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Resistor May Be Reason Late-Model GM Car DiesPosted 12/10/2001By Christopher Pittman
“My car starts and runs for 30 seconds - then it dies! What's going on?” Ever had a General Motors product towed into your shop with this complaint? And is the complaint on one of those new supercharged 3.8L engines that looks like it's going 100 miles an hour just sitting still? Does the vehicle appear to have fuel pressure when first started, but then it goes away ... as if the pump just shuts itself off? In 1997, GM began using a two-speed fuel pump system on the 3.8L Vin 1 engines. GM's reasoning: Under normal driving conditions, the engine doesn't use all that fuel. By slowing the pump down, it saves wear on the pump and reduces the amount of fuel sent back to the tank. The two-speed system uses two relays and a resistor assembly. One relay is the same fuel pump relay found on most GM cars. The second is designated as a speed control relay. The resistor is used to drop voltage going to the pump, to slow it down. When the key is first turned on, the powertrain control module (PCM) activates the fuel pump relay, which closes the contacts sending power to the speed control relay. But the speed control relay is not activated yet, which allows full voltage to the pump. As the vehicle is cranked, it again powers the fuel pump relay to start the engine. Once the engine is running and idling, the PCM activates the speed control relay, which then puts the power through the resistor assembly - thus reducing voltage and pump speed. The speed control relay is used more as a switch that can either a) send full voltage to the pump or b) send it through the resistor assembly, and then to the pump. But for the pump to work at all, both relays have to be in good working order. So what caused this problem? The resistor assembly has been breaking apart, causing open circuits. As long as the speed relay is deactivated and full power is sent to the pump, there is fuel pressure. But when the speed relay goes into speed control mode, power does not make it back to the pump, shutting it down and so the car dies. If you come across one of these two-speed fuel pump systems, check for power at the resistor assembly. If there is no power, replace the resistor. The resistor looks like a ballast resistor found on ignition systems from “back when” except it has wires coming from it and a weather pack connector. Since it needs airflow to keep it cooled down, GM has positioned the resistor on the passenger side wheel well, under the windshield washer bottle, close to the inner frame.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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