Here's a stumper for ya....

[c4c5specialist]

Ok, got some weird electrical behavior. I'm going to tell ya'll up front I think it is the ECM but I could be overlooking something more fundamental. This is on an 84 corvette. This behavior happened the day after a huge storm (just keep that in mind).

Just so you all know I'm pretty savvy with wiring and understand these circuits individually that I'm about to explain so I don't need direction there. What I do need to understand better is how these interact within the ECM to find the "cause"... unless the ECM is the problem itself.

Fuel circuit - only power going into the relay from the "O" wire (which is the direct feed from the battery). Nothing coming from the ECM. No power on either side of the fuel pump fuse either.

The fuel pump circuit uses the ECM to ground the fuel pump relay and turn on battery power to the fuel pump. CKT 465, which is the green/white wire from the relay is the ground source supplied by the ECM. Using a Test light connected to battery POSITIVE, when you turn the ignition on, probe this wire and the light should turn on for 4 seconds. Battery power on the orange wire, CKT340, is good.
That tells you that supply from the fused link is GOOD and is NOT burned out. The 2 wires that come from the same terminal at the fuel pump relay, CKT458 are the supplies to the ECM and oil pressure sensor. Just remember, that the relay is important to both fuel pump and ECM. So you MUST have battery voltage at these 2 wires BEFORE your engine will start. Your fuel pump fuse will NOT be live until the relay transfers power to the fuse.


Fan circuit - I had grounded the fan to come on automatically with the ignition switch. However, they (I added an auxiliary fan too) aren't working. Power through the fuse, however, no power to the wire going to the fan. Again, I'm thinking no power coming from the ECM to the fan itself.

Check your fan relay, that is supplied by the smaller wire coming off the battery. Your fan can be controlled 2 ways, by AC function or by the switch on the engine.

Inst cluster fuse does not work on either side...... weird.

Check your Gauges fuse below the Cluster fuse. They are both supplied off the same circuit, as is the cruise, signals, so check all of those and let us know what you have.

The car just turns over and is obviously not getting fuel. I was thinking that perhaps the oil sender went, causing the fuel circuit to shut down but that wouldn't explain the influence on the fans......

So, any thoughts you vette gurus????

Hi there,
I would check your fuel pump relay if the ground source is being supplied correctly by the ECM when you turn the igntion is on.Use the testing sequence in bold above to confirm function.
Allthebest, c4c5:bang

 

[chevyaddict]

Outstanding!! c4c5!!! I will troubleshoot all these items this weekend and let you know what I find!!!

 

[chevyaddict]

More Info

Ok, so I haven't had a chance to get over to his house to do more troubleshooting with the recommendations... but he did tell me today that when he tries to get it to punch out a code with the check engine light by crossing the A & B terminals, NOTHING happens.... Wouldn't this be an ECM failure? Even if there are no trouble codes you still should get the 1 2 1 2 1 2 sequence..... any thoughts?

 

[toptechx6]

No MIL flash

Not if the power supply or ground to the PCM is missing. You could also have a problem with the MIL lamp, does it illuminate with the key on? Another possibility is the circuit between the ALDL and the PCM, if the PCM doesn't see the "signal" to enter diagnostic mode the MIL will not flash. There is no magic shortcut when it comes to wiring unfortunately. Since you know the MIL won't flash a good place to start might be the test procedure for that condition in the FSM. Good luck.

 

[chevyaddict]

OK EVERYONE! I have this flipping car at my house now and I've decided today is the day to troubleshoot!! I know... its been awhile. I will certainly pass on what I find here to confirm deny all the awesome suggestions!!!! WISH ME LUCK!!!

 

[toptechx6]

I thought you sold that thing it's been so long! Best of luck with it.

 

[John Robinson]

No fuel

I know this may sound crazy but check the tps. I had a condition were it thought the car was flooded. I had spark and fuel but the tps shut down the ecm because it was reading a flooded condition. Changed the tps and car started right up. I went the ecm route before I discovered the tps was the problem. Incidentally I did not have any codes when this happened.

TPS ( Throttle Position Sensor)

 

[chevyaddict]

to c4c5.......

This is what you said to try:

The fuel pump circuit uses the ECM to ground the fuel pump relay and turn on battery power to the fuel pump. CKT 465, which is the green/white wire from the relay is the ground source supplied by the ECM. Using a Test light connected to battery POSITIVE, when you turn the ignition on, probe this wire and the light should turn on for 4 seconds. Battery power on the orange wire, CKT340, is good.
That tells you that supply from the fused link is GOOD and is NOT burned out. The 2 wires that come from the same terminal at the fuel pump relay, CKT458 are the supplies to the ECM and oil pressure sensor. Just remember, that the relay is important to both fuel pump and ECM. So you MUST have battery voltage at these 2 wires BEFORE your engine will start. Your fuel pump fuse will NOT be live until the relay transfers power to the fuse.

With the ignition on and the test light to the POSITIVE side of the battery I touched to the green/white wire. Nothing. I then switched the test light to the NEGATIVE side of battery and starting probing. NOTHING out of any part of the fuel pump relay. I also do not get anything out of either side of the fusible link that runs from the thick red wire that comes out of the fuel relay which connects to the orange wire which runs to a 20AMP fusible link and back to the ECM I presume?... I'm guessing this is what you are talking about that when the ECM is grounded it allows power through the relay, yes? NOTHING... on either side....so - does this lend itself more to bad ECM ground then? I'm going to check that next. Car does turn over there and there is spark through ignition. Also, the battery will die after sitting for not very long so there is obviously a power drain too....

Hey Toptech - This is the exhusband's car BTW..... He got the 84 during the divorce. I was the troubleshooting/electircal guru betwixd the two of us so there ya go!!!!

 

[chevyaddict]

I know this may sound crazy but check the tps. I had a condition were it thought the car was flooded. I had spark and fuel but the tps shut down the ecm because it was reading a flooded condition. Changed the tps and car started right up. I went the ecm route before I discovered the tps was the problem. Incidentally I did not have any codes when this happened.

Does the 84 have TPS???? What is TPS??? If this is for tuned port the 84 doesn't have that - its the cross-fire injection setup.....

 

[chevyaddict]

HOLD THE FORT!!! I think I may have just found it...... reconnecting something that got pulled apart in this circuit... stay tuned!


NO!!!!!! That wasn' it... rats! It was a contributor though..... my check engine light is now working and I have power to the circuits............

When I run that test again to the green/white wire it doesn't shut off after four seconds...... it just stays lit....... what does that mean?

 

[John Robinson]

OOps

Does the 84 have TPS???? What is TPS??? If this is for tuned port the 84 doesn't have that - its the cross-fire injection setup.....

TPS (Throttle Position Sensor)

 

[chevyaddict]

TPS (Throttle Position Sensor)

:duh I knew that.............. Thanks!!!

 

[chevyaddict]

Well, Just for the heck of it I ran the manual feed to the fuel pump.... could hear it turn on so I thought for sure it would fire and it STILL won't start. I'm going to confirm the spark thing - He told me it was getting it but I should check myself. I'm really going to be perplexed if its getting spark and at least fuel up to the motor and won't fire - then its certainly something else. Fans still don't work (although power thru the fuse), fuel pump fuse is dead on both sides too....... hmmmm

 

[chevyaddict]

FUEL PROBLEM

Ok, get this..... if I squirt some carb cleaner down the throttle bodies it tries to start. So, perhaps it is the TPS.... something is certainly stopping the fuel from entering the motor. There is power to both injectors.

SO - here is a new question (riddle)...

What sensor would simultaneously cause the fuel pump relay to shut down and also when you bypass the fuel pump to make it run still inhibit fuel from entering the engine? Although the TPS could be it, I don't think it is causal to shut off the power to the relay. I'm going to undo a fuel line and make sure fuel is at least getting up to the throttle bodies but I suspect it is....

 

[John Robinson]

This is for a 93

Throttle Position Sensor

The scan tool displays throttle position in volts. The voltage should display between .23 - .59, with throttle closed and ignition “ON,” or at idle. Voltage should increase at a steady rate as throttle is moved toward Wide Open Throttle (WOT) about 4.6 volts.

The ECM has the ability to Auto-Zero the TP sensor voltage if it is between .23 and .59 volt. This means that any closed throttle voltage between .23 and .59 volt will be determined by the ECM to be 0% throttle angle, and should display 0% when the throttle is closed. A failure in the TP sensor circuit should (MY OPINION MAY NOT )set a DTC 21 or 23.

Copied from the service manual for a 93 model

 

[Grizzly]

Ok, get this..... if I squirt some carb cleaner down the throttle bodies it tries to start. So, perhaps it is the TPS.... something is certainly stopping the fuel from entering the motor. There is power to both injectors.

SO - here is a new question (riddle)...

What sensor would simultaneously cause the fuel pump relay to shut down and also when you bypass the fuel pump to make it run still inhibit fuel from entering the engine? Although the TPS could be it, I don't think it is causal to shut off the power to the relay. I'm going to undo a fuel line and make sure fuel is at least getting up to the throttle bodies but I suspect it is....

Just browsing this thread and had a thought. What sensors would shut down the fuel if you had an accident? Perhaps oil pressure? Also would it shut down if it overheated? Neither of these conditions exsist obviously, but if a sensor malfunction said was so the ECM would shut the fuel down.

Just another place to look.................Griz

 

[John Robinson]

Good Idea

Ok, get this..... if I squirt some carb cleaner down the throttle bodies it tries to start. So, perhaps it is the TPS.... something is certainly stopping the fuel from entering the motor. There is power to both injectors.


Thats what I did and the car ran and restarted. In my case the ECM was being told the car was flooded. In that situation the ECM shuts down the power to the injectors until the engine turns over and clears itself by coughing and running. The carb cleaner trick made it run long enough for the ECM to resit itself. However when the engine ran it idled at 2000RPM. I removed the wire harness from the TPS and the rpm's dropped to around 1200 on idle. I took the after market $20.00 TPS off through it as far as I could and replaced with an OEM part problem solved. Just to add fuel to this problem I was in a garage the other day and they had a car that would not run and they were stumped. I suggested they check the oil and they told me it was 2qt low but that shouldn't stop the car from running. After another hour of shop talk to it they added the 2qt of oil it was down and what do you know it fired right. Here's a hint it wasn't a ford or chrysler product

 

[1984C4]

Grizzly, you may be on to something there with the oil pressure switch. If she is not getting any oil pressure in an 84 it will not start but maybe it will try to burn the carb cleaner while cranking since it isn't getting fuel. If this started after the rain have you looked at the ECM to see if it was in the path of a water leak?

 

[1984C4]

Dawn, You may have this from Bill already. If not it may help you out. http://misterbill.homeip.net/vette/wiring/

 

[chevyaddict]

UPDATE

Well, I finally got a chance to tinker some more with this #@!(*&% car! You guys are gonna love this.

First, the reason the fans aren't working is because the wire I grounded got knawed on by a critter and severed it. I found this when I was looking to make sure he put the fuel filter on in the correct direction. The reason I was doing that was because, even when I engage the fuel pump (I can hear it running), no fuel is getting to the throttle bodies!!! None! Zilcho! I check and there is plenty of fuel in the tank. Now, the fuel pump is new - it was replaced thinking this was the problem. So, either there is a massive clog in the line somewhere, or the strainer is blocked, or the fuel filter is blocking the fuel. Weird.

HOWEVER, this doesn't explain why the fuel pump won't turn on with the ignition. The only way I can get it to run is if I run a wire from the "g" on the ALDL to the battery......

Don't you just love this???
Dawn