The rear of my '89 Mark VII LSC wouldn't settle, which I traced to an inoperative compressor relief valve. Upon discovering the valve was not available separately but only in an expensive new or rebuilt compressor assembly, I worked out the following inexpensive fix.





As of early 1999, McMaster Carr (a very large industrial supplier) is now online and sells to individuals. In their catalog # 102, page 1653, there is a "High Efficiency Miniature Air Valve" (part # 62095K11) that sells for about $20. It's 12 volt dc, uses only 1.3 amps, and is very small. The specs say it is good for 0° F to 120° F. It has push-in ports to connect 5/32" truck airbrake plastic air line. Any large truck dealer will have this air line, it's just a few cents per foot (you need 2 feet). From the truck dealer also get a Weatherhead brass fitting; you need a 90 degree compression street ell, 1/8" pipe size, for 5/32" plastic air line tubing, and it has an inner tubing support so the compression fitting won't crush the plastic air line.

I installed a rebuilt dryer, which allowed me to cut apart the old one to see how it is built. Drill an 11/32" hole in the very top of the side of the dryer, in the thick part, which will put the hole well above the desiccant pellets inside. Drill on the rear side, just to the inner fender side slightly from straight back. Tap with 1/8" pipe tap. Wrap the brass fitting threads with Teflon tape and screw into the drier. Point the fitting toward the fender. I installed the air valve, with the ports and wiring pigtails pointing toward the rear, behind the battery on the inner fender to avoid the worst of engine bay heat. Drill 2 small holes in the inner fender and use 2 small sheet metal screws to mount the valve. I fashioned a protective cover out of a small plastic bottle, positioned with one edge down, and cut out the bottom edge for air circulation. Thread the plastic airline under the top air dryer bracket, and connect between the brass fitting and the valve. Clip the wires going to the original relief valve close to the compressor. Solder and heat shrink these wires to the pigtails on the new valve. On my 89 Mk VII, the green wire goes to the black pigtail and the blue wire to the green pigtail. I found the wiring diagram for my car in a large motor manual in the local library.

I live in MO where we have occasional below zero weather and considerable heat and humidity in the summer. I have had this on my car for over a year without a single problem.

UPDATE: Current as of August 2004 -

This is some new info I developed recently while helping someone find materials to install a remote relief valve instead of buying a complete compressor assembly.

The valve in the original article used 5/32" airline and fittings, which have become somewhat difficult to find.

The valve comes from McMaster Carr and anyone can buy direct over the internet from them. That valve now goes for about $27.00. (See original article above for page and part #) The airline is available also from McMaster Carr. Page 93, part # 62095K11, "DOT Nylon 11 Tubing", available in a 5' length for 39 cents per foot.

There are several choices for fittings, also from McMaster Carr. All listed are 90 degree fittings. Straight ones are also available.

Page 136, 5/32"OD tube to 1/8" pipe adapter, Part 51055K44, good to 158 deg F, $2.58.

Page 145, brass fitting, part 51495K232, good to 175 deg F, $2.97.

Page 147, brass fitting, part 5523K32, good to 250 deg F, $3.44.

Another somewhat cheaper alternative valve would be on page 395, called a "Polypropylene Direct-Acting Miniature Solenoid Valve" It's 12 volt dc with 1/8" female pipe fittings which would allow it to be used with very commonly available fitting and plastic airline sizes, like 1/8". It's available with either 18" wiring leads or spade connectors and is only around $18.00. Part # 7877K25 or 7877K55.

I have also realized something else since I did mine. The drier is self-renewing. It absorbs moisture as air is pumped to the bags. When the dry air is exhausted from the bags, the system is designed so that the dry air passes back through the desiccant and removes moisture. This way the desiccant does not become rapidly saturated.

After realizing this, I moved my remote exhaust airline from the top of the drier to the bottom, to allow the dry air being exhausted to pass through the desiccant and absorb moisture. To keep the desiccant pellets from clogging the small airline, I pushed a small wad of coarse steel wool into the hole before screwing in the fitting.

Although the body of the drier is not as thick near the bottom, it held for me for over a year with no problem. However, to be on the safe side I roughened the outside of the drier body with coarse sandpaper around the fitting and applied some epoxy adhesive to give a little added strength.