Keep air out of antilock brake systems

Drive it Forever

Dear Mike:

I tried to flush the brake fluid on my wife’s five-year-old SUV. I opened all of the bleeder bolts and pumped all the fluid out into four small jars. Man, that brake fluid was black. Then I bled all the calipers one at a time. I’ve gone around the truck bleeding calipers three times.

When the engine is running, the brakes are really spongy, my wife is upset and the truck doesn’t slow down very well.

Is there a secret?

T.G., Sacramento, Calif.

A: The secret is to never introduce any air into any antilock-brake system. You have some air lurking inside the ABS controller’s internal plumbing. This gives you a spongy pedal, prevents the ABS from doing its job properly and may well cause the brake-warning light to come on.

To rid your car of this air, you’ll need a scan tool that can interface with the ABS controller. It’s necessary to cycle the ABS pump and valves with the scan tool to move the air along to where it will be carried out of the system at the caliper-bleed bolts.

Generically, this involves holding your foot on the brake pedal while telling the scan tool to cycle the ABS, and then bleeding the system at each corner. The specifics of how different systems need to be treated are quite different, however. You’ll need a shop manual and a scan tool for the correct procedure for your truck.

Or maybe it’s time to throw in the towel and have a shop mechanic finish your job - if you can get the truck there without crashing.

It’s possible to flush an ABS-equipped car’s brake fluid without a scan tool. The operating principle is to keep the reservoir at least half full at all times. I use two assistants, one to pump the brake pedal while another hovers over the reservoir and keeps it topped off with fluid. Bleed until the fluid runs clear. Another solution is to use a compressed-air-powered pressure bleeder.

Now that you’ve got air in the ABS controller, however, you’ll need a scan tool to do the job properly.

Dear Mike:

I have a 1987 Jeep with a five-speed, manual transmission. The last time I had it serviced, some bozo of a mechanic at the transmission place put automatic transmission fluid in the gearbox. The truck was driven about 20 miles with this fluid installed.

Would this cause any damage to the transmission and, if so, what?

J.P., San Marcos, Texas

A: This transmission calls for a 75W-90 GL-5 gear lube, which is considerably more viscous than automatic transmission fluid - or ATF - and has a very different additive package. As amazing as it may seem, some manual transmissions in current production actually do specify ATF as the correct fill.

I wouldn’t think that a couple of dozen miles filled with ATF would burn out your transmission, although I wouldn’t recommend it. The transmissions that specify ATF are engineered to use it from the ground up. The advantage is less viscous drag and marginally better mileage.

Relax, and go to a different shop next time. When you do, be sure that all the ATF is drained before the tranny is refilled.

Dear Mike:

I just bought a Saturn Ion. The battery is mounted in the trunk next to the spare. Why?

Is this safe? Don’t batteries give off hydrogen gas? There’s no ventilation in the trunk.

H.H., Circleville, Ohio

A: There are any number of vehicles on the road with their batteries mounted someplace other than in the engine compartment. Volkswagen Beetles have had their batteries mounted under the rear seat since before World War II.

There isn’t really enough hydrogen produced, especially with modern battery technology, to produce a fire or explosion hazard. Any car with a trunk-mounted battery will have either a vented battery box or a battery that doesn’t vent hydrogen.

The advantage? More room in the engine compartment. As a secondary advantage, the battery should last much longer when it’s removed from the heat under the hood. Front-heavy vehicles also can benefit from moving the weight to the rear.

A disadvantage is the increased voltage drop in the battery cable caused by the longer wire run, but you can compensate for that by upgrading the gauge of the cable. Another downside: The positive battery cable for your Ion lists at $128. Ouch!