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Discussion Starter #1
Hi,
I have been having some very serious and confusing problems causing my 2005 Mazda 3s sedan (automatic) to not be able to start the engine. What happened was that a couple months ago, on a hot day, I had been driving for a few hours (I work as a delivery driver) and when I had changed direction to go forward after one stop, all power to the car went out completely (lights, engine, horn, etc.) but that came on again by itself about 3 minutes later before I even had a chance to open the hood. A little later as I was driving, it started to happen again but didn't go off completely. However, about 1/2 hour later, it did, I think just as I was starting to back up. This time I could not get it to come back, even with jumpstarting. I had it towed home, and the next day as I was cleaning the battery terminals, accidentaly connected the two + cable wires together without the washer inbetween, which apparently made a better connection. The power was back on to the car, but the engine would not start. The immobilizer light shows code 16 (instrument cluster can't communicate with PCU) and I cant read the OBD codes like I used to be able to. There is no cranking, and I eventually found that the problem is that the main relay is not closed by the PCU as it should be when the key is turned ON, keeping the engine fuses and ignition from receiving any power. I checked all the fuses in both fuse compartments, and all the relays with a small battery. They all work, but the computer for some reason does not allow the engine to crank. I tried reprograming the keys (although the battery had been disconnected for a long time several times before without any problem) and although it went through the programming sequence properly, it still won't start. We also replaced the started even though I was sure that wasn't it. I found a few little almost broken grounds near the headlight, but even with these connected up there's still nothing. I did some testing of various wires also as directed by mechanics, but I didn't really find anything wrong except there does seem to be missing voltage on a couple of the wires to the PCU. I'm not very good with electric charts, and so I was wondering if anyone can guide me through exactly which wires in the harness to test for voltage to determine if it's just that a surge when the battery cables disconnected from each other erased the memory, or if the whole pcu is broken, or a short or something else. If you go to the dealers they try to replace the instrument cluster, PCU, wiring harness and more for thousands of $$ when it could be just one simple wire. There is a thread on a different forum (Mazda 3 forums, called "Total Electrical Failure")that gives more info, but they weren't able to help more with testing.
Any further help would really be appreciated if you know what to test to determine exactly what the electrical problem is. I'm able to provide wiring diagrams and or pictures of the car if needed, since I really need to get this fixed as soon as possible without spending huge amounts of money.

Eric
 

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sounds like you have a pretty good idea for what electricity should be flowing where, which is a huge help. Have you looked at all at the fuseblock to see if the positive wiring going into there is in good shape? The fact that you weren't getting OBDII readings would make me think maybe the ECU has a bad fuse/poor connection, which would obviously cause starting troubles. I don't think the ECU is usually listed as having a fuse on most cars, it might be worth a look at both fuseblocks if you haven't pulled all fuses already
 

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Demon Spawn
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Hi,
I have been having some very serious and confusing problems causing my 2005 Mazda 3s sedan (automatic) to not be able to start the engine. What happened was that a couple months ago, on a hot day, I had been driving for a few hours (I work as a delivery driver) and when I had changed direction to go forward after one stop, all power to the car went out completely (lights, engine, horn, etc.) but that came on again by itself about 3 minutes later before I even had a chance to open the hood. A little later as I was driving, it started to happen again but didn't go off completely. However, about 1/2 hour later, it did, I think just as I was starting to back up. This time I could not get it to come back, even with jumpstarting. I had it towed home, and the next day as I was cleaning the battery terminals, accidentaly connected the two + cable wires together without the washer inbetween, which apparently made a better connection. The power was back on to the car, but the engine would not start. The immobilizer light shows code 16 (instrument cluster can't communicate with PCU) and I cant read the OBD codes like I used to be able to. There is no cranking, and I eventually found that the problem is that the main relay is not closed by the PCU as it should be when the key is turned ON, keeping the engine fuses and ignition from receiving any power. I checked all the fuses in both fuse compartments, and all the relays with a small battery. They all work, but the computer for some reason does not allow the engine to crank. I tried reprograming the keys (although the battery had been disconnected for a long time several times before without any problem) and although it went through the programming sequence properly, it still won't start. We also replaced the started even though I was sure that wasn't it. I found a few little almost broken grounds near the headlight, but even with these connected up there's still nothing. I did some testing of various wires also as directed by mechanics, but I didn't really find anything wrong except there does seem to be missing voltage on a couple of the wires to the PCU. I'm not very good with electric charts, and so I was wondering if anyone can guide me through exactly which wires in the harness to test for voltage to determine if it's just that a surge when the battery cables disconnected from each other erased the memory, or if the whole pcu is broken, or a short or something else. If you go to the dealers they try to replace the instrument cluster, PCU, wiring harness and more for thousands of $$ when it could be just one simple wire. There is a thread on a different forum (Mazda 3 forums, called "Total Electrical Failure")that gives more info, but they weren't able to help more with testing.
Any further help would really be appreciated if you know what to test to determine exactly what the electrical problem is. I'm able to provide wiring diagrams and or pictures of the car if needed, since I really need to get this fixed as soon as possible without spending huge amounts of money.

Eric
you touched 2 positive wires together on a live circuit? have you checked the main fuse? should be 80 or 100a fuse check that first if this is blown it will prevent everything from working as a safe guard for these things. otherwise it sounds like you f-ed the ecu. when you put the key in the ignition and turn to "on" what happens? dash lights come on, nothing etc? if nothing check the fuse if you get dash lighting get a code reader and scan the codes on the obd port and see if it gets a reading while the ignition is on, if it gets a failure to communicate the ecu is fried and will need to be replaced. and computers are ROM and very difficult to accidently just erase like the BIOS on a computer it is READ only and has to have special procedures done to unlock the read only and be changed, and no a tuner is not actually over writing the rom it is modifying what already exists. touching wires like that closed the loop on a positive feedback and mostlikey sent way too many amps back through the ecu and fried the rom or processor.
 

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Discussion Starter #4
Well I meant that it seems that one of the two wire connections that connect at the positive terminal of the batt. Somehow disconnected, and this may have caused power surges that damaged the pcu or something. There is only a click when starting (from the CIRCUIT relay I think) even though we replaced the starter to be sure. Is there a way to bypass the immobilizer so I can see if the pcu itself is ok?
 

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Discussion Starter #5
I finally got it fixed after more than 2 months!! All that apparently was wrong was that the fuse box plug was pushed up slightly by pieces of pine tree seeds or something that I forgot to clean when I had disconnected it! I'm so glad to be able to finally use her again, but now unfortunately the headlights and dome lights flash like a police car when off (but not when you turn them on) but I'm sure I'll be able to fix that, and it might have something to do with the alligator clips I was using as ground wires that I haven't properly fixed yet
 
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