Fuel Gauge Question

98 Pontiac Grand Prix GTP...
Just replaced my fuel pump thanks to you guys! Thank you guys so much for all the information that you provide to help DIY's like me. Having said that, now my fuel gauge is on full all the time. It is only half full, but yet reading full. Is it a possibility that the brand new delco sending unit on the pump assembly is bad? Or do you guys know of another possibility? Any help will be very much appreciated! Sorry if there is another thread out there somewhere that relates and answers my question. I searched for over an hour and couldn't find the same problem anywhere.

Your post has been moved from Tips & Maintenance to the regular tech part of the Grand Prix forum.

Darn newbie...sorry about that BNaylor. Thanks again for helping me out and keeping me straight.

How about it anyone? Got any idea of what tha?

GTP Dad, you've helped me out tremendously in the past...got any idea?

You sure you don't have our identities confused? If you desire his help then send him a PM message.

:popcorn:

You're right BNaylor, I may have you two confused. But either way I'm just looking for some advice or anything to go on to remedy this problem. Have YOU ever heard of a problem like this? Is there a possibility that I've wired the harness wrong to make this happen? The gauge reads full whether the ignition switch is on or not. I do apologize if I stepped on any toes though!! Both of you guys seem to know pretty much everything there is to know about the workings of GM vehicles. Thanks again for any help you or anyone else may be able to offer.

No big deal. What was your original issue for changing the fuel pump assembly? To clarify why did you have to do any wiring of the harness other than normal connection.

If you disconnect the electrical connector at the top of the fuel pump which has the fuel pump input and the fuel level sending output what does the fuel gauge do with ignition off or on?

The car would just turn over and never would fire. No other connections were made, but I wasn't sure if the plug to the new harness could've been crossed on the purple and black/white wires to make it do this. The gauge stays on full no matter what.

Was this a GM/AC Delco fuel pump assembly? You'd have to check the wiring against the FSM wiring diagram to be sure. So if you disconnect the fuel pump electrical connector the gauge stays on full? I'd have to double check but the gauge should read far left when that is disconnected. Maybe there is an issue with the IP or a grounding issue since the gauge reads with ignition off.

Do you have a DMM (meter)?

I'm sorry this IS NOT a Delco...it's a Bosch! A mechanic at my local Chevrolet dealer is telling me that it HAS to be a DELCO for the sending unit to work properly. The issue is it being a GTP(so he says). He seems to think that it having the supercharger that it is different. Although EVERY wiring schematic that I can find for the 3.1 all the way to the 3.8 SC says the same thing. I'm having a hard time believing that this is the case. BUT, I'm not a GM mechanic either!! I do know though that this pump assembly don't worky! The pump works like a champ, but the sending unit don't. I have realized though that the gauge does move while the switch is on. In other words, the gauge has moved since Wednesday when I put the pump in. I had the pump out again a little while ago and I'm on empty and the gauge is now reading 3 quarters of a tank. I also hooked the old pump back up and it read correctly. I moved the float with my hand and the gauge moved accordingly. So maybe he's right. All I know is that the Delco is about double the price, but oh well I gotta do what I gotta do. But I hope for his sake that he's right, otherwise he'll be taking a special ordered USED pump back and giving me my cash back! I'm sorry for wasting your time with all of this dialogue, but if you might have an idea of WTH...I would appreciate it. I would love to tell him Monday when the pump comes in to the dealership that I don't need it any longer, and that the Bosch works just fine. Thanks again for your patience and your eagerness in trying to help.

Wow! That sounds like a real mess. Bosch? :uhoh:

Since it works with your old fuel pump assembly installed I guess you can rule out any wiring or IP problem. I would have to agree with the mechanic. The pump to include fuel level sending unit are unique to your GTP and the resistance of the wiper on the sending unit has to be within a certain range in which the Bosch unit is not providing. For example you cannot use a fuel pump assembly for the 1998 GT or SE models in a GTP. The GTP AC Delco part number is MU227 for the complete pump assembly and SK1091 for the level sending unit by itself. The GT/SE part numbers are MU232 and SK1195. So you can see there is even a difference between models.

Another option if the form factors and mounting points of the sending units are exactly the same is to transplant your old level sending unit to the new Bosch unit since the fuel pump itself is working. Or just get an AC Delco fuel level sending unit part number SK1091. We have a source that supplies the correct AC Delco fuel level sending unit for around $85 plus shipping. Last and possibly best option is to take the Bosch fuel pump assembly back to place of purchase and get whatever refund you can get. Good luck and let us know how it turns out.

OK, got the AC Delco in and same problem...? It is the MU227. I'm at a loss. Plug the original back in and it works!! The satisfaction of knowing that the Delco won't work either doesn't feel so good after all. Has anyone gotta clue?

For the sake of trying, I've gone to a local auto parts store and tried just about every fuel pump they have. Airtex, Bosch, and AC Delco all do the same thing. Plugged into the pump and sending unit harness with the float all the way down they all read 1/2 tank. So the sending unit is about a 1/2 tank off then. And like I said earlier, if I plug in the original the gauge works properly. It's almost like the resistance of the sending unit is different or something. I'm really stumped...I can't believe that I'm the only Grand Prix owner that's had this problem. I'm sure I'm not but it just seems like it. The local Chevy and Pontiac service departments have never heard of the problem either. Oh well, I'll keep you guys updated. If anyone has any ideas I'd appreciate you sharing them with me. Who knows you may have something that I haven't tried. Thanks guys for allowing me to vent and write on here.

Yeah this is crazy. Have you checked the wiring color coding versus pins to see if they match at the connector both fuel pump side and the connector at the wiring harness?

This is an odd one. Have you actually ohmed out the new versus old senders with a multimeter to see if they read the same off the car? The reason I ask is that the old unit may have a distorted plug (due to overheating when the pump failed), and only the old pump/sender plug fits tight with the vehicle harness.

Hey guys I finally gave up! I ended up swapping the original sending unit with the one on the Bosch pump. Yes, I checked to make sure that the wires matched up, and yes, I checked to see if the original was ohming different and it was. But, the original was correct and ALL the others would only go as low as a half a tank when it was empty. EVERYTHING works like a champ now!! I guess we just need to chalk this one up to MY luck and move on without even trying to understand the madness. Thank you again for ALL the help that you guys at least tried to give. I will definetly continue to check out all of the stickys in times of DIY chaos!! Thanks again!!

Thanks for the feedback. I forgot to mention that in most cases when you install a new fuel pump pod or just the fuel level sending unit you are supposed to check calibration. This info is contained in the FSM under replacement of the fuel pump assembly. If it is out then you bend the float to bring it into specs so the gauge reads properly and PCM knows to turn on the low fuel light and for the DICs with trip computer. Resistance is checked at empty, mid level and full and there is a specified output or resistance values.