fuel pump blues

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From a battery drain to the fuel pump blues

YUKON EXHIBITS MYSTERY Y BATTERY DRAIN

A customer rolled in (or should I say was towed in) with a 2002 GMC C Yukon, complaining of a constant battery drain. The engine won’t start in the morning, she charges the battery, starts and runs it, but after parking it for a few hours, the battery’s dead again. To make a long story short, the charging system was working f ne, but there was a parasitic draw somewhere.

We traced the problem to a faulty blower motor processor for the autohe auto- matic temperature control system, which was causing the blower motor to run continuously, even with the key off.

The faults were traced to the automatic climate control processor (actually a resistor) and the blower motor. As it turns out, the processor was fried, so we initially assumed that replacing it would do the trick. However, during investigation, we noticed that the blower motor was bad, which in turn seems to have taken out the processor. The processor was obtained for $280 (which seems way too much for this little guy). After replacing the blower motor and processor (easy access), the repair was complete.

HONDA CHARGING

Even though Honda has used the Electronic Load Detection (ELD) System since 1988 on the Civic and 1990 on the Accord, there is still a lot of confusion on how this system operates and controls the charging system.

The ELD system inputs the electrical load to the engine control module (ECM), and then the ECM controls the output of the alternator under various electrical loads, and is capable of shutting the alternator off completely under very low or no load

situations. situations. When the vehicle is f rst started, the alternator may not begin charging for up to three minutes.

Before performing any tests on the charging system, be sure to put an electrical load on the vehicle’s electrical system by turning on the headlights or turning on the blower motor or rear defogger. The ELD unit, located in the underhood fuse box, must sense current f ow through the fuse box so it can correctly indicate the electrical load on the vehicle to the engine control module (ECM). The ELD unit has three wires leading to it, a 12-volt power supply wire, a ground wire and a signal wire. On the signal wire the ECM will send a reference voltage of approximately 4.5 to 5 volts to the ELD unit. The ELD will pull this reference voltage towards ground depending upon the amount of load on the electrical system. With no or little current f ow through the ELD, the ECM will see approximately 3.5 volts on the signal wire. At this time, the ECM can ground the control wire at the alternator and the voltage regulator will turn off the alternator without illuminating the charge light. If the current f ow is high, the voltage on the signal wire will be close to one volt.

The ECM will then see a high electrical load and stop grounding the control wire so the voltage regulator will allow the alternator to charge.

Disconnecting the control wire from the alternator will disable the ELD control of the charging system, thus allowing the alternator to charge normally. The most important thing to remember when testing the charging system is to be sure to add an electrical load (such as headlights). (Information courtesy of Identif x Inc.)

FUEL PUMP BLUES

Last December I ran into the car from the deep dark underworld. It was a 1987 Chevrolet Celebrity, 2.5L, VIN R, automatic transmission with A/C.

Symptoms: Car ran great 90% of the time. The other 10% it stalled with no restart, or if it did restart right away, it would stall again or run very rough with no power. The tech tried to get the best description possible from the owner of when the problem occurred and on what type of road surface. The owner’s “wealth” of information amounted to, “I don’t know — it just happens. Fix it!”

Following the rules of good car repair, the f rst step is to try to duplicate the symptom. After driving the car for an hour, the technician was not able to do this. Nothing on the scan tool or fuel pressure gauge indicated a problem so the tech removed all the test equipment and started work on another car. When he brought the car into the shop two hours later, the problem was duplicated to a T. Of course! (FACT: When no tools are hooked up to help f nd the problem, the problem will always show up!)

Maybe it was a lack of fuel. He sprayed some carb cleaner at the air cleaner and the car picked up and ran much better. But after hooking up the fuel pressure gauge again, the car had 12 psi, restarted, and ran great. Next he checked for a shorted injector.

Resistance and current f ow of the injector were OK, but the readings were on the low end of the specs. If it wasn’t a shorted injector, then the ECM must not be allowing enough fuel delivery.

Since injector resistance was at the low end of the spec, the tech replaced the injector thinking it might have damaged the ECM. After replacing the fuel injector and the ECM, he returned the car to the owner. That same afternoon, the owner brought it back with the same problem. The tech disconnected the EGR valve to eliminate the possibility of it sticking in the applied position, installed a fuel pressure gauge, grabbed a f re extinguisher “just-in-case” and set off on a second test drive. This time he saw the fuel pressure drop to 7 psi. He replaced the fuel pump and got the same results. He replaced the fuel pump again and the fuel pressure regulator and fuel f lter.

Same problem. Finally the shop owner asked the tech to call our hotline. After listening to everything he had tried, I assumed that the replaced parts were good and asked him to check the fuel pump circuit. The voltmeter registered good power supply to the tank. The ohm meter checked the ground at 5 ohms, so that circuit seemed OK. Often checking ground circuits with an ohm meter will not load the circuit enough to f nd a ground problem. Since this was an intermittent problem, the circuit may have been f ne when tested.

I had him check the ground circuit with a voltmeter and do a voltage drop from the ground to the case of the alternator. Why the alternator? Remember, this is the voltage source with the car running.

The technician found a 0.5 volt drop with the car running, but intermittently it jumped to around 6 volts. At the same time, the fuel pressure gauge dropped to around 7 psi. I suggested that the tech examine the ground in the trunk for the fuel pump — it was corroded.

With repairs made to the fuel pump ground, the car ran f ne with no other symptoms.

It took two phone calls to our hotline to repair this car. Total time: 11 minutes! (Information courtesy of Identif x Inc. See www.identif x.com.) ●