QuestionQUESTION: Hi, I have a 95 Eclipse GS (manual trans w/ 140k miles) that I have been having problems with. the 30A motor fuse keeps blowing immediately after the car starts. I've heard that a bad fuel pump or fuel filter can cause the ignition fuse to blow (the computer telling it to blow), is this true? Because I have tested the fuel line and it has low pressure before and after replacing the fuel filter. Could this be the source of my problem even though it isn't the ignition fuse? I've checked and rechecked everything I can, relays, wires other fuses, everything. The only two options I can think of now are the computer or the fuel pump. And I can't readily check the computer because I can't move it to an Autozone or anything. Thats it for now. Thanks for your time.
ANSWER: To help you track this down, I need to get a better indication of which fuse you are talking about. My 94 (1G) doesn't list any as "motor". Are you talking about the fuse high and to the right in the underhood box or the one that is in near the middle in the underhood box (next to the radiator fan fuse)?
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QUESTION: the "motor" fuse on my car is the one in the very top left (when looking down on it) corner of the underhood box.
ANSWER: The fuse you are talking about powers the ignition coil pack, the vehicle computer and the fuel pump. There is another 30A fusible link, and it is for the ignition switch, and powers the accessories, most which have their own fuses. The MFI fuse is what we are talking about and since there are not any additional fuses in this circuit to enable you eliminate circuits, You have a couple of ways to find the problem.
The proper way would be to use an amp clamp and POWER PROBE and check the components included with this Circuit Breaker (FUSE). The circuit that is pulling more than or near 30A is the culprit and needs to be replaced.
WITHOUT an amp clamp, the best way to diagnose this is to jumper the ASD relay and start to remove associated components. The ASD relay should be the second relay over by the Master Cylinder. The one closest to the master cylinder should be the fuel pump relay. Since (as you stated) the fuel pump has low pressure (checked with a pressure guage?), it is likely the fuel pump is internally shorted and working poorly. If the coil or fuel pump disconnect doesn't repair it, (and the relay has been jumpered taking it out of the loop) then you are going to have to look at the engine control computer (ECU) and it's components. The ECU provides power for many components and one of them could be shorted as well. If you get to that point, I would start checking resistances of the individual components within the MFI.
Another quick option for checking the fuel pump circuit would be to remove that relay, disconnect and place a FUSED (20A) jumper between the Black-Blue wire and the Red/Black wire (the two thickest wires). If, when you now start it that FUSED jumper now blows, you know it's the fuel pump circuit. The quick way to test that would be to use a POWER-PROBE and put the test point onto the black/blue wire and apply 12V with the + button. If it blows the internal circuit breaker on the tool (10A, but holds almost to 20A) then you have found your problem.
The right tool makes it super easy, but the amp clamp costs $75 and figuring out how to use it with a meter, and the POWER-PROBE costs just over $100 for the one without the digital meter.
There are your cheap way and the proper way to verify that indeed it is a bad fuel pump.
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QUESTION: thanks for the detailed response i'll be following it starting tomorrow :) I have another question however, before I read this email I was doing some more testing and found that the secondary resistance in my ignition coil pack (between the towers) is about half of what it should be. It read 12 K ohms and the Chilton book I have said it should be between 20-27. The plug checked out normally. Do I need to replace the ignition coil because of this? Thanks again.
AnswerNo. I hardly ever use resistance as an indicator unless I want to check to see if something is shorted. The secondary side won't cause your fuse to blow, unless it was connected to your primary side, and the resistance is too high for that. I see people change parts out for wrong resistances all the time without solving a problem. After the vehicle is running, you can do a coil output test to determine if the coil(s) can support 20+ kV (unless you run it at the track under higher boost/nitrous). I suspect it won't be an issue.
