: When a fuel sender fails - what happens?



turbojimmy
08-10-11, 11:32 AM
The gauge in my '94 reads full all the time. The factory service manual doesn't really offer any troubleshooting for the sender. I see that the sender and the fuel pump share the same ground. Since the fuel pump works, I don't think it's a ground issue.

I have had some funky wiring problems with the car - mainly random corrosion at various connectors. They have been external (well one was inside the door, but pretty much external) so far. The fuel pump connector looks good.

So I know that the senders become unreliable over time, but when they fail completely does the gauge show "full" all the time?

sven914
08-10-11, 01:53 PM
Generally the fix is a new sender. However, the gauge is controlled by resistance; when there is high resistance on the signal wire, the gauge will read full. So, the wire from the sender to the gauge may be shorted to ground.

turbojimmy
08-10-11, 05:45 PM
Generally the fix is a new sender. However, the gauge is controlled by resistance; when there is high resistance on the signal wire, the gauge will read full. So, the wire from the sender to the gauge may be shorted to ground.

Hmmm....maybe I'll pull the instrument cluster out first. It's not too hard and if there's something shorted back there it should be obvious.

Thanks!

HUF
08-10-11, 06:22 PM
^^^^
I would not bother removing the instrument cluster. The sending unit on these cars is well-known and common issue.

turbojimmy
08-10-11, 07:26 PM
^^^^
I would not bother removing the instrument cluster. The sending unit on these car is well-known and common issue.

Yes, but what happens? Do they all make the gauge read full? I know they become unreliable, but I don't want to drop the tank if there's some other electrical issue. I've stumbled on enough electrical problems with this car so far that I'm wondering if the sending unit really is to blame.

brougham
08-10-11, 07:51 PM
I've never heard of the sender being a problem before. It shouldn't be too hard to find out what wires it is and test it without dropping the tank. The I wouldn't bother with the dash either, it's probably just a broken wire somewhere.

turbojimmy
08-10-11, 07:54 PM
I've never heard of the sender being a problem before. It shouldn't be too hard to find out what wires it is and test it without dropping the tank. The I wouldn't bother with the dash either, it's probably just a broken wire somewhere.

It just occurred to me that I test the resistance right at the plug for the sender. I think the manual might have the range in there somewhere. I have the whole back end taken off it so the plug is easy to get at without getting under it.

HUF
08-10-11, 08:22 PM
I've never heard of the sender being a problem before. It shouldn't be too hard to find out what wires it is and test it without dropping the tank. The I wouldn't bother with the dash either, it's probably just a broken wire somewhere.
It is a very common problem on 1993-1996 Fleetwoods, as well as ImpalaSS, Roadmasters, etc. It is the same part.
Read this up, and you'll see what I am talking about:
http://www.impalassforum.com/vBulletin/search.php?searchid=3071693

brougham
08-10-11, 08:34 PM
It just occurred to me that I test the resistance right at the plug for the sender. I think the manual might have the range in there somewhere. I have the whole back end taken off it so the plug is easy to get at without getting under it.
That would at least tell you if the problem is the sender or something else.

brougham
08-10-11, 08:41 PM
It is a very common problem on 1993-1996 Fleetwoods, as well as ImpalaSS, Roadmasters, etc. It is the same part.
Read this up, and you'll see what I am talking about:
http://www.impalassforum.com/vBulletin/search.php?searchid=3071693

Those seem to be typical problems with any older car- either ground issues or the sender sticking.
I don't know what is different but Cadillacs had different senders then Caprices and Roadmasters.

HUF
08-10-11, 08:51 PM
It is pretty much the same part with minor differences. The failures are similar as well. Owners of 1994-1996 Fleetwoods with not properly working sending unit will probably chime in soon.

turbojimmy
08-10-11, 08:59 PM
So I checked the resistance of my ignition key just to make sure I'm doing it right. I got 1130 ohms, which is consistent with "Pellet #5". Good info for future reference. Which, by the way, is why I post seemingly random dribble here. I know I can search on it years from now and retrieve it.

According to the manual, the black/white wire goes to ground and the purple wire feeds the fuel gauge. Knowing I'm using the multi-meter correctly, I unplugged the sender harness and hooked the leads up to the purple and black/white wires on the sender. This should measure the resistance through the sender, right? I got nothing. The multi-meter didn't do anything which means it's wide open (very high resistance), right? Given this, the gauge would be reading full which makes sense.

If I had a short to ground, the resistance would be very low and it would read empty. So, for giggles I grounded the purple wire and turned on the key. The car exploded and set my house on fire. Actually, no it didn't. The gauge, predictably, went to empty with that purple wire grounded.

So given all of this, I'm confident that the gauge and wiring is okay and that the sender is indeed bad.

I rest my case.

Thanks for the tips y'all - I just needed some thought provoking posts to get me on the right track.

HUF
08-10-11, 09:20 PM
^^^^ Glad you did not have to remove the cluster. :) While there, check out the pump wiring.

scurling
08-12-11, 07:52 PM
I'm sure you have read quite a lot on this issue. I have two 94' Fltwds. ONe had the pump/sending unit changed by a previous owner. I changed the pump on the other one myself. Dropping the tank is really no problem, 2 rear strap bolts and the fuel line disconnects. Just make sure the tank is about empty to reduce the tank weight. After market pumps and sending units are available these days, so you don't have to pay $500. for a dealer replacement.