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Good Morning everyone, I am in need of some master electricians or people with common sense which I lack. This 85 Chevy Corvette I have is an automatic with TPI. When I go to turn it on, no fuel pump can be heard, no dash, but the middle console(switches to change from english/metric, oil pressure, trip) works with lights, the power windows work. the headlights work. Hooking up the fuel pump directly works and will pump, so I went to go check the fuse. No power on either side. Im not sure exactly where to check the fuse's power source. I checked the fuel pump relay plug, the leftmost pin is getting 12v, im not sure what else is suppose to be getting power(if youre looking at the plug from the front of the vehicle). Im looking for some guidance on where to check for a fusible link or something.
Now onto the dash. The funny thing with this is that it looks completely off, but if you put a light up to it, you can see its on and trying to display, but is randomly displaying and turning off, and never brightnening to actually see it. I dont know if these two are related in power source, but I thought I'd ask. THANK YOU FOR ANY HELP. Jesse
Update, I tested the fuel pump relay and it’s good! I’m not getting power to the 3rd pin in the relay from the plug and I’m wondering if anyone knows where that runs to. I’m getting close
Take a look at this schematic. Hopefully it helps. Try using a jumper to bypass the relay to check the fuse and pump motor. Also, it appears that , with the ignition off, you can put 12 volts to pin G of the diagnostic port to power up the fuel pump temporarily, to check operation. Keep in mind that when you turn on the ignition, the ECM powers up the relay for about 3 seconds to build pressure in the fuel system, then once the engine fires up, the oil pressure switch takes over. Good luck!
. Keep in mind that when you turn on the ignition, the ECM powers up the relay for about 3 seconds to build pressure in the fuel system, then once the engine fires up, the oil pressure switch takes over. Good luck!
thank you so much for that schematic! New fuel pump relay and oil pressure switch and the pump turns on! Only… during the cranking :/
so it seems like the ECM isn’t sending the signal to prime the system. Any info on that you know of or am I looking at a replacement?
Does it start and run? (It's not clear whether it does or not.)
If it does not start, is the Check Engine Light ON with the key ON? If = NO, the ECM is not powering-up, and there will be no "prime" because the ECM isn't on.
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The Check Engine Light is NOT on! That was super helpful thank you!!
I did check all the fuses and they are all okay and ECM has power in the fuse, did not check ECM for being powered directly. However, I went to go plug in my ALDL for the first time and it’s not giving me any power to my code reader. Does that mean the ECU isn’t giving power to the plug, meaning the ECU could be dead?
I'd check the wire and terminals first. Make sure you have continuity between ECM and relay.
Im going to check this soon, just need to find a long wire haha.
Originally Posted by IHBD
GAGES fuse supplies power to the Check Engine Light. If this fuse is blown or not powered, the light won't come on even if the ECM is powered. A quick check of this circuit: Turn key to RUN. Does the SEAT BELT light and the warning tone come on for about 10 seconds and go off? If = YES, this circuit is intact. (The Audio Alarm Module is powered from the GAGES fuse.)
The ECM fuse supplies power to the ECM and also to the Knock Amp. Pink/blk wire. The knock amp is under the hood, on the passenger side, adjacent to the blower motor. on the outside of the blower case. About 3X3x1/2" thick. With the key in RUN, check for power to ground on the pnk/blk at the knock amp. If = YES, the ECM is probably powered at its A6, which is the "turn on" input.
There are two Hot at All Times battery inputs to the ECM. This power comes from the fuse link near the battery. There are 2 orange wires, one in each of the ECM connectors for this circuit. (There may be other orange wires, but you're only interested in the two power wires.) The power wires are in cavities B1 and *16. So the wires of interest are at the very ends of each connector. Check that they are powered at all times, ie with the key = OFF.
The OBD I Assembly Line Data Link (ALDL) connector does not have "power" in it for a scan tool or reader. OBD I scanners have separate power cords from the battery or lighter plug. Once you get the CEL to come on, codes can be read by jumpering cavity A&B together, turning the key to RUN, and any stored codes are flashed-out on the CEL. (This is colloquially known as the "paper clip method".)
Check the required circuits as outlined above. Eliminating possible faults keeps narrowing the possibilities until you find it.
The ECM did have power at both areas with the key OFF that you mentioned IHBD, so power is getting to it. Now the weird thing with my knock sensor is that it has a red and a black coming off it IF its the correct thing Im looking at. It comes off the side of the motor , inbetween the headers that you have to squeeze to remove yes? Well if thats the case it has been tampered with so I couldn't really get a correct reading on it.(If this is not the right thing, I couldnt find anything that looked as to what you described, Im not always the best when it comes to finding things. If there is any picture you can find im very much a visual learner HAHA)
Now what you mentioned previously that I did not answer. The car does NOT start but will every so often randomly hit and sputter a bit but never get close to even a rough idle. When the ignition is in the ON position both the "Door Ajar" and "Parking" Brake lights come on, but no check engine light nor seatbelt light(The seatbelts have been tampered with, it came with two extra seatbelt cases or whatever so could be the reason...). The dash is also extremely dim to the point you need a flashlight to see it, so I cant really rely on that. I did triple check the GAGE fuse and it is in tact and operating to my knowledge based off the two lights that are on in the middle. I would love to try the code reading trick once I can actually get the light on. Do you think maybe a blown bulb??
Did you check the pink/blk wire at the ECM cavity A6 (6th from an end) with the key in RUN? This is the "turn-on" input. If no power here, this is the place to begin.
The knock AMP is the other place this circuit goes. I suggested it because it is easier to access than the ECM. The knock amplifier (not the sensor on the engine block) is located directly below the Right Side Rear hood latch bracket. Has 4 (maybe 5) wires going into the connector.
The knock amp pink/black wire has 12.1v with the key in the on position!
I dove into the distributor because it seems like fuel injectors aren't getting pulses(fuel rail is hitting 50psi while cranking, but still wont start, but when you spray quick start in the intake it starts immediately) so I went to go check the ICM. On the two prong side, positive is getting 12.2v while the ground has 5.8v. I just wanted to make sure it was good so I replaced it, but still nothing, On the many pronged side, only one out of the 4 or 5 wires is getting pulses of power. Are any other wires suppose to be getting power???
Also I tried the paper clip trick on the ALDL and the check engine light wont even turn on to blink . Let me know what you think. Just thought I'd let you all know about what I have figured out!
Last edited by Jstuck8; Sep 27, 2023 at 09:22 PM.
Reason: update with more info
If it fires on quick start, the ignition system and module are functioning.
When you spray quick start does it STAY running? If = YES, the Cold Start Injector (CSI) isn't working, The injectors are.If = NO, the CSI still isn't working, but the regular injectors aren't either. It should fire quickly on the CSI, but will won't stay running if the main injectors aren't functioning.
The CSI is very important to ease of start for the first start of the day. ("Cold Start" means that the engine and ambient temperature are the same. Ie, it hasn't run for overnight or more.)
Check the purple wire in the CSI connector for 12Volts with the key in START. The CSI is only energized with the key in CRANK. Put the auto shifter in gear, or don't push the clutch pedal and the engine won't crank, but will still power the CSI in START. If no power, there is a fuse (CINJ) behind the Driver Information Center.
If the Check Engine Light won't turn on, that is a separate issue. When you remove the DIC to check the CSI fuse, that would be the time to check the bulb. Perhaps it has been removed by a PO to pass inspection or something?
Has this car ever run correctly under your ownership? Ie, this 'won't start' is a new development or did you purchase the car this way as a project? If bought as a project, there may be multiple issues. Including plugged injectors.
With spraying quick start in it, it will NOT stay running, and dies after a second. Tonight when I get home I will check the CSI.
This car DID run when we got it, it had electrical problems due to a fire in the back of the motor by the distributor. I’ve repaired all wires to my knowledge and it started just fine. Then one day the fuel pump stopped priming and the starter went out, replaced the starter and still not fuel pump. It was strong when it use to start. Gear to gear driven, cam lobe, and Andesite rollers on top end.
the car was complete when the guy before me acquired it. Then he put his unknowing hands on it 😭 so now I’ve been trying to fix everything. Thank you guys for the help!
With spraying quick start in it, it will NOT stay running, and dies after a second. Tonight when I get home I will check the CSI.
This car DID run when we got it, it had electrical problems due to a fire in the back of the motor by the distributor. I’ve repaired all wires to my knowledge and it started just fine. Then one day the fuel pump stopped priming and the starter went out, replaced the starter and still not fuel pump. It was strong when it use to start. Gear to gear driven, cam lobe, and Andesite rollers on top end.
the car was complete when the guy before me acquired it. Then he put his unknowing hands on it 😭 so now I’ve been trying to fix everything. Thank you guys for the help!
update:
CSI was not getting power, checked behind the dash and yep fuse was blown. Changed it out getting power on both sides. Still won’t prime the fuel pump/start the engine and the CSI plug still isn’t getting 12v while CRANKING or in START :/
update:
CSI was not getting power, checked behind the dash and yep fuse was blown. Changed it out getting power on both sides. Still won’t prime the fuel pump/start the engine and the CSI plug still isn’t getting 12v while CRANKING or in START :/
also the CEL bulb is not blown
a little FYI -- i had this happen do to dumb **** stupidity. the ECM is grounded at the rear of the LH cylinder head. if you attempt to power up the system without that ground, at least in my case, it blew the ECM circuitry the provides the 2-sec fuel system prime. you can crank on the starter until hell freezes over and the engine won't start. just sayin'.... in my case, i attemped a fuel pressure test and missed that ground during a top end rebuild.
a little FYI -- i had this happen do to dumb **** stupidity. the ECM is grounded at the rear of the LH cylinder head. if you attempt to power up the system without that ground, at least in my case, it blew the ECM circuitry the provides the 2-sec fuel system prime. you can crank on the starter until hell freezes over and the engine won't start. just sayin'.... in my case, i attemped a fuel pressure test and missed that ground during a top end rebuild.
I will definitely check this, I think I saw this spot when I was looking around. I’ve always felt like this car has had ground problems, just my spider sense. Wonder why it worked great at first and then just stopped randomly. I’d like to buy a new ECM but I just want to make sure it’s the problem before I slap anymore more money onto it
I will definitely check this, I think I saw this spot when I was looking around. I’ve always felt like this car has had ground problems, just my spider sense. Wonder why it worked great at first and then just stopped randomly. I’d like to buy a new ECM but I just want to make sure it’s the problem before I slap anymore more money onto it
just for a little extra help and food for thought -- here's the wiring diagram from the factory service manual --
I checked the grounds at the ECM, one was getting 9~10v and the other two were getting 12v and I was getting power. Took an injector plug out and tested, was getting no pulse but a constant 12v. No pulse, no prime, no CEL, I decided I’d take a shot and get the ECM
Changed the PROM and when I turned the key the CEL illuminated!!! First crank and it roared to life. Hell. Yeah. Happy to see it bark. Now, it only runs for 5 or so seconds and then just dies. I’m thinking timing is the issue, PO put a MSD top on a stock distributor bottom, and I think it’s offsetting the timing since the hooks don’t line up with the screw holes, just a thought. But I want to say thank you to everyone and their little pieces of invaluable knowledge. I guarantee I’ll have problems in the future to let you all know about lol. Never a dull moment with chevys(love em and hate em).
85 Chevy Corvette Fuel Pump/Electric
. Let me know what you think. Just thought I'd let you all know about what I have figured out!
