Non-Z diagnosic question - Stumped

[Ricklandia]

Thought I'd throw this out here as I've had no luck finding an answer anywhere else, and we all know Z-folk are prime with knowledge...

My daily is a '91 Miata (1.6L, PS, AC). All summer long, I run the AC with no troubles. Winter has finally arrived here in S. FL (if you can call it that), at least it's cool enough to turn off the AC and roll down the windows. Now comes the strange part...

If I turn OFF the AC, under certain circumstances I lose my power steering. What makes it more difficult to understand is that it doesnt happen all the time. For example, I can putz around town all day and it doesnt happen. However, on my drive to work, I put about 20 miles of highway driving (80MPH), followed by 2 miles of slower traffic (40MPH) before I make my first sharp turn (on ramp to another highway). That's when I notice the power steering gone. I then put another 10 miles or so in semi-highway driving (sometimes stop and go, sometimes not). When I get to my destination, most times the power steering has returned, ...but sometimes not. At the end of the day, stroll to the parking lot, fire up the car and... bingo, power steering is working fine.

Get this, it has NEVER happened on the reverse trip home! Same route, same distance.

Now, just to add a little MORE mystery... this is not new. Last year, the same thing happened. Same scenario, same circumstances. The thing is, I never did anything about it as last winter wasn't much of a winter at all, and I ended up running the AC fairly regularly throughout the "winter". But on the odd occasion I DID turn off the AC, the same thing would happen. This year, from April to this week (Nov 20), the AC has been on consistantly and the problem never appeared. Not once.

I have been rattling this problem over and over in my head for over a year now and I still can't figure out why it would do this. No chance I'm taking it into the shop, I can see the dollar signs rolling in the mechanics eyes already; new pump, new lines, new Johnson Rod... you know the story.

I'm open to ideas as this one has me stumped.

BTW, the power sterring fluid is full, has never leaked, and no signs of foaming. Hoses look fine, belt is fine. Basically, everything appears to be as it should, everything functions as it should.

[hls30.com]

Purely a gut reaction-no knowlegdge of a Miata other than where the gas and key go- but see what is going on with your AC clutch-if the PS and the Ac are on the same belt....any issue in the clutch could become an issue with the PS pump...

Have someone sit in the car and turn the ac on and off and see what happens...

Will

[sblake01]

Also, you might check the belt tensioner if it is shared by the ps pump and compressor.

[JLPurcell]

I also agree, I would first take a look a the compressor clutch and bearings, if the AC and PS share the same belt.

[zbane]

And while you are checking into this issue, why not take tha Alfa to work?

I would suggest the Z, but I wouldn't want you to associate the Z with work, the Z is for enjoyment after all.

Just a quick question-is your PS supposed to be at 100% all the time, or does it assist more at lower speed, less at higher? I had an 88 Honda CRX with this feature-city driving=full power. Highway driving=no PS assistance. If your Miata has the same feature, and the issue is not solved by the aforementioned ideas, perhaps it is a sensor failure, or some other fault in the electronics that "speak" to the PS system. FWIW (probably not much)

[Ricklandia]

And while you are checking into this issue, why not take tha Alfa to work?

I would suggest the Z, but I wouldn't want you to associate the Z with work, the Z is for enjoyment after all.

Just a quick question-is your PS supposed to be at 100% all the time, or does it assist more at lower speed, less at higher? I had an 88 Honda CRX with this feature-city driving=full power. Highway driving=no PS assistance. If your Miata has the same feature, and the issue is not solved by the aforementioned ideas, perhaps it is a sensor failure, or some other fault in the electronics that "speak" to the PS system. FWIW (probably not much)

Hmmm... now that's something I hadn't thought of. I just assumed it was on 100% of the time. The steering on the Miata is tight enough (and quick enough) at highway speeds that it's almost a drawback (ie too quick). I'll have to do some research on that but I have a gut feeling it's on 100% of the time. As for the AC clutch, that's something else to consider. It take it your thoughts are that when the AC clutch is OFF, it's not free-wheeling as it should? Something else I never considered, simply because I can feel the clutch cycling on/off during normal conditions but never heard any belt squeeling at all. In fact, with the AC off I still do not hear anything odd, but I hadn't thought to actually look. What makes it so frustrating is that it only seems to happen after the long run TO work, but never on the return, or "around town" driving.

In fact, traffic was very light this morning going to work, and it lost PS again but this time is stayed "lost" when I arrived at work. I'm assuming that this was because traffic was light enough that I never really stopped (for traffic) at all.

As for the Alfa... "Marcello" is my wife's baby. Ironically, although it's physically larger than my Miata on the outside, it's smaller inside. Enough so that I can't comfortably drive the car. It's not so bad to make a quick trip to the grocery, but any more than that and it might be dangerous. The steering wheels literally presses on my thighs so a tight turn become like a juggling act.