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Topic Review (Newest First)
12-23-2012 02:53 PM
cobalt327
Quote:
Originally Posted by vicrod View Post
If memory serves GM did not use an inertia switch for crash shutoff of the fuel pump. They relied on the engine oil pressure to maintain the power to fuel pump. The idea is that, in a crash, if the engine is not running the fuel pump will also not run.
I believe Ford used an inertia switch.
To test the system install a fuel pressure gauge on the injector manifold.
Key on starts a 2 second prime run of the fuel pump.
Engine cranking, oil pressure increases and runs the fuel pump.
Engine running, oil pressure = fuel pressure.
There is a gray wire to the fuel pump. If this wire is broken run a new one.

vicrod
Thanks for the reminder.
12-23-2012 08:26 AM
joe_padavano You all really need to read the factory service manual and look at the factory wiring diagram.

1) There is no inertia switch on an 87 Chevy truck

2) There is only one oil pressure switch that controls the fuel pump. Again, this is used once the engine is running and there is oil pressure.

3) The relay is only used to prime the system prior to oil pressure rising. The relay is controlled by the computer and is only activated for about two seconds after turning the key to START. Yes, as I noted above, a good troubleshooting method is to remove the relay (which is on the firewall) and jumper across the pump terminals to get the pump to stay on.

4) The only other switch in the fuel pump circuit (other than the ignition switch) is the tank selector switch if the truck has dual tanks. That's it. This is a very simply circuit.

By the way, if the relay (or computer activation of the relay) goes bad, it is often possible to start a TBI truck by cranking the engine long enough to build oil pressure. That will finally turn on the pump and operate the TBI. This can take quite a while.
12-22-2012 10:20 PM
496CHEVY3100 Go to passenger side under hood remove large plastic cover marked fuel pump -fuel relay unplug relay install jumper wire across where you unpluged relay with ignition on you should hear pump run with gas cap removed or have assistant hold head close to cap,if it runs you found problem ,my 1988 chevy does have a low oil pressure cut off switch it has 2 sending units one under distributer one beside oil filter the other in front of filter is knock sensor ,i had to replace both on mine before pump would run ,if i turned mine long enough to build 18 lbs oil pressure aprox 60-90 seconds it would start hope this helps
12-22-2012 07:53 PM
vicrod
oil pressure

If memory serves GM did not use an inertia switch for crash shutoff of the fuel pump. They relied on the engine oil pressure to maintain the power to fuel pump. The idea is that, in a crash, if the engine is not running the fuel pump will also not run.
I believe Ford used an inertia switch.
To test the system install a fuel pressure gauge on the injector manifold.
Key on starts a 2 second prime run of the fuel pump.
Engine cranking, oil pressure increases and runs the fuel pump.
Engine running, oil pressure = fuel pressure.
There is a gray wire to the fuel pump. If this wire is broken run a new one.

vicrod
12-22-2012 06:13 PM
Hristamaki Where would the reset button be located?
12-22-2012 04:38 PM
cobalt327
Quote:
Originally Posted by Hristamaki View Post
I have recently had a problem with my truck it is a 1987 chevy r10 with the 305 Tbi for some reson I am not getting any power to my fuel pump I replaced the fuel pump relay, checked fuses, and checked my selector valve. Not sure what else would be some help would be great thanks!
Somewhere on that vehicle is an "inertia switch". It stops the pump in case of a turn over or wreck. It can switch off the power if it's bumped, etc. It'll have a simple reset button that's pushed to reset it.
12-22-2012 03:20 PM
joe_padavano
Quote:
Originally Posted by 1939P7 View Post
I'm not sure if you have the oil pressure switch that is hooked up to the fuel system.. I couldn't load the the pic here but will post it in my photo's
There should be an oil pressure switch, but that is unlikely to be the problem. The TBI system uses the oil pressure switch in parallel with the relay. The computer energizes the relay to operate the pump for about two seconds when you turn the key to START. This provides pressure to the TBI injectors before the oil pressure rises enough to close the oil pressure switch. Once the engine is cranking, the oil pressure switch closes, bypasses the now open relay, and provides current to the fuel pump.

Now, are you testing the system correctly? There should be +12V at the relay feed, but you WON'T get anything to the pump unless either you just turned the key to START (that two second timer) OR you have oil pressure that closed the switch. The only way to test the pump and wiring is to remove the relay and temporarily jumper across the two pump contacts in the relay socket. If the pump turns on, the circuit is working. If not, use a test light to trace the wire from the relay connector to the tank. The selector switch is a likely candidate, as are all the connectors in the harness. If you now get 12V at the tank, check the pump and ground wire at each tank.
12-22-2012 02:47 PM
1939P7 I'm not sure if you have the oil pressure switch that is hooked up to the fuel system.. I couldn't load the the pic here but will post it in my photo's
12-22-2012 02:37 PM
1939P7
Quote:
Originally Posted by Hristamaki View Post
I checked the power going to the pump by unplugging the harness before the tank and it had 0 power the selector valve is what switches the two tanks I put a jumper wire there so I can only use one tank on my driver side. I also tested my fuel pump twice by hooking it up to power and it sot out gas just fine
run a wire from the relay to the pump and don't use the harness wire at all just bypass it ... if you get fire in the hole somewhere the frame or something rubbed a wire to the fuel pump....
12-22-2012 02:29 PM
Hristamaki I checked the power going to the pump by unplugging the harness before the tank and it had 0 power the selector valve is what switches the two tanks I put a jumper wire there so I can only use one tank on my driver side. I also tested my fuel pump twice by hooking it up to power and it sot out gas just fine
12-22-2012 02:07 PM
1939P7 By chance do you have dual fuel tanks ? I have seen the switch go bad .
some early GM's had wire harness problems as the the early blazers where the harness goes just below the toe board of the fire wall . Have you had someone turn the key and go to the tank to see if you can hear the pump running remove you gas cap and and put your ear up to it and had have a helper turn the key on It could just be a bad pump , did you check the ground wire in the harness where the wires go to the pump ? If you give it a short shot of starting fluid and it kicks over I would pull the pump the only other thing could be is a clogged fuel filter .... Fuel pressure regulator ..
12-22-2012 01:49 PM
Hristamaki My relay has power, so would that mean my problem is between my relay and my fuel pump?b but what is there besides wires and grounds?
12-20-2012 04:27 PM
T-bucket23 start first to see if the relay is being energized. If you still have the old relay take the cover off it and see if it energizes when you turn the key on. there are a lot of things that could be wrong. You need to start at the relay and work out from there.
Did you test the fuses with a meter? I have seen several blown that look fine.
12-20-2012 11:29 AM
1939P7 Did you check the wires under by the fuel tank harness .. heres a link to steer you towards the testing. Hope it helps you..

Fuel Pump Problem-- NEED HELP!!!
12-20-2012 09:59 AM
Hristamaki
My truck will turn over but will not start any ideas?

I have recently had a problem with my truck it is a 1987 chevy r10 with the 305 Tbi for some reson I am not getting any power to my fuel pump I replaced the fuel pump relay, checked fuses, and checked my selector valve. Not sure what else would be some help would be great thanks!

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