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95 chevy 350 computer problem(s)

I have a 84 Suburban 3/4 ton, 4 WD,with a 95 trotlebody fuel injected 350 with computer. It has been running great for the past 125k miles with just routine main.It just quit running like someone turned the key off at 50 mph. I pulled it over, waited a few minutes and tried to start it. The first thing I noticed is I didn’t hear my electric fuel pump cycle.It cranked but not a hint of starting. I thought well the fuel pump went out after 14 years. A good sam came by and gave me a ride home. I got some tools together and went back. Still not starting. I checked the wire to the fuel pump, mounted on the frame, and the wire was dead.I hot wired the pump to the battery and it cycled, but it still wouldn’t start. So I went home got my neighbor , loaded up the beast on a trailor and brought it home. THAT NIGHT ANOTHER NEIGHBOR TOLD ME THE SAME THING HAD HAPPENED TO HIM AND IT WAS THE MODULE IN THE DISRIBUTOR. So I got a new module and put it in, still not starting.

Next I checked the wire to the fuel pump. It had been spliced in 3 places so I replaced it. Still no juice.Next I replaced the fuel pump relay.Still no juice. but the circuit is intact from relay to pump. I called the mechanic who put the motor in for me 14 years ago (he retired and now sells Toyotas)we’re still friends and he came by. We checked the coil and distributor, it has a good blue spark. But the 2 injectors are dry as a bone.Didn’t have a pressure guage to check pump pressure but it seemed to have good pressure when we cracked open the line.

He thought that the computer went out (it’s original) so I got a computer. When I was installing it I noticed that the fuse block red light was not on.The fuses were good and they checked out with the tester. I took the light out and tested it and it was good.So I hooked it back up.

I plugged in the new computer, but the red light does not come on, and the fuel pump doesn’t cycle and it doesn’t start.

The next suggestion I got was that the oil pressure sending unit is bad and telling the computer not to let the engine run. So I replaced that little sucker.Still cranking but not starting.

I think there must be some sensor telling the computer to not let the engine run.Somehow it’s tied into that red light not working too.

I would gladdly appreciate any ideas or suggestions.9

Well, yes, you have a great problem with another time wasting thought coming up. Some people like to hot wire fuel pumps (I would say to never do that) and I think the pump has been damaged. There could be other problems like maybe the timing chain got loose and slipped a few teeth. If the sprocket fell off or the camshaft broke…I would be interested to know if the rotor turns when the engine is cranking. All my wrong thinking is just another way to say that you may have to start at square one and waste a lot more time. Some things to check: center electrode in distributor cap; they can crumble. The rotor can be damaged. I don’t believe my suggestions will help but you never know when some oddball minor part will embarass you. Frayed wires on the ignition pickup were a problem once (on an Escort). The car would start and idle, but when you stepped on the gas, the vacuum advance would move the breaker plate, separate the frayed wires and the car would stall. Neat, huh? Good luck with yours and never hot wire a fuel pump. Usually, back yarders will try to check the pump before they put the tank back in and it works then but never again.

I am getting an idea that there isn’t any power to the computer and/or to the throttle body injectors. Have you found battery voltage on both sides of the fuses you have checked? What does the ‘noid’ light show for the injectors? Is there voltage at one side of the injector plugs?

Get back to us with the voltage at the feed to the fuel pump relay, at the voltage feed to the computer, and the voltage feed at the injectors. We will go from there.

There may be a bad engine sensor causing the problem. If you don’t have a service manual to guide you I suggest you purchase one. Check for any codes stored in the ECU. The problem could be the with the oil pressure circuit, the ignition switch or in the main power buss. It may also help to check the compression to make sure that is ok.

I hope you see, by now, that you can’t fix it by merely changing more parts.
The two fuel injectors are turned on by the engine computer (ECM). The ECM turns the fuel injectors on when it gets the proper signal from a number of sensors…one likely one is the rpm signal from the distributor.
You can see from the wiring diagram that there are several places on the ECM which should have 12 volts from fuses. You need to check that 12 volts is there at those terminals.
To get more details on which signals, you need where, you need the repair manual for the 1995 Suburban.
Here is the wiring diagram. Scroll down to Fig. 17.

Thanks for the suggestions, but your first statement was the most helpful.As for the fuel pump,as I mentioned in lines 6 & 7 is that the pump is mounted on the frame, not in the tank, and the lead to the pump is dead.

Thanks, I think it’s a bad sensor too, but I didn’t work as a mechanic before I retired. I asked my friend , the retired mechanic, about ECU codes and he said that this system was so ‘primitive’ as compared to todays ECU’s that it wasn’t capable of the diagnostics like todays computers. Is this true or false, if false I wouldn’t have a problem getting an analyzer to check them. I haven’t checked the compression of this motor, I haven’t had any reason to in the past 'cause this motor has performed flawlessly until this happened.But you have my curiosity twiged now so I’ll check it out, after I get it going again.

Thanks I’ll do that.

I’ll measure the voltage at the fuse block. I just used a light tester and both sides were good.

It looks like you are going to change some more parts to try to fix it.
You said yourself that a power indication light doesn’t now glow. Don’t you want to work on that angle?
The wiring diagram I showed you a link to shows where (what fuses) power for different parts of the engine management system come from, and go to. Someone could disconnect the battery, disconnect the ECM (engine computer) electrical connector, reconnect the battery, turn the ignition key on, and check for 12 volts at the fuses and at the terminal of the ECM connector. There are several terminals on the ECM which should receive power (12 volt). From the ECM-1 20A fuse to he ECM terminal A6. From the IGN-E 10A fuse to the Fuel Pump Cycle Module (this may not be getting power, or a ground) to the ECM B2 terminal.