ls1mx5

Ok here goes-

I am in the process of finishing my ls1 conversion into my 1990 miata... The wiring is almost done. The engine, harness, and ecu are from a 1998 z28 6 speed... The motor cranks, but does not start. Ecu has mil on, but does not let me read the codes using a mac scan tool. I have power at all of the correct pins, and grounds also check good. I also installed a vats bypass circuit. I just can not get spark. I was hoping someone might have an idea on what I'm missing... How can I tell if the ecu is bad? Does anyone have any other ideas? What am I missing?

TIA!
Karl

LS1Sandrail

Do you have fuel pressure the entire time you are trying to start? Are the injectors getting pulsed? Hook up a pressure guage and a noid light and find out. I think the PCM just grounds the injectors and they get 12V from the switch. Are the injectors hot with the switch on?

If none of the above is working, the next place I'd look is the VATS

Old Drive-ability

I can try to help you but it’s been a while.
The fact the starter is turning may say the VATS is working fine. The ignition at start up may not need a healthy ECM. I would listen for the sound of the fuel pump cycling on for the two seconds it should when the key is on and the engine is not running. IF you don't here the pump or relay click on and off I would wonder about the ECM but as I said most G.M. systems don't need an ECM to make spark at startup. I would then surely do as suggested test the injector pulse while cranking. If the injectors were being pulsed you would surely smell fuel at the tip of the exhaust. Normally as I remember even if the starter works or is jumped the vats will run the injectors for a few seconds and I don't think it gets in to the ignition system.

John_D.

iTrader: (2)

How do you know if it's fuel vs. spark?

I believe VATS turns off fuel, not spark.

Spark can be off if the cam sensor is telling it to fire on the wrong stroke. Although I thought the pcm will try to fire on opposite cycles to get back on track, if it doesn't get a hit on the initial try.

I've heard of people wiring the coils wrong too. I think this is in reference to getting the front to rear reversed on a bank.

It would be helpful if you could get something connected to it to get any codes from it.

I'd check key-on fuel pump activation, fuel pressure while cranking, and double check the coil wiring.

slow

iTrader: (7)

with vats not disabled my car will run for a couple seconds before shutting off.

With a dead pcm, it would crank, have spark, but no injectors firing.

Ryan,

ls1mx5

hey guys-

thanks for the info... i need to double check vats... fuel pump is on a seperate circuit at this point. i can get the pump to go on and off... the starter system is basically just connected to the factory miata wiring... do i need to connect the neutral safety switch to the ecu??? i do not smell any fuel, and have had no pops through the headers yet... i have a spark plug connected to a coil, and touching a valve cover bolt. NO SPARK! plenty of cranking amps... one thing that worries me about the ecu, is that i bought it from a junk yard and have no idea if it is good or not... the only way that i might have hurt it, is that on the first 2-3 times cranking the engine over, i forgot to have the large ground strap connected... so, the starter circuit pulled ground through the ground strap on the drivers side, and got hot enough to cook some of the loom off of it. could this have hurt the ecu as well??? the ecu was just grounded through its small wires? i really do not want to have to buy an ecu as a guess fix! i did wire up the check engine light, and that goes on when cranking, i still have egr and o2's disconnected... so i think the ecu might be working... however, when i connect the mac scan tool, it can not get a signal to pull the codes out!?! any ideas? all of your input is appreciated... im so close yet so far...
thanks again
karl

Jones'n

Is the Crank Position Sensor wired in correctly and pluged in? That can cause no spark. Also, check that the computer grounds and powers are all good.

kossuth

iTrader: (12)

Before you get too crazy and think the PCM is bad or something check to see if the coils have power. The fuel injectors and coil packs for both sides of the motor use a single 16 gauge wire on each bank to feed the coils and injectors. It's the H pin on the main coil pack connector which is a pink wire you should check for voltage. It should be a constant hot.

ls1mx5

ok- a little more info... i am using the complete factory harness that came on the engine. as far as i can tell everything is connected correctly. i do not have egr or oxygen sensors connected at this time. i also eliminated a/c compressor... i do have a few connectors that i have not yet figured out, but as far as i can tell all of the critical connectors are plugged in. i will check coils for power. thanks for the info! keep it coming... does anyone have a spare 98 ecu? i wouldnt mind trying that to eliminate that variable.

Jones'n

Be sure that one of those few connectors isn't the crank position. It is a 3 wire sensor just above the starter. That one NEEDS to be plugged in to get spark, and I know sometimes people don't really see it, so they don't plug it in.

kossuth

iTrader: (12)

Ah, thats another important piece hehehe. That or the cam sensor and it probiably won't fire. You can get away with just about anything else being unhooked just to see if it starts or not, but those two are a must

ls1mx5

Jones'n- THANKS! I did miss the crank sensor!!! I'm sure that will help once I figure out how to get the connector on! The engine is in the car, and there is almost no room around the starter... I will need to get the car back up in the air to TRY to contort myself enough to plug it in! This is very promising... The only thing that still has me worried about the state of the ecu is that I still can not get the scan tool to communicate with it! ANY IDEAS ON THIS?!? THANKS AGAIN!

kossuth

iTrader: (12)

Not sure which pin on the 98 ECU is the class II serial data port, but you need to hook that into the DLC plug for the scanner. It is supposed to be on pin # 2 on the connector. Pins 4-5 have to have a ground on them, and pin 16 is supposed to have a constant hot on it. Some tools need to have power to them in order for the tool to function properly. Hope this helps.

NC98Z

iTrader: (1)

I had a cpu from [ a major tuner company, don't want to **** nobody off I don't have to] a well known tuner replaced when the tuner unit I bought burnt up mine due to the tuner unit being bad. They sent me back a new cpu with their program and the car would crank but not fire. Had to relearn the cam and crank positions. $80.00 is the going rate a the Chevy place for a relearn, or a complete reflash. So if I ever get another hand held tuner Unit I will be setting at the Chevy Dealers when I plug it in my Z28.If it won't run at least I can beat the towing fee!

ls1mx5

FYI- ITS ALIVE!!!

after about 15 trys with the car spitting and sputtering it finally stayed running! it sounds awesome! it seems like the ecu needed to relearn the throttle sensor and assorted other sensor for a few minutes... the scan tool can now read codes, and everything looks/sounds good so far... the car shoots flames and sparks out the open headers! now i only need to figure out 500 more little issues before i can take it for a ride... thanks again for all of the help! that crank sensor was buried! all better now... soon...