Kingman Posted May 24, 2010 Share Posted May 24, 2010 This is for my step dad's 1999 Chevy Tahoe 5.7L. It's had a problem with not starting when it's wet, which is why it's been sitting for almost a year. I needed to borrow it a few weeks ago when my throttle cable bound up and since the weather was "supposed" to be clear I went ahead and took the risk. It ran great for the dry day, but the next morning it was raining and wouldn't start. It hasn't started since. The problem lies somewhere within the fuel pump circuitry, you can't hear the fuel pump working. I put in a new fuel pump relay which is tested to be working fine, however the fuel pump will not run. The relay as 12V going to it on both the constant and ignition power sources. The pump it's self is only getting 5.03V to it. I'm not sure if GMs run 5V to the in-tank pump or if it should be a full 12V, and AllData is of no help there. I've poured fuel down the throttle body and it fires right up so it is a fuel problem. I don't want to be replacing the fuel pump yet, because I think if it was the pump it wouldn't work at all, instead of only when it's wet outside. If it's supposed to be only 5V then that basically means the pump is shot, but if it's 12V then I need to start tracing voltage drops and/or find something that would cause it to drop signal. I'm also not sure if there's a security system that could be shorting out and cutting the fuel off. Has anyone owned one of these that's had a similar problem or have an idea of what else I could look for? Thanks for any advice... Link to comment Share on other sites More sharing options...
nunya Posted May 24, 2010 Share Posted May 24, 2010 The passlock security system will let it start and run for like 3 seconds. It will still make the fuel pum work though. Can you get to the plug at the pump? Sometimes if it's the original plug they won't be fully connected anymore or get some moisture in them screwing with the connection. They changed designs on the plugs (replacement pumps/float assemblys come with a new plug and pigtails that you can cut the old plug off and splice the new one in) for that reason. Link to comment Share on other sites More sharing options...
5523Pathfinder Posted May 24, 2010 Share Posted May 24, 2010 Can you jump the relay and give the fuel pump voltage and see if it runs? Link to comment Share on other sites More sharing options...
ahardb0dy Posted May 24, 2010 Share Posted May 24, 2010 I would run a wire from the positive on the battery directly to ( from what I read on another site the grey wire goes to the fuel pump), and see what happens, I believe the fuel pump should run on 12 volts. Link to comment Share on other sites More sharing options...
shift220 Posted May 24, 2010 Share Posted May 24, 2010 The pump runs off of 12v. Try replacing the fuel pump relay in the fuse box with another relay (from a/c or something that fits) and try it then. Could just be a bad relay. Link to comment Share on other sites More sharing options...
ahardb0dy Posted May 25, 2010 Share Posted May 25, 2010 from his first post: "I put in a new fuel pump relay which is tested to be working fine" Link to comment Share on other sites More sharing options...
shift220 Posted May 25, 2010 Share Posted May 25, 2010 Where did you measure the voltages from? If the relay pin 30 is getting 12v and its only getting 5v at the pump then it lies in the wiring on the way to the pump. I'm going to say its a bad pump (perhaps shorting out on the inside and killing voltage). Even if the pump was getting 5v, you would hear it slightly whining. Check the ohms across the pump, if its shorting out then you'll know right away bad pump. I'm not sure the range it would be if it's still ok. Link to comment Share on other sites More sharing options...
shift220 Posted May 25, 2010 Share Posted May 25, 2010 You can stick a short wire with stripped ends in the slots where the relay sits. Put it in slot 30 and 87. This will bypass the relay and give power directly to the pump. If the pump works then you know its in the switch circuit (which the computer controls). Link to comment Share on other sites More sharing options...
Kingman Posted May 25, 2010 Author Share Posted May 25, 2010 I tested the pump voltage in two places. At the relay - 12V constant on one pin and 12V with the ignition on, as well as at the clip that attaches to the pump, with the pump unplugged. I had to drop the fuel tank to do so. I was thinking about jumping 30 and 87 earlier today, but didn't want to in case for some reason GM runs a lower voltage to their pumps and I feared I might fry something. In inspected the plugs on the pump and the clip, no corrosion at all and everything is solidly in place. I even shot some contact cleaner on everything and it didn't make a difference. Since it's 12V I will jump those tomorrow and see what happens. If there's still 5V at the pump then I'll need to trace some wiring. I hate tracing wiring. Link to comment Share on other sites More sharing options...
Kingman Posted May 25, 2010 Author Share Posted May 25, 2010 (edited) Wow, jumping 30 and 87 didn't turn out so well. Blew fuses, melted the relay block a little, burnt my fingers on the red-hot wire, and got no where. That alone indicates a bad pump...no resistance. But I fully dropped the tank and realized I was measuring voltage from the wrong damn clip. So to correct myself, the pump has 12V going to it. Bad pump... too bad they're $200 or more. New fuel pump ordered and hopefully it'll fire up. Thanks for the help guys! Edited May 25, 2010 by Kingman Link to comment Share on other sites More sharing options...
adamzan Posted May 26, 2010 Share Posted May 26, 2010 That generation gm truck is common for fuel pumps going out. My old boss replaced 2 of them in his silverado and a guy at work had one die recently as well. Link to comment Share on other sites More sharing options...
GrimGreg Posted May 26, 2010 Share Posted May 26, 2010 That generation gm truck is common for fuel pumps going out. My old boss replaced 2 of them in his silverado and a guy at work had one die recently as well. All GM pumps are notorious for failure. Link to comment Share on other sites More sharing options...
shift220 Posted May 26, 2010 Share Posted May 26, 2010 Its actually funny that you post this now as I'm replacing a dead pump on my Sonoma as we speak. Not the stock one, but a higher flowing one I replaced it with. I did the same thing to check voltages and found the 5V wire which is for the fuel level sensor. I thought of you finding the 5v wire and it came together. Mine died nicely though, jumping the relay did nothing but bring power to the pump. Are you sure you got pins 30 and 87? Link to comment Share on other sites More sharing options...
Kingman Posted May 26, 2010 Author Share Posted May 26, 2010 The pins look like this on mine. : . : I jumped the 2 on the far left, as that's what the relay says 30 and 87 are. There are 2 87s... 87A and 87. I jumped 30 to both 87 and 87A. Link to comment Share on other sites More sharing options...
pathfinderfisherman Posted May 27, 2010 Share Posted May 27, 2010 The pins look like this on mine. : . : I jumped the 2 on the far left, as that's what the relay says 30 and 87 are. There are 2 87s... 87A and 87. I jumped 30 to both 87 and 87A. well let me know what happened maybe i can help I used to be a GM nut Link to comment Share on other sites More sharing options...
Kingman Posted May 27, 2010 Author Share Posted May 27, 2010 New fuel pump in, runs like a top!! Has a lot more power than before...probably because it's getting fuel! That thing was a bitch to replace. Holy sh*t. Now the check engine light is on tough. Link to comment Share on other sites More sharing options...
nunya Posted May 27, 2010 Share Posted May 27, 2010 You get the pump/float assembly COMPLETLY set in place and sealed? If not it will kick an EVAP code Link to comment Share on other sites More sharing options...
Kingman Posted May 28, 2010 Author Share Posted May 28, 2010 Well that's debatable. It's set in and secure, but the light is still on... It's weird going from something that has power to gutless. Link to comment Share on other sites More sharing options...
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