??? - 85 corvette - no 2 sec fuel prime - bad ecm?
 

spent most of the day troubleshooting a fuel pressure problem on my 85. THE PROBLEM - I'M NOT GETTING A 2 SECOND FUEL RAIL PRIME WITH THE INITIAL "KEY ON." I have checked the fuel pump relay as well as all associated power and grounds. electrically, everything checks. I can jumper or by-pass the relay and get pump to turn on. ran a couple checks on the relay itself - even swapped out a new relay - still no 2 second prime signal. the engine will start after a few cranks, mostly because the oil pressure switch will by-pass the relay and apply 12 V directly to the pump. once started, the car runs like a champ - fuel pressure around 34psi. no "service engine" and i'm assuming no codes. it seems like it boils down to the ECM. from the ECM voltage chart in the FSM, pin A1 (wire that goes the relay), with key on, I should get 12VDC for 2 seconds, then zero volts. here's what i'm getting - key off .02V, key on .67V for about 2 seconds, then around 12.5V (battery voltage). I would think the 12.5 volts after 2 seconds would activate the relay, but it doesn't, which has me kind of stumped. seems like things are backwards. so my question - bad ECM? :( I really don't know much about the ECM, but i'm thinking there an issue with the FMD (fuel management device) circuitry. really hate to run out and buy another ECM, and just start swapping out parts - so, any other checks? am I over looking something? :(:(:(

Did you test this 12V at the ECM Connector A Pin 1 or at the Relay?

Also check ECM Connector B Pin 2, it also goes to the relay, says Fuel Pump Sig. on my diagram.

Should also have power at Pin G of the ALDL connector.

Did you check your Ground Circuit for the Relay? Do a voltage drop test on both it AND the power feed to the relay. Should be under .2V or so. Better even yet to do it in the mV range and tell us that #. You HAVE to do this under a loaded circuit so try the test with everything plugged in and then turn the key to on, and note the reading.

Backprobe connectors and such as necessary.

If you want a simple test, do this. Back probe ECM A Pin 1. Take a test light and attach to battery ground. Touch back probe. Turn key on, does it light for 2 seconds? Does it light? If yes, ECM is ok. You have a wiring or ground issue elsewhere.

If not check ECM, powers and grounds. Also ask yourself, "What does the ECM NEED to see to control this output to the pump?" Do those things happen?

You could do the same test light test at the relay to test your wire to the control side of the relay. Pin C Green White wire. If it lights, (brightly) your wire is good.

To check ground attach test light to battery positive and touch pin B with the relay out-black/white wire. If it lights, your ground is good.

A incandesant test light should draw enough amperage to run turn on a relay so this is a valid test. Control side of relay does not draw much amperage.

Hope this helps.

I like your attitude. Any idiot can turn a wrench or be a parts changer. Don't be a parts changer.

Sounds like you've got a FSM, so that's good. Are you looking at the Schematic diagram? Are you measuring the voltage on the coil of the relay? Those are funny numbers, that sound more like they might be for something else. Where are you taking your measurements? Try removing the relay and measuring at pin C of the relay socket. Do you have "ground" at pin B of the relay socket?

:cool:

If you just attempted to power the fuel pump through the G terminal of the ALDL and the pump DID NOT RUN couldn't you then likely consider that either the ECM or the RELAY was at fault? Considering that all of the connectors are clean tight and connected to the correct components.

Continue the balance of the tests after this attempt?

thanks for the input. got a good night's sleep - thing's a little clearer this AM. to answer 93Rubie, all electrical checks were done under the hood - haven't checked anything directly at the ECM yet. thanks for the other suggestions. formulating a battle plan this AM. BTW, I don't wanna be a parts changer, but I am a wrencher. I am however, still an idiot - :D :rofl:

thanks WVZR-1 - didn't apply 12V at pin G of the ALCL connector, but pin D of the relay itself, and the pump came on. all my tests have eliminated a relay problem. i have a back-up relay, and as is said swapped out the relay - same problem. damn, did I just admit i'm a closet parts changer - :eek: :rofl:. will continue troubleshooting things today....

Hot Rod Roy - :thumbs::thumbs: - BINGO!!! there's something you said here that has me thinking. :iagree:, FUNNY NUMBERS, BUT from a "sequence of events" thing, the ECM may be doing it's job. now that I think about it, the numbers were .02V, .67V, and maybe 12.67V and not 12.5V. note the .67 numbers. now I did not pay attention to the polarity of my readings, but if I had a short or something feeding battery voltage back into the circuit somehow, a negative or reverse potential - maybe something like a power and ground reversed - a, one would cancel the other, condition might exist. i'm going to do is double check the voltage and pay special or close attention to the polarity. there were only a couple places where I did any harness repairs - maybe I got something bass-ackwards - :eek:

anyway guys, thanks for the help and suggestions! this is the only real issue with the car. solved the cooling fan problem, and the raspy exhaust is caused by leaks at both ends of the magnaflow cat. I always felt there were issues with the fitment anyway. not sure my "old-man" ears can handle the noise anyway - may just go back to stock!

update - after troubleshooting the dog crap out of this thing, it seems that the problem IS within the ECM itself. anyway, replaced it with a delco remanufactured unit - problem solved. not sure what I did, but I must of F-up something during my top end rebuild. :eek:

Something took out the driver circuit in that ECM. You should get a Meter and put in in Amps and in series with that Fuel Pump Relay Control circuit and measure how many amps are going into the ECM.

If it is over 750milliamps, you WILL fry your new ECM in short time.

I suppose and old ECM could fail, but do you want to assume or know? I would want to know my circuit is NOT feeding too much amperage into that ECM. Just my 2 cents.

It doesn't work that way. The ECM takes what it needs and no more. Each pin has circuitry to protect itself from surges, shorts and other bad stuff.

:iagree: - somewhat, but as I said -:iagree:, but they don't protect themselves from idiots :D. I know exactly what I did, but i'm taking this one to the grave :hide:. BTW, a little FYI - never, never, never power up anything without first having the ECM grounded. ...just sayin' - :thumbs:

That will do it. Lesson learned. Imagine going something like that one a newer car....thousands of dollars is possible. :willy:

Cliff is right they will protect themselves but even without the help.....older ECM's did not have the protection newer ones do. The logic is simply not in them.

I've got to chalk that one up to murphy's law, and you are absolutely correct - lesson learned. :rock: