I've got a 99 Blazer 4WD LT that all of a sudden wont start. My wife made a few trips yesterday and everything was fine. This morning I got in and it would turn over but not start. Using a little starting fluid it ran for a couple of seconds so that told me the ignition system was most likely not the issue. So far I have checked the relay and fuse under the hood. Fuse is ok and swapped horn relay with the fuel pump relay. If I turn on the key then go under the car I do not hear the fuel pump running...even put my ear to the fuel tank. I checked the relay sockets with the key on and it has 12 volts on the 2 right side sockets. I figured my next step would be to test to see if I had voltage at the fuel pump and was able to remove the 3 pin connector from the top of the tank. I was, however, unable to get a reading because I had trouble holding my probes on the connector which only has a couple of inches of slack. My questions are:

1. With the key on should there always be voltage to the fuel pump? I read something about 2 seconds somewhere else.

2. What are the three wires? If I were to jury rig a couple of wires and connect them directly to the fuel pump to test it which wires and what voltage should it be? I'd hate to burn up a perfectly good fuel pump or the fuel gage by connecting the wrong wires.

Thanks
Ken

 

Work through all the steps in 96+ Fuel System Diagnosis.

 

Thanks for the link. There's lot of helpful info. From looking at the link it appears that the fuel pump gets turned off if the car does not start so keeping the key in the on position does NOT keep the fuel pump on unless it gets VCM pulses (whatever that is). I'll have to have my wife turn the key while I have my ear to the gas tank. Almost all of the diagnostics requires the car running which can not be performed.

"When you turn ON the ignition, the vehicle control module Vehicle Control Module (VCM) energizes the fuel pump relay which powers the fuel pump ON. The pump remains ON as long as the engine is cranking or running and the VCM receives reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after the ignition was switched to the ON position or if the engine stops. The VCM also turns ON the fuel pump for 2 seconds when you turn OFF the ignition."

OK..I had my wife turn the key to on then start for about 4 seconds while I was under the car and heard nothing from the fuel pump. Since I can't hear the fuel pump I'll assume it's not working and testing the fuel pressure is a moot point. Tomorrow I'm going to try to make something that will allow me to test the voltage at the fuel pump.

 

If the tank is empty I would just drop the tank and pull the pump then hook it directly to the battery and see if does anything. If nothing then its obviously bad. Aside from a fuse or something I do not see what could have happened to stop voltage from going to the pump. It really isnt that hard to pull the tank. I pulled mine when I put in a new exhaust and it probably took less time then what you have put into trying to test the wires at the pump...Just my 2 cents

 

Thanks, that's what I want to try but without removing the tank which is about 1/4 full. Looking at the 3 wire connector on the pump I could most likely solder some pins on the end of wire and insert them into the pump wiring block. i just don't know what the three wires are. I assume 1 is 12v, 1 ground, and 1 for the sending unit. I've spent a few hour looking things up and can't seem to find that little bit of info.

If I can measure the voltage on the connector by doing sort of the reverse an putting 3 little female sockets on wires and one of them is 12v then I would be pretty sure that the pump is dead. On the other hand if I don't have 12v then the pump may be good and I have a bad wire or ground somewhere....or the control module, relay, fuse or something else. I really don't have the money to put it in the shop and being a one vehicle house it's hard to pick up new parts. The link below is the best I could find for the wiring but it doesn't show the pump connector.
http://oi32.tinypic.com/3039tev.jpg

 

I gave up and had my Blazer towed to the garage. I was able to hook a ground wire to the engine and test the voltage at the fuel pump. With my wife turning the key I measured 0 volts, gnd, a 5 volts on the three wires. Assuming that the 5 volts is the gauge. I checked every fuse and relay and couldn't find the problem. Thanks for everyone's help!

 

I too have a 99 blazer LT. And would you believe it currently has the same problem.
Here's what I know so far. The three wire connector on top of the fuel tank is a pressure sensor. The fuel pump has a 4 wire connector.
You can test the fuel pump with or without the key from the fuse panel under the hood. It helps to have some one in the car if trying to start, but not absolutely necessary. With a fuel pressure gauge in place you can verify the fuel pump operation without having someone in the car. Prepare a wire to jump between the 12V input to the fuse panel and the fuel pump test point (small hole at the front of the fuel pump relay - try using a paper clip to push in the hole). While turning over the engine via the ignition key, the engine should start and run after you connect the jumper. CAUTION - DO NOT LEAVE THE JUMPER IN PLACE FOR MORE THAN ABOUT 10-15 SECONDS (it can draw up to 20A and may smoke a small gauge jumper wire).
My blazer had a bad fuel pump, but even after replacing the pump it won't start without doing the test jumper. More will be forthcoming as repairs progress.
Hope this is of some help to you.

 

Found a bad relay socket for the fuel pump relay this morning. First tested the fuel pump relay by using jumpers from the relay socket connectors. The car started and ran fine. Then bent the legs on the relay slightly outward and placed pieces of unmelted solder into each socket connection for the fuel pump relay to make a snug fit. I must have screwed up the socket connector leading to the fuel pump while swapping and testing. Works AOK now, but will have to remember the crazy socket problem during any future problems. Also found eazier way to jumper to test the fuel pump. Remove the fuel pump relay and jumper across the fuel pump relay socket connectors diagonally from passenger side rear socket connector (should have 12V ALWAYS present, even with key off) to the driver side front socket connector (this feeds to the fuel pump). This is very easy to accomplish with just a single paper clip or small piece of wire.
Good luck with your Blazer, hope the shop you're using knows what it's doing and is able to fix it for you. Just couldn't spend the kind of money the shops around here were asking for a fuel pump replacement. Really learned valuable info fixing it myself.

 

Ihave the same problem on my 99 & did the same as you jumping the 87-30 holes but i have to thump the tank to get the pump to run & then it starts & runs good . ILL try to bend the relay pins like you said i dont quite under stand the solder part though. does the solder melt ?Is there a way to take fuse block apart?

 

That is a problem that others have experienced before. I haven't looked at an s-series fuse box in quite a while, but I believe you would have to basically destroy it to get it opened up. Best bet is probably to replace it with one from a similarly optioned truck at the junkyard. There were two basic levels of fuse boxes; standard & up-level.