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Re: fuel troubleshooting question Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Sat, 5 Jan 2008 16:27:24
In Reply to: fuel troubleshooting question, jp, Sat, 5 Jan 2008 14:24:00 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The fuel pump fuse is AFTER the fuel pump relay. Looking at the Bentley book (page 371-66), the fuel pump is 101 at the bottom. It gets power from the fuel pump fuse (30 amps, in fuse box 22A). At the top of that fuse, you'll see one wire that goes up to the fuel pump relay (102) that actually supplies the power. The other wire on the top of the fuse goes to the heater for the lambda (O2) sensor - that's part number 136.
When the engine cranks, the engine control computer closes the fuel pump relay by grounding pin 85 of the relay. Pin 86 of the relay coil gets power from the main relay (229). When the fuel pump relay (102) closes, it supplies power from the battery to the fuel pump.
So, if you aren't getting power at either side of the fuel pump fuse, then there are a few possibilities. One would be that the fuel pump relay (102) isn't closing. Or, it is closing, but the relay contacts are dirty, and don't pass enough current for the fuel pump motor to run. In the latter case, I'd expect to see around 1 or 2 volts on the fuse. If you're seeing 0 volts on the fuel pump fuse, I'd say the relay isn't closing. If you're seeing a volt or two on the fuse, then the relay contacts are dirty. Either replace the relay, or pull out the relay, pop off the case, and clean up the contacts.
OK, say you crank the engine, and the fuel pump isn't getting power. First do a simple test - turn the key to ON, but don't start the car. Do you get ABS, Brake Fluid, and Battery lights? If you don't, the problem is the ignition switch. A common problem is for the contacts in the ignition switch to get dirty. Sometimes they make good contact, sometimes not. If they don't make good contact, you don't get power on the +15 power bus (+15 is just the name, all the power busses are 12 volts). The +15 bus is powered when the key is ON or START. The +15 bus powers the engine control computer, the dash lights, etc. If the +15 bus is dead, the ECU can't turn the fuel pump relay on.
OK, assume that the dash lights come on, but there's no power at the fuel pump fuse. Pull the fuel pump relay out, and jumper pins 30 and 87B on the socket. See if the engine cranks and runs. The fuel pump should run - if the engine doesn't run, then I'd guess you have an ignition problem, specifically the hall effect sensor. Why? Because the engine control uses the pulses from the hall effect sensor to know that the engine is turning, and it won't power the fuel pump unless it knows the engine is turning.
There are a bunch of other possibilities, but I'll bet you find that the times the car doesn't start, you don't get dash lights, and the problem is the ignition switch. Not a guarantee, but a decent guess.
posted by 76.204.14...
, Sat, 5 Jan 2008 16:27:24 <-- Viewing This Message
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