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Q: Car won't start

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I hooked the battery up backwards. Blew the 100a alt fuse and two others. Before I changed the 100 amp fuse car would start, but no lights and it wouldn't go into gear. Once fuses were swapped and tested lights worked but now the car won't start. I've tested all relays (except starter relay because it's in an ackward place) and they checked out fine. When the car is in the start position all dash lights and service lights illuminate but no response when I try to start it. I've bought a new battery so I'm sure it's good. please advise.

My car has 167000 miles.
My car has an automatic transmission.

A: Whenever a car won’t start, it’s important ...

Whenever a car won’t start, it’s important to differentiate between a crank no start and a no crank. It sounds like you have a no crank because you are testing the starting circuit and you have replaced the battery. In your situation, you should test every fuse in every fuse block you can find. This should be done with a test light and it doesn’t take a lot of time. Simply touch the test light to the small metal piece that is exposed on the back of the fuse. There will be a couple of fuses that can’t be checked in this way, but they will be designed to be tested visually. Testing relays should be done as part of checking a specific system. Contrary to popular belief, the only good way to test a relay is by using it in the circuit you are testing. This will often take some thought to set up a test with jumper wires.

With your situation, the results of what has happened is going to be random and unpredictable. I can give you some diagnostic tracks to follow, but they are simply a means to an end in this case.

Moving on to the no crank, if you have anti-theft, there will be a starter cut relay. Not all of these have a security system so determining if you do would be my first order of business. You will most likely have an indicator on the dash showing a security feature. If you do, there are a few techniques that work on most cars. Lock and unlock each door, including your trunk or rear hatch, three times. Most of the time you will only have to do this on the drivers door, but often it will be another door or hatch that will reset the security system. A second method is to disconnect the battery, turn the key to the run position, reconnect the battery with the key in the run position. If neither of these work, you will need to do some deeper diagnosis.

If those didn’t work, I would find the starter cut relay (if you have a security system) and use jumper wires to activate the rest of the system. This requires a wiring diagram and some knowledge of how these systems work. Be warned, if you have an anti-theft system and you are testing at the starter cut relay, there are modules involved that will not react well if you happened to short them to ground or supply power to the wrong pin. So be careful and I don’t suggest you proceed if you are not certain about these systems. If there isn’t a starter cut relay, remove the starter relay and do the same with jumper wires. Again, this will also require a wiring diagram.

I would do most of my testing at the starter relay. From there you can confirm there is power from the ignition switch when attempting to crank the motor, you can supply power with a jumper wire to the starter solenoid, it should crank when you do, and you can confirm the fusible link and 30A main fuse is good. If one or more of these fuses or switch is not working, you will need to follow the power from its source for each circuit to the point of failure.

You may find there are more than a couple system that have problems, so be patient and use a systematic approach. If you find this process is too daunting, you may want to have a certified technician, such as one from YourMechanic, come to your car’s location to diagnose the starting issue and perform the correct repairs.

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