1996 Caravan Fuel Pump/Sender Unit Question

I have just replaced the fuel pump assembly in the warped tank of my 96/3.0
carvan with one from another 96. They look identical. Fuel guage reading
is now incorrect - reads too low, which I believe means resistance from
sender is too high vs the old one. Does anyone know if different spec
senders were used on these units? Is there anyway to change the calibration
on the guage? I assume there is some voltage regulation in the circuit
somewhere, and I wonder if there is any adjustment possible.

 

Is this a trick question? You did say that the tank is warped, right?

Possibly the float rod on the "new" assembly got bent during the
changeover, or perhaps the old one had been "tweaked" to compensate for
the mishapen tank (and if you're the orginal owner, you may say that you
know that not to be the case).

Ignoring that issue, you might consider trying (throught trial and error
with, say, a near-empty tank) to bend the float rod to make it read
correctly - but you may have to choose between a reasonably accurate
reading with the tank "full" or with the tank near "empty" (near empty
would be the logical choice).

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

...

CC replaced thousands under warranty. Our 96 was changed out in 98
IIRC

 

The way you worded your original post ("I have just replaced the fuel
pump assembly in the warped tank of my 96/3.0
carvan with one from another 96..."), I took it to mean you had and
still have a warped tank and that you changed out just the sending
unit/fuel pump assy.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

This is correct. I still have the warped tank in the van. I purchased the
van used in 1999, and the original owner failed to have the tank replaced
under warranty. It will hold about 12-13 gallons. I replaced the fuel pump
assembly with another one which I got from a wrecking yard, and was told
that it was from a 1996.

The pump works fine, but the guage reads between E (when half full of gas)
and about 1/3, when full of gas. The float assembly moves ok on the
replacement unit. It just seems to me that the resistance range of the
sending unit is different than the old one I removed.

For what it's worth, I don't think the warping affects the reading. While
the inward warping of the tank does push the pump assembly vertically (vs
the mounting plate), it is on a spring which allows this this movement.

I just wondered if there was an option to flash the PCM when the tank is
full, indicating that the voltage the computer is seeing from the sender
means 'Full'.

Thanks for the replies.

Mike C

 

LOL!

ca...

 

Oops - Tom - I confused you with the OP.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my address
with "x")

 

OK - here's what I think's going on: The fuel level sender is at its
lowest resistance when the tank is full - I don't know the specs. on
your van, but typically it would be on the order of 60 ohms (float full
up position). It goes to something on the order of 1000 ohms when the
tank is empty (float all the way down).

I think the wiper is slightly corroded (adding resistance into the
reading). This makes sense because:
(1) It has been sitting unused for a while (no wiping action to clean it
off) possibly in fuel, possibly in air, possibly in mixture of air and
moisture or fuel and water, and
(2) Higher resistance pushes the reading to towards empty (higher
resistance => lower gage reading). This would also have the effect of
compressing the gage scale downward (towards 'E'), which you are also
seeing.

Take it out, and carefully remove any covers from around the resitance
element and wiper. Very carefully clean both. Be careful - those wires
on the resistance element are very fine and delicate (meaning "easy to
break", in which case you throw it away). Try using some TV tuner
cleaner (Radio Shack).

To monitor your progress, read the resistance across the sensor (via the
two connector pins) at float full-up and full-down positions before you
start, and then as you clean it. I'm betting you will see the
resistance come down to the levels given above, and it will start
reading right.

Another type of problem that you see on sending units is broken
resistance element wires (they can completely wear thru from a lifetime
of cycling), but the behavior you are describing is not consistent with
that. I have no doubt that the wiper and/or resistance element wire
wiping surfaces are corroded - it fits the history of sitting in a junk
yards for an unknown period of time and the symptoms you have described.

I can't emphasize enough how delicate those wires are. You might try
just moving the float up and down a bunch of times (to wear thru the
corrosion) after squirting TV tuner cleaner (or electrical contact
cleaner) onto the resistance element and wiper without otherwise
directly contacting the element or wiper with your hands or any tools.
The other thing to watch out for when fooling with the wiper is
springing the wiper back a little so it doesn't make good contact with
the proper amount of pressure against the resistance wires - another
reason to avoid directly messing with it.

Let me know how it goes.

Sorry for the confusion earlier on in the thread - I confused Tom Ruta
with you when he posted.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")

 

....

No worries Bill.

 

Brilliant. Thank you Bill. I will bet you are correct, and I had not
thought of corrosion on the sender. This would explain the symptoms, as I
see essentially higher resistance through the range. The range you mention
is correct - on my old sender it is 80 to 1000 ohms. I did check that the
part number on the senders were the same before I installed it - but I never
checked the actual resistance. Aghhhh.

I am going to take the lazy approach initially - pour in some fuel additive
and go through a few tanks of gas to see of the movement of the wiper will
clean the contacts itself. If not, out comes the tank again.

Thanks again - your post was extremely helpful.

Mike C

 

You're welcome. Sorry you have to drop the tank to get to it.

Bill Putney
(to reply by e-mail, replace the last letter of the alphabet in my
address with "x")