key in start position and no CEL - bad ecu maybe

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kendem
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Got a 91' LE and yesterday the engine cranks but doesn't start. I noticed my cel was not on when key in start position. Pulled the ecu out and sure enough the red light was off. So either my ecu died or no power going to it. I checked all the fuses and relays , even the ignition relay and all looked good.

Any suggestions on how to tell if the ecu is really bad?


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VStar650CL
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The MIL has its own dedicated 10A fuse in the cabin fusebox and should function even if the cluster doesn't, so I'd say a dead or unpowered ECM is a good guess.

kendem
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Any way to test the ecu directly, the fsm has no steps to do that

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VStar650CL
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Not besides the blinky lights. That's part of what they're for. If power and ground are both good but no blinky will light, it means a dead ECU

kendem
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Can I remove the ecu from the harness and test the terminal harness directly to see if i'm getting power as its too difficult with the ecu attached. If so which pins on the harness would you suggest to test with?

Last night, with the key in the start position I did test pin 46 with the harness attached and it was about 11.4 volts, I used ground from the negative battery terminal.
Test my battery directly and I get 12 volts. I don't know why the difference.

I might remove the ecu and apply battery voltage directly to pin 46 and use pin 48 as ground would that work?

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VStar650CL
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Here's the breakdown I see from the FSM.
OR, 34 - +12V at all times
B/W, 47, 109 - +12V at all times
B, 13, 107, 108, 116 - Must have ground. Engine and chassis grounds must also show good connection
B/R 36 - +12V with key in On or Start

If that all checks good, the ECCS Relay still needs to turn on for the ECM to wake itself up. The B/W on 47 and 109 should go to +12V with the key on if the relay is working. If not, check for +12V on R/B at 4 with the key off, if not then the relay coil is probably open circuit.

kendem
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Ok, thanks i'll check those things you suggested. But the eccs relay was the first thing I checked based on the fsm and it was ok. I even switched it to use the fuel pump relay (as they are identical relays) and still ecu didn't come on so I doubt it is a bad relay.

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VStar650CL
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Yes, but the ECM needs to activate it. It won't do that without an ignition signal and can't do it if there's a disconnect in the wire from the relay coil. That's why you need to see 12V at pin 4.

kendem
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VStar650CL wrote:
Wed May 18, 2022 10:23 am
Yes, but the ECM needs to activate it. It won't do that without an ignition signal and can't do it if there's a disconnect in the wire from the relay coil. That's why you need to see 12V at pin 4.
Correct from what I can see, the eccs relay is dependent on the ECU not vice versa. So I believe my relay is good. The fsm says pin 4 should have 12 volts for only 1 second after the key is turned in the OFF position. Does that mean it should have 12 volts when the key is in the ON position?

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VStar650CL
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No, that's backwards. The ECM drives the low side of the coil with an open-collector output (i.e., it can only drive ground to the wire or go open-circuit, it can't drive 12V). So you should see 0V on 4 when the relay is on, 12V through the relay coil when the ECM shuts itself off by going open-circuit.

kendem
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Ok so my ecm won't turn on, do I still test voltage on pin 4?

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VStar650CL
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The purpose of the ECCS Relay is so the ECM can turn itself on. That's so it can do EONV testing in the middle of the night while the car is parked and you're asleep, but it should also turn itself on whenever it sees an ignition signal on pin 36. So you need to check 36 first, if that has no power then the relay is irrelevant. If 36 does have power, then check 4.

kendem
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Tonight when I get home I will put the key in the ON position and check pins 36(igition switch) and 34 (start signal) and if both are 12 volts then I know power is going to the ECU.

Assuming 36 has power and with the key still in the ON position then check pin 4?

The fsm says turning the key to OFF will cause pin 4 to have 12 volts for 1 second.

kendem
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Correction, pin 34 only has 12 volts when the engine is started.

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VStar650CL
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I wouldn't bother checking 34 unless the ECCS Relay turns on. The start signal is irrelevant unless the ECM is waking up.

kendem
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Last night I pulled the eccs relay out and turned the key to ON position and checked the 1 and 2 terminals where the relay sits in the relay box and no voltage, I don't know if there should be voltage when the relay is out. I don't really understand the wiring diagrams too well.

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VStar650CL
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The two Red wires on the ECCS Relay should have +12V at all times. If they don't, one of the two Brown fusible links is blown.

kendem
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When i came home i disconnected the neg battery and removed the ecu harness and tested all of the ground pins,107,108, 116, 6, 13,17, 49 and 39. They all tested good. Then i tested pin 36 with the key in the ON position and got 12 volts.

Then put the harness back on the ecu and key in ON position but no voltage on pin 47. It is almost impossible to test those pins with the harness attached because of the tight and crowded wires. I was about ready to give up and get a new car, I've had this car for almost 32 years and done all my own work.

After i read your last post i immediately went out to my car and pulled both the brown fusible links AND SURE ENOUGH ONE WAS BLOWN. funny thing was that i checked the other 3 links and they were good, don't know why i didn't check the brown ones.

I hope i can get the link at the parts store tonight, I'll let you know how it goes.

I can't thank you enough for all the help!

kendem
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Now where in the world do you get a replacement fuseable link?? Can you get any 25 amp fuseable link and just wire it to the terminals.
I saw this on amazon and it looked identical to mine https://www.amazon.com/Bussmann-BP-FLN- ... B009WQPOGI
but it said it would not fit a nissan 240sx 91'

Any suggestions

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VStar650CL
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You're most welcome. You can use anything (fuse or fusible) with a 25A rating if you don't care about appearances. If appearance matters, you can construct a new one using fusible link wire and the old terminals, several places on Amazon and eBay sell link wire. BUT, get a fuse holder and some 25A fuses for temporary testing and be prepared in case it blows again. Then you'll be looking to find a short circuit and not an open.

kendem
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New fuseable link and still nothing. Pins 47 and 38 have no voltage with key in on position, very frustrating. I hade such high hopes.

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VStar650CL
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Bad Ignition Switch, maybe? B/R on pin 36 should get 12V power straight from B/R at the ignition switch, no fuses or intervening connectors.

kendem
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I already tested pin 36 with key on and it had 12 volts

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VStar650CL
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Pins 38, 47 and 109 are all on the same wire with a splice, and will have power only if the ECCS Relay turns on. Put a lamp between any 12V source and R/B on pin 4 with the key on (back-probed, checking it with the connector off won't tell you anything). If that lights then there's a problem with the ECCS Relay or the relay wiring and I'll help you trace it. If it doesn't light then the ECM is either dead or has a dead relay driver.

kendem
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I checked pin 4 and it has 12v with key on, i don't have a lamp like you mentioned

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VStar650CL
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It shouldn't be showing 12V, it should be going to ground. Since the ECM is getting the Ign signal on 36, it means it either can't or won't wake up. You can try jumping ground to pin 4 with the key on and see if you get blinky lights. if so the ECM is alive but the relay driver transistor is blown. If not, the ECM is a dead horse.

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VStar650CL
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Let me clarify that, 4 should go to ground with the key on. With the key off it should read 12V through the relay coil, as I mentioned earlier.

kendem
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Ok I'll try that tomorrow, what is the relay driver transister? I did check the eccs relay and it works

kendem
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The eccs relay is not clicking not when the key is turned to on

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VStar650CL
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The ECM uses an N-transistor internally to ground the relay coil. If that transistor is blown, it's possible the ECM is functional but can't turn on the relay to wake itself up. It's a stand-alone transistor on many of the old ECM's and can be replaced by somebody who's good with solder to get it working again. That's why grounding pin 4 with a jumper will tell you something, you're making the jumper do the transistor's job.


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