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Discussion Starter #1
Hi all.

My 04 MDX sat for maybe 2 months and the battery drained. When connected to a jump starter it cranks but won't start.

I notice that the green immobilizer key light on the dash blinks and doesn't go off (both during cranking and the II position). Also the fuel pump doesn't seem to prime when the key is in the II position.

Can a bad battery be the culprit (even though there was plenty of power from the jump starter during cranking)?

Will a brake code from the dealer work with the master key? (procedure calls for a non-chip key, but my key isn't being read anyway)

Any thoughts on what to check for?

Thanks for your time!
 

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Have you tried a different chip key?

It is possible that the ECU lost the registration of one of your keys but not the other.
 

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Discussion Starter #3
Unfortunately I only have this one key. Would it be worthwhile to contact a locksmith to try and reprogram the key/ECU?

Thanks.
 

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I may be way off base here, but my ex's Acura TL had this problem and it turned out to be a "ignition swich harness" / cable that had to be swapped out. It may not apply in this case since it's a different model, but no harm checking
 

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Discussion Starter #5
I'll check on the ignition switch harness. A new battery was put in but the same symptoms persist.

Anyone know of any fuses that might cause this?
 

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Fuse 13 in the passenger's side under-dash fusebox powers the immobilizer receiver (and a bunch of other stuff you'd probably notice wasn't working, including the driver's seat control and navigation if your MDX has it).

Fuse #6 in the driver's side under-dash fusebox seems to be involved, too...

There's a notation in the manual about Fuse #92, in "Auxiliary Fuse Holder #2", but you're on your own finding that one... ;-)

The fact that the immobilizer key light flashes continuously does suggest that the system DOES see the key, but that it's not a recognized key, so either the key transponder died, or the immobilizer system lost its memory with the battery out (most likely).

I looked through the system troubleshooting info in the shop manual, and it's clear that you're going to need to hook the car up to an HDS (Honda Diagnostic System) computer to make any progress on troubleshooting it... there just isn't much beyond fuses above that's likely to fix it, other than some diagnostics / reprogramming with an HDS. Of course, there are plenty of other reasons to buy an HDS system, since they can save you a fortune by breaking your dependency on the dealer to do SO many other things to your car, and it's a great help in troubleshooting and testing. There are plenty of cheap HDS clones out there... I can't say which ones are good and which ones aren't though... I bought mine a couple years ago, and since then some very inexpensive options have cropped up. You should be able to do some quick research on the options online... or maybe even find an MDXer in your area who'd let you borrow their HDS, or come over and hook it up to take a look at your car.
 

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Fuse 13 in the passenger's side under-dash fusebox powers the immobilizer receiver (and a bunch of other stuff you'd probably notice wasn't working, including the driver's seat control and navigation if your MDX has it).

Fuse #6 in the driver's side under-dash fusebox seems to be involved, too...

There's a notation in the manual about Fuse #92, in "Auxiliary Fuse Holder #2", but you're on your own finding that one... ;-)

The fact that the immobilizer key light flashes continuously does suggest that the system DOES see the key, but that it's not a recognized key, so either the key transponder died, or the immobilizer system lost its memory with the battery out (most likely).

I looked through the system troubleshooting info in the shop manual, and it's clear that you're going to need to hook the car up to an HDS (Honda Diagnostic System) computer to make any progress on troubleshooting it... there just isn't much beyond fuses above that's likely to fix it, other than some diagnostics / reprogramming with an HDS. Of course, there are plenty of other reasons to buy an HDS system, since they can save you a fortune by breaking your dependency on the dealer to do SO many other things to your car, and it's a great help in troubleshooting and testing. There are plenty of cheap HDS clones out there... I can't say which ones are good and which ones aren't though... I bought mine a couple years ago, and since then some very inexpensive options have cropped up. You should be able to do some quick research on the options online... or maybe even find an MDXer in your area who'd let you borrow their HDS, or come over and hook it up to take a look at your car.
Thanks for the reply.

I'll take a look into those fuses.

Regarding the blinking continuously immobilizer, it does that with the dummy mechanical key too (I used it to enter the brake code)

I got the immobilizer bypass brake code from the dealer and entered it, with the green key blinking at the end to confirm successful entry. But the car still doesn't start - only cranks. The fuel pump doesn't seem to come on in the II position either. The immobilizer light still keeps blinking as before, like the key isn't being recognized. (The immobilizer light is supposed to stay on solid green after bypass code entry)

Does this rule out the key reader being at fault?
 
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