Here's a stumper for ya....

[chevyaddict]

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.

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.

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

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????

 

[casper]

Hi Dawn

Are you sure you have spark when cranking?
Does the fuel rails pressurize when the ignition is switched on?
Any problem codes?

casper

 

[chevyaddict]

Well, I didn't bother troubleshooting the spark because if it isn't getting fuel it certainly isn't going to fire and if there is no power going through the fuse then there isn't any power getting to the fuel pump. There is no fuel at all... but the answer is not that simple in my opinion given some of the other problems. Fuel relay and fuel pump are both new, FYI.

 

[casper]

Hi
Tried bridging the fuel pump relay / providing direct power to the pump
casper

 

[Black Bart]

I think the ECM only provides a ground. I would check the ground wire on the ECM and make sure it is grounding good.

 

[toptechx6]

Fuse link?

There are exceptions but for the most part I agree with Black Bart, the ECM normally supplies a ground based on input from the various sensors. Newer C-4's have maxi-fuses that supply sub circuits, I believe your 84 may have a blown fuse link.

 

[G Winter]

If the 84 is anything like the 90 the fan relays have power to them at all times. The actuator coils on the relays have power to them when the ignition is on. I agree with Black Bart and toptechx6. You may have one or multipal burned fuselinks, or bad grounds. Have you checked voltage? Even a bad connection on the battery ground cable could give you trouble.If I remember right the ECM shuts down without proper voltage.I found this out when my alt. died last yr.

Glenn

 

[chevyaddict]

Ok, this is great advise you guys. So, where would this fusible link be for the ECM? I know there are some directly behind the battery. Its tough to get to those and check them without moving the battery itself. I don't think the battery ground is bad because the car turns over nice and fast. Where would the ECM ground be located??? Under the dash??

 

[toptechx6]

fuse link, ground locations.

My car is newer and I have no experience with older C4's, your best bet is to get the correct service manual. As a general rule, fuse links will be under the hood near the battery, no one ever said engineers think about servicability, sorry. On my 93 the ECM ground is near the oil filter close to the back of the block on the drivers side. Personally I'd check the fuse links first, remember once you find one that has blown you'll still need to determine the cause. You will need a wiring diagram to sort that out, another good reason to have a service manual. Good luck, let us know what you find.

 

[chevyaddict]

Oh yes, I have two manuals on the car so that is good! I'll check to see if the ground is there but there has been alot of electrical changes between 84 and 93 so it may be located elsewhere...

Also, just so you all know the car is getting spark to the plugs.

 

[toptechx6]

Service manuals

That's great chevyaddict, having the manuals will help a lot. You are right that the cars changed a lot between 84 and 93. I just thought of something and hope you won't take offense, trust me it happens to everyone. Any chance you grounded the wrong wire while in the process of trying to make your fans run all the time? It may not have failed immediately but just weakened a fuse link that has since let go, just an idea you might consider when looking for circuits to trace. Best of luck.

 

[Black Bart]

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.

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.

The ECM will provide a 5 volt reference signal to the sensors like the TPS but I can assure you it does not power the fans or fuel pump, it only provides a ground. If you did some wiring on the fans and got them wired wrong and fed power back at the ECM it is toast. Did all these problems start right after you changed the wiring on the fans. Please don't take offense but I have no idea how capable your wiring ability is and just trying to figure out what may have happened. Hard to diagnose wiring when you are not their to do hands on checking.
You could make a jumper wire and run it direct from the battery to the wire from the relay to the pump. This will by pass every thing and the pump should run and if it does try starting the engine and see if it will run.
Does the 84 have the oil pressure switch in the fuel pump line my 89 does if it has one that may be bad the jumper wire will tell you if bypassing that will allow it to run.

 

[toptechx6]

PCM toast?

We seem to be using the same logic process trying to determine what happened, I was hoping for a brighter outcome but you could easily be right Black Bart. A fried ECM wouldn't explain the lack of power to the fuses though so the fuse link is still a possibility. I'll keep my fingers crossed it isn't both issues.

 

[Robertwav1]

One other thing to add to all this. You mentioned "after a bad storm". Rainstorm? On another vehicle, in a trough, in the floor, I found a bad power wire connection....it was green powder laden acidic dust from water. 3 or 4 positive leads were connected together here and the only way I found it was with a chassis manual. Thought it might be worth mentioning.

 

[John Robinson]

Don't Laugh

I am not familiar with the engine set up on the early C4.
Does it have a distributor with a distributor cap? If so moisture in the cap would prevent car from starting. It is not uncommon for the cap to have a crack that lets water condensate inside the cap and short out the ignition.

Since your problem started with moisture remove the cap and see if it is wet if you have one.:confused

 

[gmjunkie]

Ok, this is great advise you guys. So, where would this fusible link be for the ECM? I know there are some directly behind the battery. Its tough to get to those and check them without moving the battery itself. I don't think the battery ground is bad because the car turns over nice and fast. Where would the ECM ground be located??? Under the dash??

Well that's where there at!! You will have to remove the side fender and Battery!!!I've changed allot of Fusible links,I keep 3 or 4 rolls of different Ga. fusible link in my Tool Condo!!!! They can Blow any time!!!They can get wet and blow,I've replaced allot of them!!!! Be sure you use the same Ga. or amp.!!!!!!! They can corrode and blow form Age!!~! I usually replace them all while I'm there!! There's also should be a couple ground connections behind there Too!!!~!!! And Look for Green connections!!~!!

 

[chevyaddict]

That's great chevyaddict, having the manuals will help a lot. You are right that the cars changed a lot between 84 and 93. I just thought of something and hope you won't take offense, trust me it happens to everyone. Any chance you grounded the wrong wire while in the process of trying to make your fans run all the time? It may not have failed immediately but just weakened a fuse link that has since let go, just an idea you might consider when looking for circuits to trace. Best of luck.

No no, the fans ran like that for years so that is not the case but thanks for the suggestion and no offense!! We all make those silly mistakes at times.

 

[chevyaddict]

One other thing to add to all this. You mentioned "after a bad storm". Rainstorm? On another vehicle, in a trough, in the floor, I found a bad power wire connection....it was green powder laden acidic dust from water. 3 or 4 positive leads were connected together here and the only way I found it was with a chassis manual. Thought it might be worth mentioning.

Hmmmm, this might be something to check out for sure actually. The weather stripping has been out of the car for some time in hopes to complete the bodywork to paint it. Yes, the interior got wet a bit so this is certainly something to check into..... THANKS!!

 

[chevyaddict]

I am not familiar with the engine set up on the early C4.
Does it have a distributor with a distributor cap? If so moisture in the cap would prevent car from starting. It is not uncommon for the cap to have a crack that lets water condensate inside the cap and short out the ignition.

Since your problem started with moisture remove the cap and see if it is wet if you have one.:confused

Yes, this is what we thought first thing.... but the car is getting spark so that isn't the problem and that wouldn't affect the fuel nor the fan systems but thanks for offering it up!!

 

[Black Bart]

TOPTECHX6 is probably right it may be a fusible link. If you do like I said and run a wire direct from the battery to the side of the relay that goes to the fuel pump and the pump runs then look at you Manuel and find out what link feeds it. Sometimes you can tell by looking but I have seen links that were bad and looked good. Check the links one by one eliminate what it could be and you will find the defective part. After you run the jumper wire let us know what the results was and we will go from their.
Sometimes on older cars like what we have someone has did wiring on them that later on will cause a problem. Chasing these kind of problems can be a nightmare.