Power Steering pressure check - how? Crosshatch plug?

Posted: Sun Dec 30, 2007 7:57 am

If I wanted to check my Hydraulic pressure in the system on my 1991 Audi 90 Quattro 20V.. How would I go about doing it? Is there a provision for this testing, or do I have to take off one of the fittings and attach a device there?

What is the purpose of the cross hatch plug on the top of the pump? I know they are known to leak; mine is leaking at the moment and I have a new replacement with O-ring. However, if this is the port which you can test pressure at, I will test it when I replace the plug.

Does anybody know what the best approach is? And if the cross-hatch plug isn't used to testing pressure, just out of my own curiousity, does anyone know what it's intended use is for? Why is it there?

I ask because my power steering assist is not as powerful as it should be. My 1991 Audi 80 had twice the assist that this car has. The brakes feel the same way, and since the brake system is assisted by the hydraulic system, I assume the hydraulic system is what is at fault. I know it is providing a decent amount of assist though because with the car off, it is much more difficult to steer.

Posted: Sun Dec 30, 2007 10:34 am

If it's "not as powerful" then first thing would be to check/replace the belt to make sure the pump is getting driven properly. If you have a leak then that won't help as you will lose pressure. Fix the leaks first.

While you are at it, bleed and change the fluid.

Posted: Sun Dec 30, 2007 12:23 pm

You might want to check the tyre pressures as well. Low pressure results in a very heavy feel to the steering. Also, worn front tyres will give a slightly heavy feeling to the steering.

Ian

Posted: Sun Dec 30, 2007 2:22 pm

Don't these power steering systems self-bleed........?? I thought that if you drained and replaced the fluid and then steered to full lock on both sides a number of times with the engine at idle the system bled itself. Or am I wrong ......??

I agree with the other posters though. Fix the leaks, make sure the belt tension is correct (replace belt if necessary), check tyre pressures (low pressure has a huge effect on my Quattro Coupe), change the fluid and bleed the system.

While you're at it do the "test" on the bomb with the engine stopped. See how many pumps of the brake pedal it takes for it to go noticeably stiff. The bomb will be giving quite a bit of assistance to the system with the engine at idle IMO so a failing bomb could be a contributory factor.

Posted: Sun Dec 30, 2007 3:03 pm

No, you aren't wrong, they self bleed by turning lock to lock after refilling.

Also worth noting, level should be checked with the engine "running"

Jas..
Cool

Posted: Sun Dec 30, 2007 3:23 pm

Yes are self bleeding - by bleed, I was referring to the procedure for changing the fluid. Wink

Posted: Sun Dec 30, 2007 8:21 pm

Well It has a new belt, and I have put new fluid in because I did a rebuild and drained all of the old stuff. Tire pressures are fine.

If the bomb is failing, how would that contribute to the symptoms? Isn't
it for pressure assist after the engine is off? In any case I suspect the bomb does have a problem because the pedal goes stiff after a couple of pumps if the engine is not running, and the light stays on except for when i pump the pedal, it goes out but then comes back when i stop. From another post somebody told me if the switch is connected backwards, the light will work in reverse how it should, so I assume it is wired incorrectly.

So, since the bomb is almost definately dead, I would prefer to check the presure in the system first. I prefer to locate the problem before replacing parts. If the bombs weren't so expensive I would do it now but since they are, I will tend to that problem later. However, in the meantime, if the
pump is not providing adequate pressure, then I would like to fix this, as
it is not good and I have a spare pump on my 1991 Audi 80 engine that
I've saved. Though I don't want to go through the trouble of swapping
the pumps if that is not the problem.

So, how do I check the pressure? I could just change the bomb, but it is only a one way guarantee - if the problem persists, I still need to check
the pressure, because it is possible the bomb is bad AND the pump is bad. Checking the pressure seems like a simpler diagnostic that I could do first.

Site Moderator Joined: Jan 31, 2003 Posts: 9144

You need specialised equipment for testing the hydraulic fluid pressure due to the high pressures (~140 bar). I would advise as above - fix the leak, fill the system and check the tension on the belt.

UltraUser Joined: Mar 18, 2005 Posts: 1570 Location: Gwent

I would point you towards 20v.org:

http://20v.org/brakprob.htm#servo

The servo should be tested before the bomb. More good info there.

If you end up replacing parts eg servo or bomb, which were leaking away some of the pressure in the system. There is a good chance the system pressure will return to normal with good parts in there.

If you really want to test the system pressure, the pressure switch on the hydraulic servo would be a good place to fit a gauge/adaptor. Another possible tap in, is the outgoing side of the bomb - banjo fitting. Best to get a hydraulic engineers to do this for you?

Sorry to say that a 10v pump won't be any good, it is not a tandem pump like the 20v. If you were considering a vacuum servo conversion, it would be a good part to have, especially if you still have the rest of the vacuum servo system!?

Posted: Wed Jan 02, 2008 2:42 am

Ah.. I knew they looked similar, I was going to do a check and make sure they are the same before I did the swap if it came to that, but I guess I don't have to even check yet.

thanks for the tip

Posted: Wed Jan 02, 2008 8:57 am

Before jumping in and testing the pressure you need to remember one basic fact about hydraulics.

Hydraulic Pumps DON'T generate pressure, they only generate fluid FLOW.

It is the resistances in the system that generate the pressure. Therefore you need to check all the components that create restriction in the system (and fix them if required) before you come to the conclusion that the pump is U/S. It could be that the pump is actually fine and the problem lies with one or other of the parts in the system is not creating the "correct" restriction. In those circumstances the system will never generate the required pressure even with a pump that is in perfect working order.

Remember that if it is the bomb that is the root cause of the problem there is a guy in the USA who reconditions them. Check the threads here. It has been mentioned before.

Just a comment regarding Scottys post. The fact that the power steering is heavy and also that with the engine off the brake pedal goes solid after a couple of pumps would seem to suggest (to me anyway) that the problem lies in the hydraulic side of the system. If it was only the brake pedal test that was giving problems I would agree with you that the servo would in that case be a distinct possibility.

Posted: Wed Jan 02, 2008 11:16 am

if the bomb is dead then how about this guy for one
http://sites.internet.lu/folders/customautocraft/typeB3-B4_80_90_S2_RS2.htm

Posted: Wed Jan 02, 2008 5:51 pm

The pump leak is unimportant to the overall pressure issue - to affect the pressure, the leak would have to be enormous (at "all fluid expelled from the system through the leak in two minutes" kind of levels).

If the light is flashing as you describe (albeit in the reverse to normal), then the bomb is dead, and may be why the steering feels heavy, or it might be that the FLOW (see above) provided by the pump is poor - but a leak is only of cosmetic interest (unless it's huge, or bad enough to damage other parts of the car, through wetting them with fluids)

PS: The purpose of the cross-hatch covers on the pump is to allow internal parts to be fitted and internal chambers to be drilled into what is a solid block of steel. The normal way of making similar parts (like a water pump) would be to make two castings and then bolt them together with a gasket in between, but with the pressures involved in this part, this would not work for long, so it's easier to make a hole and then fill it with a steel plug and suitable (small) seal.

UltraUser Joined: Mar 18, 2005 Posts: 1570 Location: Gwent

This link has a sketch of the hydraulic system, it may help?:

http://www.sjmautotechnik.com/trouble_shooting/susp.html#hydoil

Posted: Fri Jan 04, 2008 4:29 pm

That layout is useful - it shows that the bomb is relevant to the operation of the braking system, by keeping the pressure in the system for a number of brake pedal depressions, when the pump is not running (as we know).

But it doesn't appear to be part of the circuit for the steering, so heavy steering (which is not caused by over-sized/under-inflated tyres) could be caused by poor flow from the pump, or a faulty rack, but not by a tired bomb.

UltraUser Joined: Mar 18, 2005 Posts: 1570 Location: Gwent

Spot on, the pump is actually 2 seperate pumps coupled together, hence the tandem pump designation. Thinking about it, low brake assist pressure AND heavy steering, is quite likely to be something common to both systems, maybe a slipping belt or wrong type of fluid has been used?

Posted: Fri Jan 04, 2008 9:25 pm

If the hydraulic diagram is correct then the two sections of the pump share a common suction line. Note that there is only one line from the reservoir to the pump but 2 return lines, one from the power steering, the other from the brakes.

So any contamination from the power steering will find its way into the brakes (at least as far as the master cylinder anyway) and vice versa.

And I don't see any servo in this system. The high pressure oil from the bomb acts direct on the piston in the master cylinder ..........

I agree with Scotty, if the steering is heavy it's nothing to do with the bomb if that circuit diagram is correct. The only common factors that would affect both systems are the pump, drive belt, and the hydraulic fluid.

Going back to our original poster's problem, as the two hydraulic systems only share one common line, the suction into the pump from the reservoir, whats the betting the fault might lie in that line or in the fittings that connect it to the reservoir or to the pump .........??

UltraUser Joined: Mar 18, 2005 Posts: 1570 Location: Gwent

Yep could be the shared suction line too, if leaking it could let in air= aeration, if part blocked = vacuum causes cavitation, both would make the pump noisy though.
The 'Servo' is also referred to as an 'amplifier' and in that schematic it is the 'hydraulic booster'

More here:

http://www.sjmautotechnik.com/trouble_shooting/brakeTSB47_92_2.html

I think ours are like on page 3?

Posted: Sat Jan 05, 2008 1:37 am

That is not a servo as I understand it. It is a hydraulically-assisted master cylinder to give it it's correct title. Move the brake pedal and it opens a port allowing high pressure oil from the bomb circuit to act on a secondary piston in the master cylinder which produces a higher applied force on the hydraulic circuit feeding the brakes than could be achieved with pure pedal pressure alone. I agree the effect is the same as a vacuum servo, but that master cylinder aint one IMHO.

Anyway, enough of being pedantic. At least we all now know much better how the system works and what is likely wrong with matey boy's 90 Quattro in the US of A ........... I vote for something stupid in the hydraulic circuit between the reservoir and the pump, especially as the engine has been out of the car recently. Maybe even the pump itself, but I can say that in my experience with hydraulics it's unusual for a 2-section pump to fail both sections at the same time. Is the hydraulic circuit even connected up right ..........??

Posted: Sat Jan 05, 2008 7:20 am

Hey Guys,

Thanks for all the comments. Yes, It's all connected correctly; and the engine was out of the car recently but the car acted this way when I bought it.
I bought this car with a wiped out timing belt, so I had a complete head job done (ALL new valves, keepers, stems, etc.) and of course, a new timing belt, water pump and tensioner pulley. It's driven like this ever since day 1, and I drove it for over a year this way, it has never fluctuated or gotten worse or better.

After reassembling the engine after doing piston rings and bearings, the car drives exactly the same way. The hydraulic fluid was changed, I bought new mineral fluid (CHF 11S). The only fluid I haven't changed is the brake fluid. I bought some super rated blue brake fluid but haven't put it in yet.

I do have the hydraulic fluid reservoir from my old 1991 Audi 80 10V still;
perhaps I could install that and mount it high up to establish no possibility
of air bubbles to test the given theory... I doubt there are any leaks besides the cross-hatch weep because for the year I drove it, I never
had to add fluid. I am guessing the hatch started leaking recently.

Scotty: Thanks for the map, I found this once before but never took
a hard look at it to realize these points.

Type17: Thanks for the explanation on the hatch plug, it makes sense to me now.

I do know I was told that the steering rack was changed just before the timing belt busted, and when I bought the car. Perhaps they did something incorrect with installing the steering rack and this is causing the heavy steering? In addition, possibly the brakes need to be bled/bomb is bad and they are responsible for the brakes problem... I guess it could be the causes to both problems aren't one in the same? I guess it's possible they are isolated incidents, individual to eachother?

Posted: Sat Jan 05, 2008 1:44 pm

If I were you I would concentrate on the common components in the system that can affect BOTH brake assist AND steering. That would be the fluid reservoir and the suction line from it to the pump, the pump itself, and the pump drive (the belt in other words).

If it were me I would be looking at the hydraulic fluid feed from the reservoir to the pump as a first step, and also the reservoir itself. If you read the article the guy says about the difficultly of completely cleaning out the reservoir and thus removing all the "gunge". Any possibility of a partially plugged connector or line that is restricting the supply line to the pump ......?? Pull them all off and check ........ if it had a failed steering rack replaced previously maybe the system was not fully cleaned out .....??

Next would be the pump. However note my comments that it is VERY unusual for BOTH sections of a 2-section pump to fail at the same time. If both sections are apparently failed the cause is often something external to the pump. Although contamination circulating in the hydraulic system from a previous failure (the rack replacement just before you bought the car) could possibly affect both sections of the pump, so don't completely rule out the possibility of a pump failure.

If you draw a blank with the reservoir and the supply line then look at the pump. It is only a hydraulic pump and will not be hard to pull apart. If it is fcuked already then you'll do no harm stripping it down, I'll have a look at mine later and give you a few pointers how I think it should come apart. IIRC it is ia vane-type pump so once it is in pieces it will be obvious if there is anything wrong with it, the inside of the body where the vanes run will be all scored like a ploughed field as a result of the contamination that's passed through it.

It cannot be the bomb at fault IMHO because you are also experiencing hard steering, and as we can see the bomb has nothing to do with the steering circuit. Fix the common faults first is my advice and then check the bomb later.

Posted: Sat Jan 05, 2008 3:19 pm

Posted: Sat Jan 05, 2008 8:25 pm

Well done that man ............ Almost as good as a Caterpillar workshop manual ........!!

Posted: Sun Jan 06, 2008 8:56 pm

that's useful - I might have ago at the old S2 one as bought a ZF repair kit at Biggar two years ago! Very Happy

Posted: Sun Jan 06, 2008 9:36 pm

moved to the technical section by me- thanks to Jas for copying the original text on strip down etc. and re-uploading the photos....