1971 SDV New Fuel Sending Unit problems

[MrPublicTime]

The gasket on my '71 SDV fuel sending unit was leaking when I filled-up the tank, so upon the advice of my mechanic replaced the whole thing as the gasket by itself was not available and it's specific to '71 and '71.  Unfortunately they didn't save the old working sending unit!

We're now on the second new fuel sending unit (from Caddy Daddy) with the same problem:  The unit works fine out of the tank, hooked-up to the gage which reads properly in the dash, Ohms read 0 - 90 just fine, but when installed with a full tank, only reads to just over 1/2.

I read about grounding the tank to the frame, corroborated by Caddy Daddy, so I did that very thoroughly, and still does not work.

Any ideas??  Thanks!  Ian 8532

[Big Fins]

You need a very clean ground from the sending unit to the frame via #12-14 gage wire.

[TJ Hopland]

Yes the ground needs to be attached to the sending unit then to the frame usually near the back bumper.

Other issue could be the float not floating for some reason?

[Big Fins]

I never saw gaskets on the sending units. They were always machined surface to machined surface. Not that I have a habit of pulling gas tanks. That's a real PITA job. Another drain plug the manufacturers should have continued with.

I guess human greed and thievery helped to eliminate that.

[TJ Hopland]

What does the 71 look like?    I think 73 was when they went to the o ring and lock ring style which I think is more or less how they still do them even though its all plastic now.

[Cadman-iac]

  The older cars have a flat rubber or rubber impregnated gasket between the sending unit and the tank to seal it. The screws go through the gasket as well as having their own sealing washers.
The 60's up to whenever they started with the in-tank pumps used a rubber O-ring to seal the sender to the tank and a metal ring that rotates to grab 3 tabs that keep pressure on the unit and O-ring.

Yours should have the O-ring style.

  Rick

[TJ Hopland]

I was also just thinking that maybe the float is hitting something inside?   If the 71 tank is like a 73 inside the filler neck extends probably about half way into the tank where it attaches to some sort of support structure.  The sending unit isn't much further back so if the float wasn't to the side it could hit that pipe and or structure. 

I think the early 70's was when some additional emissions and safety laws kicked in so they did seem to be doing a lot of experimenting and one year only stuff then.   By 73 they seemed to have worked it out because a 73 tank pretty much looks like a 80's tank.   

[Cadman-iac]

  I was thinking the same thing that TJ just mentioned.  It's hitting something inside the tank. Did you get a chance to compare the old unit with the new one to make sure that the arms were the same length and shape  as well as pointing in the same direction?
There is a baffle in some of the later tanks, but I'm not sure if they used any in 71. The last car I parted out was a 72 coupe, and I don't remember if I kept the tank or not. I'll look for it and see if it's got any baffle in it if I still have it.

Rick

[MrPublicTime]

Thanks all...  Yes, there are, in fact, two ground wires now running back to the frame by the filler - one from the sending unit and my additional ground from the lip of the gas tank which I abraded to nice clean metal before adding the ground wire.

Unfortunately I don't have the old sending unit to see what the float compares to new and my go-to Honest John's is out so ...  If anybody has an '71-'72 fuel sending unit to sell me that would be great. 

It's hard to imagine it's hitting something in the tank, but who knows.  Both new units behaved the same - got to just over 1/2 and stopped and won't go down until the fuel level drops.

Thanks again!  Ian

[Big Fins]

It's hard to imagine it's hitting something in the tank, but who knows.  Both new units behaved the same - got to just over 1/2 and stopped and won't go down until the fuel level drops.

Thanks again!  Ian

That is a confusing statement. Of course it won't go down until the tank level drops. Judging by the rest of the thread, it may be an adjustment in the float level or an unknown resistance in the wiring.

[71 Fleetwood]

Meester Public - Here is my 1971 fuel sender.  Hope it helps!

[71 Fleetwood]

The other side...  also Caddy Daddy

[71 Fleetwood]

Sorry to pile on, but it looks like the new rod to the float is shorter.

[MrPublicTime]

Yes, it does look a bit different, but only a bit.  And and doesn't seem like enough to impede the float travel to 1/2.  How did the Caddy Daddy sending unit work-out in your tank?  I got a quote from Bob's Speedometer to rebuild an original fuel sending unit $350.00 (!).  I sorely wish my mechanic hadn't thrown my old working unit away! 

Again, the unit hooked-up to the wiring out of the tank (under the car) worked perfectly (lifting the float moved the needle full range), but installed only moves the needle to just above 1/2 and I have 2 ground lines to the frame.  One from the sending unit and 1 from the tank lip.  Two different Caddy Daddy units, same results.

[wheikkila]

One question? What does a 70 float look like. Maybe you car had a earlier style. The down side to not checking the new part against the old part. And keeping the old part till the job is finished.
  Thanks Wayne   

[71 Fleetwood]

Mr Public - Mine now indicates 3/4 full.  I don't trust it so I note 200 miles to the next fill up. 
I need easy math so actual mpg is out the question 

Footnote - All of the senders that I have installed in classic M*stangs were incorrect too.

[MrPublicTime]

Well, I have done further research and came across a guy with a Jeep and the exact same problem.   Like me, the sending unit worked perfectly out of the tank to his wiring/gage.  He discovered the new float arm was not bent at the correct angle to match the fuel level so he tinkered and solved it.  Now we may be able to connect the dots...

When I left the car with the mechanic, I thought the tank was on fumes as dropping a tank with gas in it would be annoying and difficult.  After the job was finished I filled it up.  However it only took 20 gallons (both times!) and I know the tank is rated for 27.  The first time I thought it was credit card limits (20 gallons exactly!) So... I think there was 7 gallons in the tank when 'empty'.

I'll try bending the arm up a bit to get a better reading of the fuel level.  Probably mock it up with the sending unit on a table elevated to match the depth of the tank and work-over the arm travel to match.  If I only had my old unit for exact comparison!

I'm sure I'm not alone, but I need to completely 'manage' the work being done on the car I've discovered.  Mechanics seem to be mostly replacement artists and unwilling to research and problem solve... 

[35-709]

"I'm sure I'm not alone, but I need to completely 'manage' the work being done on the car I've discovered.  Mechanics seem to be mostly replacement artists and unwilling to research and problem solve..."

You can be certain you are not alone. 
Mechanics who understand and can work on our old cars are few and far between, if you or I had to pay them to "research and solve", their bill would be astronomical because you would be first paying for their education.  They could no doubt tear apart and repair a Honda, Toyota, a modern Cadillac, etc. in nothing flat, but everything slows to a crawl when presented with an old car whose mechanical features they are unfamiliar with.  Then they know either you or they are going to pay for the education --- guess who the loser is.  Why many of us do our own work and understand our cars as best we can.  The way it is with old cars.
 

[bcroe]

Again, the unit hooked-up to the wiring out of the tank (under the car) worked perfectly (lifting the float moved the needle full range), but installed only moves the needle to just above 1/2 and I have 2 ground lines to the frame.  One from the sending unit and 1 from the tank lip.  Two different Caddy Daddy units, same results. 

As I discovered decades ago, to read accurately, the float must have
just enough range to touch the top and the bottom of the tank.  That
involves both the length and angle of the float arm.  All mine have
been recalibrated and are quite accurate.  Bruce Roe