This is great information and I wil be running tests shortly now that I have a vacuum tester. Thanks--I'll update you as I go. Does it make sense to cut the vacuum supply line to test for vacuum, and then use a joiner? Otherwise is there a better way to tap into this line? Perhaps it would be obvious if I unbolt the unit but from the drawings it isn't clear that there is an accessible connector on the unit, and I'm not keen to disturb anything more than I have to.

 

No, you can just unplug the "vacuum source" hose from the Vacuum Controller, connect that hose into the Tester instead. I would try to minimize vacuum joints -- Volvo does not use sizes found at standard Auto Parts store (Autozone, Advance). Generic hoses are either too tight, or too loose.

 

I was able to practice on a scrapyard S90 in Windsor, CA, and also came home with a vaccum controller and an ACC controller for under $20 on half-price weekend! I dismantled the glovebox area, including the lower trim and the airbag on the scrapyard car, and tried to remove the evaporator cover / vacuum reservoir to try to figure out how to trace the vacuum supply tube from the firewall to the controller, but without success.

Back home, on my car, I disconnected the vacuum manifold from the controller and tested the vacuum supply--no supply under any engine condition; didn't move the needle at all. So it seems my problem is in the supply between the engine side of the firewall and the vacuum controller--the bit I don't know how to access / inspect / work on.

Any thoughts on how to proceed from here?

 

As shown on Vacuum Diagram I linked above this is the vacuum path: vacuum tree on intake <--- check valve on firewall <--- reservoir <--- controller.

You should go downstream -- check that connections at the reservoir are good. Then make sure that check-valve at firewall is installed properly, and it works, and so on.

Maybe someone worked on the engine and unhooked something? You need to trace it now. Again refer to the documents I have uploaded. There should also be an annotated photo the with the vacuum valves at the firewall.

Now that you have the Diagram and the vacuum Gauge you should be able to troubleshoot.

 

Humility is a wonderful thing. Time to fess up--I got the problem resolved, and it was simpler than I could have imagined. I was getting 20 inHg of vacuum at the firewall, but it took about 15 seconds to build up. I assumed that was normal. It now seems that the only reason I got to the specified vacuum there was due to leakage through the check valve closest to the vacuum tree on the inlet manifold. The check valve had been reversed by my mechanic during engine replacement. The flow would perhaps never have given usable vacuum in the reservoir. I shall be talking to my mechanic!

Easy fix, and although I now know a lot more about the vacuum system, it is a reminder to double-check the basics and think through the probable causes before major surgery.

 

Check valve should not be leaking. Glad you fixed the problem. My dad used to say "in 95% of the cases it's the obvious thing".

Also, it sounds like your engine system is healthy.