Mexican suspension

toddao · Post by **toddao** » Tue Mar 22, 2011 10:33 am

After the hydraulic system re-build the car pumps up and stays up overnight but this morning it displayed some strange symptoms :

At the front it dropped to the lowest setting then suddenly sprang up to the highest one then back to the low after a few moments and so on, just like a Mexican lowrider.
The whole time the lever was in the normal position.

Something wrong with the front height corrector I suppose but can anyone explain this or point my towards a fix? Thankyou.

( I don't want to be doing the Mex car bump thing as I'm driving!)

Linegeist · Post by **Linegeist** » Tue Mar 22, 2011 12:45 pm

It's what's known as a 'runaway closed loop servo' effect Todd. (See, you really wanted to know that didn'tcha?)

Basically the suspension corrector linkage is maladjusted. When the car rises to the correct height it goes a tad too far, which triggers the corrector valve's 'Drop it quick' response.

Of course, when it DOES drop, that goes too far as well, which triggers the "Arrgh! Get it up, get it up!!!" response. (Don't ask me how I know this - but it kept a French village in stitches for about 10 minutes on my Mk1, one Saturday lunchtime

Most embarassing - it looked like the BX was trying to bonk a 2CV in front of it...........

I think you'll find the answer lies in the HC linkage guv'.

Post by **docchevron** » Tue Mar 22, 2011 12:47 pm

Either that or it's full of air, which has the same effect.
Have you tried cycling the suspension up and down a few times?

toddao · Post by **toddao** » Tue Mar 22, 2011 5:28 pm

Thanks for the replies. I'm pleased to say that it's not the 'runaway closed loop servo' effect Bob! Though that sounds very exciting. I was hoping that I wouldn't have to venture under the thing again after a winter getting well aquainted with the underside of it..

I pumped it up and down a few times as per Doc's suggestion but it continued with its Mexicali performance. Then I put it in high and cracked the regulator bleed open and closed, then went up and down, up and down, repeated the bleeding and suddenly.. it stabilised. It does appear to have been air, a bit liked trapped wind I suppose!
Quick fix thankfully.

That's why they call him The Doc!

Linegeist · Post by **Linegeist** » Tue Mar 22, 2011 9:52 pm

Air? Now that's something I've just learned!!!

I didn't know it could cause that effect ..... but I can (dimly) see how.

But how does air get into a pressurised circuit.............?

I have a headache.

Post by **docchevron** » Wed Mar 23, 2011 1:07 am

Todd's had the hydraulics in bits all over the systems, so, even though the system is self bleeding, aside from th brakes obviousley, one assums it'll sort itself, but if large enough amounts of very compressable air gets in the system it can take ages to shift, hence give weird issues with the suspension in as much as air in the system will compress, whereas oil wont.
Sorry Todd, I would have mentioned opening the bleed but assumed you would have upon reassembly of the system..

toddao · Post by **toddao** » Wed Mar 23, 2011 7:04 am

Yes, I did bleed it upon reassembly but I thought a bit of that could help along with the up and down business. I'm just glad that it worked and just hope that it doesn't go all Mexican on me again while I'm happily rolling down the autobahn ( though that's still a weldathon away) owing to more trapped wind!

DavidRutherford · Post by **DavidRutherford** » Wed Mar 23, 2011 9:19 am

What would be very interesting would be to find out what priming method was used on the production line, as that would have been starting from a completely empty system, and needing a complete system fill. I can't imagine a funnel being used to fill the suction line on every single production car.

I wonder if there's any information "out there" about the new-car priming sequence, as it would certainly save a lot of fiddling about when people do major surgery on the hydraulics.

Post by **mat_fenwick** » Wed Mar 23, 2011 10:23 am

I know that a lot of cooling systems (nowadays at least) are filled by vacuum - which also acts as a leak test. The coolant will only start filling once a certain vacuum level has been reached.

This caused a HUGE problem in my last job where we supplied water pumps to Jaguar - the company who supplied a retaining ring to the company that manufactured the seals we bought in changed their press tool, and although this part was still to drawing it was slightly different. So although the pump still passed all our (positive) pressure tests, when a vacuum was applied they all leaked. Meaning that all the vehicles on the production line had to be filled and bled by hand...

Amazing how a tiny change so far down the line can have such large effects.

Linegeist · Post by **Linegeist** » Wed Mar 23, 2011 12:15 pm

As an aside to this - I invested in a Sealey vacuum brake bleeder some time ago .... See here .....................

This thing has solved no end of problems, including drawing air locks out of my eldest daughter's Renault's cooling system and drawing lubricating oil through motorcycle cables - not to mention bleeding (non BX) brakes in mere minutes.

Best forty quid I've spent since I paid to have the mother-in-law's broomstick sabotaged.