94 c1500 no fuel pump signal. Cranks but not start.

[Alexcantsleep]

Ok guys and gals, here’s the situation...this one has me stumped.


I’m a funeral car collector so I’m pretty familiar with gm but I recently bought a Chevy suburban hearse conversion, which is a tad different than my caddies. I’ve had one other suburban but it was a bit different (had a 6.5 in that one).

So, I’m a nutshell, I’m a car guy but don’t know these trucks inside and out well, at least not yet.


This one is a 94 c1500 with 154k on it. The guy I bought it from told me it needed a fuel pump. He had taken the tank out (and left it for a bit) and couldn’t figure out how to install the new pump so it sat a while, he put the tank back in and he sold it to me, cheap. The tank was a little crusty inside from being left open and the sending unit looked really bad so I put a new tank and entire sending unit/pump in it. Blew out the lines,installed a new fuel filter and hit the key and it fired right up and ran like a champ. And I had heard it prime like normal..So for a few days I started it and ran it some, drove it around my property etc. i changed the shocks, changes the oil, tranny fluid and axle grease, new tensioner and belt, air filter and a light bulb but the motor was running great, maintaining temp, no noises, etc..

So today I take it to the tire place to have tires put on it. It drives fine the whole way there, no issues at all. the shop calls and says the truck won’t start.. it’ll crank just fine but not start. The truck had a cheap, aftermarket alarm on it and I figured it was faulty, triggered, etc...so I drove back over and I can’t make it run. The alarm arms and disarms fine and looking closer, none of the circuits connected to the anti theft options where even hooked up.

So I have it towed home and I’m taking a look at it.

The first thing I did was spray some carb cleaner in the Tbi and it cranked right up so it seems to have spark.

So I painstakingly removed every trace of the alarm system, which as appeared, wasn’t connected to anything that would make it not run. No change. I tried a new fuel pump relay, checked my grounds and connections, no change. So next I tried to see if I was getting current to the pump. There’s a grey wire from the relay to the pump and no matter what, I wasn’t finding any voltage there. There’s a small, black fuse marked “fuel pump fuse” connected with two grey wires as well and it was fine. When you turn the key on, this fuse shows to have power for a few seconds and then stops. But in either case it’s like the fuel pump relay isn’t receiving signal to do it’s thing.

So next I ran the codes, and there were none. So we seem to be good as far as the ECM is aware.

So I’ve been digging online. I found one video of a guy who had to replace the distributor because an internal module failed. It too just wouldn’t start one day and he said the module on the distributor was not allow ring the fuel pump or injectors to operate but didn’t mention it this would also affect spark. I’ve also heard a few people mention the tbi module. And of course, the fact the fuel pump only has 40 minutes of drive time on it doesn’t mean the pump isn’t bad but I figured that was hopefully less likely. I sure hope so anyway since on the way I stopped and added 40 gallons to my tank.


But, like all problems, the best advice comes form experience so I was hoping someone might know this issue or have some advice on what to do/try next. Everything seems normal. The truck is all stock and hasn’t had any mods done to it (Aside from the vinyl roof and casket floor).


Thanks everyone!

 

[Erik the Awful]

Check to see if you have power at the fuel pump. Also make sure your ground wire is good. There have been reports of brand new pumps failing, especially with cheap replacements, but make sure you have power and ground before tossing more parts at it.

 

[PlayingWithTBI]

Check to see if you have power at the fuel pump. Also make sure your ground wire is good. There have been reports of brand new pumps failing, especially with cheap replacements, but make sure you have power and ground before tossing more parts at it.

^^^X2 - Here's a step by step tutorial to check your electrical system too.

https://easyautodiagnostics.com/gm/4.3L-5.0L-5.7L/distributor-mounted-icm-tests-1

 

[Alexcantsleep]

Check to see if you have power at the fuel pump. Also make sure your ground wire is good. There have been reports of brand new pumps failing, especially with cheap replacements, but make sure you have power and ground before tossing more parts at it.

I used my test light and didn’t find voltage going to the pump itself. There’s a grey wire from the relay that runs to the tank and so checked it in several places for voltage but never found any. I rechecked my ground at the tank and that seems to be on. Just don’t seem to be getting power to it.

I also thought of something else..
The night before i had changed my oil, is it possible the newer oil has had something to do with the oil pressure switch? My gauge did oil pressure was working but i seem to recall reading the switch can cut signal to the fuel pump?
I’m going to check my ignition module today. It shows that if this is bad we also won’t have spark? Mine seems to have spark but I’m not ruling anything out.

 

[PlayingWithTBI]

My gauge did oil pressure was working but i seem to recall reading the switch can cut signal to the fuel pump?

The oil pressure switch will bypass the pump relay and send voltage to the pump once it sees over 20PSI (IIRC). They're wired in parallel not series.

 

[PlayingWithTBI]

When you turn on the key, engine off, does the pump run for 2 seconds?

 

[Alexcantsleep]

I’m not hearing the pump prime at all anymore. Is the voltage through the fuse that I’m getting in the first few seconds potentially the voltage that should be going to the fuel pump? I wonder if maybe it IS a bad, albeit new pump.
So with the key on, after a few seconds, is it normal to have no voltage to the pump of the engine isn’t running?

 

[Alexcantsleep]

OK, so I just went back out there to check, if I use my test light on the gray leads that go to and from the fuel pump fuse as well as the gray wire from the fuel pump relay that runs down to the tank I’m getting voltage there when we turn the key on for a few seconds and then it turns off. So if we’re getting voltage to that circuit for a few seconds when the key is turned on but the fuel pump is it kicking on does not indicate a bad fuel pump?

 

[Erik the Awful]

If you have power coming out of the relay with the key on, but no power at the pump, you have a wire problem between the relay and the pump. I think there's a connector under your brake master cylinder and possibly another connector before the tank, but you can also troubleshoot by putting a jumper wire between the relay and pump to see if it runs.

 

[Alexcantsleep]

So we went again yesterday. We ran power directly to the pump. You could hear a very faint “clunk” when you initially put power to it but it won’t run. I’m guessing this pump, despite have less than 30 minutes of runtime is faulty. I’m thinking I’ll try to hunt up a Delco pump if I can find one.