Fuel Gauge Reads Empty After New Engine
 

Almost 2 years ago Pepboys made a series of mistakes which ultimately ended up in them repacing the engine in my 98 Blazer. After the swap was complete I noticed the fuel gauge was low, very low. I thought how curtious was that to leave me with NO gas. So I tried to go fill up immediatley and I only got maybe a gallon in before it was topped so then I knew it just was not working. I told Pepboys it worked fine prior to the new engine and they said they did not touch anything dealing with the gas gauge. Meaning it is coincidental it happen to just fail while it was in their possesion.

I guess my question is I believe the wire from the fuel sensor is shorted so it always reads empty. I am guessing that wire runs through some main wire harness that they DID touch. Does anyone know the path this signal wire takes to the computer?

I may just be in denial that my sender unit really did fail but I find it hard to believe it would fail and show empty when it really had a full tank. Usually I thought they would get stuck in one position during normal use. I have just put off this repair too long because I have no idea where to start. Luckily my Blazer has the trip computer option so I have just been resetting it when I fill up.

If I know where I can isolate either end of the signal wire(from sender or to the gauge). I can read the resistance of the sender and attach a resitor to the gauge to see which works or does not work.

Oddly enough - I came searching for a similar issue: My gauge reads FULL after an engine swap.

Interesting yours is the opposite condition, maybe it isnt a wiring issue, but that the computer needs to be recalibrated after an engine swap? Just thinking of possiblities. I just hate to drop my tank and buy a new pump/sender to find the problem is not fixed.

I'd check the fuse box/relays under the hood. They may have removed that or disconnected the ground under it. Oddly when my engine got replaced I had no fuel, no headlights & a couple other things missing as well as the engine wouldn't start. No fuel, no spark, no starter. That little ground that supplies the fuse box negative power gives all those relays a ground in order to actuate.

I just looked at it, but all the fuses and relays are ok. I did not see any grounds attached to it as you mentioned. On one side of the box(closest to the front of the car) there is a post marked B+ which I assume is attached to the battery. On the other side(closest to the rear of the car is another post with nothing attached.

I was able to trace the fuel pump wire from the top of the fuel tank all the way to the upper left quadrant of this fuse/relay box, but not the sender wire.

One thing to note that I did not mention before is that the PCM is also recieving an empty tank signal because it says the range is always 0 miles.

I am still at a loss as to where to continue looking.

Mine also reads full after reconnecting the battery but then after about 10 seconds goes to empty. One the next startup it will show empty from the beginning.

lunghd, Is this similar to your problem?

Ok I am almost 100% sure it IS a ground problem. I was able to jack up the rear and get to the connector on top of the tank today. I was able to insert a 100 ohm resistor between the purple wire(sender wire) and the solid black wire(fuel pump ground), turn the ignition on and get the gauge to read 1/4 tank full. Same test with a 180 ohm resistor read 3/4 tank full.

There is a black with white stripe wire that I believe is the ground for the sender unit but it is not connected grounded. I have no idea where it is supposed to ground, but I am sure that is why I am getting no signal to the gauge/computer.

I appologize for the numerous posts but I just got the fuel gauge working again. I went ahead and connected the BLK/WHT wire under the car that travels along the frame to ground. This is only a temporary fix but proves to me that it is just a ground. When I get more time another day I will try and trace this wire towards the engine bay and find out where is SHOULD be grounded and fix it right.

Does anyone know where the BLK/WHT wire from the sender signal reference is supposed to be grounded?. I suspect it is probably under/inside the dash.

The wiring diagram that I have shows this wire running back to the PCM, but not before running through SP422 (splice pack) which is located just behind the transmission cross member on the inside of the frame. The diagram also mentions something about SP423 which is located near the cross member just behind the fuel tank.

Your diagram is correct it does splice at the rear of the tank and rear of transmission. So the BLK/WHT wire connects from the fuel tank connector all the way to the PCM. Do you know which connector(color) on the PCM? Does the purple sender wire also go to the PCM along with the BLK/WHT one?

Both run to the PCM. The PPL wire (circuit 1589) runs to connector C3, cavity 13 and the BLK wire (circuit 470 - was BLK/WHT at one of the splice packs)runs to connector C3, cavity 19.

Thanks for the info, I think this will help me find the root of the problem. I am beginning to think me grounding it allows the computer to have a reference voltage to compare with the sender wire and work again, but maybe it is really not right. I will take a look at this tomorrow, thanks again.