Tech Tips

TechTips is a monthly feature in AutoInc. Each month, specialists who field calls for autoline Telediagnosis® , a technical hot line service, share some of the most common vehicle problems and their solutions.

Bright Test Light, 12 Volts ... Right?
Recently I ran into a very perplexing problem on what would normally be a very simple, straightforward system to diagnose and test. One of our subscribers called about a Dodge van that stalled when coming up to a stop sign.

He had checked all of the basics, including EGR. He even had the idle speed turned up to 1000 RPM and it would still do the same thing. I asked him to unplug the torque converter lockup solenoid. Sure enough, the problem went away. The next step was to check the inputs to the ESA computer to see why it was keeping the solenoid energized. The speed sensor, idle switch and coolant temperature sensors all tested good, so I had the tech go to the Part Throttle Unlock (PTU) relay and check the voltage on all of the wires when the problem occurred to see what the computer was doing to the relay to control it.

He called me back even more confused than before! He gave me the voltage readings from all of the wires. The wire that the computer would normally put a ground on did so until vehicle speed went below 45 mph and then it went to 12 volts. I said it sounded like normal operation. He said, "Yes, except the output wire to the solenoid also still has 12 volts on it, keeping the solenoid energized." Simple enough. The relay was sticking, causing the problem. Replace the relay and the problem should be solved.

He called me back two days later to tell me he had ordered and installed the new relay and the vehicle still had the same problem. I once again suggested that he get readings on all of the wires on the relay -- but use a voltmeter this time. I had discovered previously he had been giving me voltage readings from a test light! Easy mistake to make -- bright test light, 12 volts ... right? Not in this case (when the vehicle acted up). He did see the control wire go to 12 volts, but the other side of the pulling coil was at 18 volts. When I suggested that he check the charging system, he found that yes, indeed, it was overcharging. I asked him to replace the Electronic Voltage Regulator and the charging system operated normally. He test drove the vehicle and the stalling problem was gone.

What had happened was that the voltage drop across the pulling coil in the relay was enough to keep the relay energized even when the computer released the ground on the control wire.

By Tom Vadnais, autoline Chrysler, Mitsubishi, Hyundai specialist (20 years experience)

The Subaru Pneumatic Suspension System

The Subaru pneumatic suspension system is really not the diagnostic beast it appears to be once you understand how the system processes fill and drain commands. The system consists of a reserve tank-fill valve assembly, an air compressor-drain valve assembly and four suspension stations along with all the required plumbing.

Each suspension station on the vehicle consists of a pneumatic shock unit, a fill/drain solenoid and a height sensor. The height sensor sends signals back to the suspension ECU indicating the level of each suspension station. In the case of a right-front low indication, the suspension ECU will activate the air compressor, energize the reserve tank fill valve and the fill/drain valve on the right front shock. When the shock reaches the correct height, the system will then de-activate itself and await the next command.

It is important to note that while the system can process fill or drain commands to one or more suspension stations at the same time, it cannot process a fill and drain command simultaneously. This is because the drain path for bleed air actually must return through the compressor assembly. In the case of a right-front high signal, the suspension ECU will again energize the right front fill/drain valve, but this time also energizes the drain valve in the compressor assembly, allowing the bleed off of air in the effected shock.

It is easy to see that since the system uses the same plumbing for both fill and drain, only one type of command can be processed at any one time.

By Jim Newkirk, autoline import specialist (18 years experience)

© 1995, Automotive Information Systems.

Autoline Telediagnosis® with its 29 phone specialists handles more than 12,000 phone calls per month from shops all over North America. As an ASA member, you're already signed up to use the hotline service at a discounted rate. Call (800) 288-6210, Monday - Friday, 7 a.m. to 7 p.m., Central Standard Time.

ASA Main Page
AutoInc. || More Info About AutoInc. || Tech Tips || Tech To Tech
Stat Corner || Chairman's Message || News Briefs || News Briefs Extra
AutoInc. Magazine ®, Vol. XLIII No. 12, December 1995