Toyota MR2:

AM2 fuse blows immediately

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#1 4/28/2010 7:40 PM

daled
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Registered: 4/28/2010
Posts: 4

AM2 fuse blows immediately

Toyota MR2 1985 won't engage starter fuse in engine compartment marked AM2 7.5amp will not hold -- not even a 10amp will. Somehow this cuts electricity to the small wire at starter. The starter relay checks out OK with an ohmmeter. Could the voltage regulator in alternator be the culprit?

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#2 4/29/2010 12:17 AM

TechHelp2
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Registered: 11/24/2009
Posts: 1299

Re: AM2 fuse blows immediately

Does the fuse blows when you try to start the engine or it blows the instant you plug in a new fuse?

Get yourself a short checker at the local auto parts store, if you can't get one you can get a 10 amp circuit breaker. This takes the place of the fuse so you don't have to keep replacing the fuse when it blows. This also helps protect the circuit from overheating and starting a fire. Then proceed with diagnosing the problem with the key on or in the start position -- what ever blows the fuse.

With the short checker in place it will start popping if the short is present. Start disconnecting components one at a time. When you find the short, the breaker in the checker will stop popping. This will put you in the area of the problem.

Start with the big items first. Disconnect the starter from the circuit. If the breaker is still popping then disconnect the alternator. If it's still popping then continue disconnecting or removing relays or components till it stops.

The circuit goes from the AM 2 fuse to the ignition switch, and comes out and branches off to the clutch start switch at the clutch pedal to the starter relay. The black and white wire coming out of the relay goes to the starter solenoid. 

The circuit also includes the engine main relay, cooling fan relay, injection relay, EFI relay, and so on. You should have a wiring schematic to do this. If you don't have a repair manual with the schematics, you can get one with a subscription to the ALLDATAdiy.com.

Last edited by TechHelp2 (4/29/2010 12:22 AM)


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#3 4/29/2010 7:48 PM

daled
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Registered: 4/28/2010
Posts: 4

Re: AM2 fuse blows immediately

No I am not using the ignition. The fuse pops as soon as the battery cable is connected. I have replaced starter before I found the fuse issue. Ignition system checks out OK with the ohmmeter and so does clutch switch.

Toyota MR2 Recalls, TSBs and Service Information >> ALLDATA

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#4 4/30/2010 1:39 AM

Administrator
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Registered: 9/27/2008
Posts: 1279

Re: AM2 fuse blows immediately

A lot of times it involves disconnecting components and wiring harness connectors until the fuse doesn't burn. That will narrow down the circuit to help you find the exact location of the short to ground.

Often times it's not a component at all causing the problem, but a wire that's pinched or the insulation has worn through forcing the bare wire is contact a sharp metal surface (ground).

If you're not real familiar with DC diagnosing, it may be an uphill battle that requires a technician to figure out.


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#5 6/22/2010 7:04 PM

daled
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Registered: 4/28/2010
Posts: 4

Re: AM2 fuse blows immediately

still popping  7.5 AM2 fuse  , checked alternator found volt.regulator bad replaced it but  i think that was a result of my lingering original issue ,so far the only way i can stop the fuse is to disconnect the right side plug on my ECU in the trunk, what a pain......

Toyota MR2 Recalls, TSBs and Service Information >> ALLDATA

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#6 6/23/2010 1:37 AM

TechHelp2
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Registered: 11/24/2009
Posts: 1299

Re: AM2 fuse blows immediately

If it stops popping when you disconnect the ECU, then the problem may be an internal short inside the ECU.

This could be and expensive part. Maybe you should get another opinion on this problem to confirm this; unless you have a spare you can swap with to see if the fuse stops popping.

To play it safe I would get a second opinion by an Automotive Professional. It could be cheaper and safer in the long run.


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