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Re: Fuel pump and sending unit

PostPosted: Mon Mar 22, 2021 11:38 am
by gt350fme
I recently rebuilt my fuel pump unit mostly because the fuel level cable contact was intermittently failing at the bulkhead, the rubber bulkhead was also cracked leaking.
My internet searches found a cheap fix for the bulkhead connection and seal in the following pics.
Replacement wire and connectors should be soldered and shrink wrapped.
Also of note for carbureted engines is the fuel pump pressure variance of the many stock in tank fuel pumps out there, some are lower pressure some higher, for a stock engine pressure is not an issue, but if a larger performance engine is used 5-7 psi is a minimum, racing applications need even more pressure and flow.
Hope this helps.

Re: Fuel pump and sending unit

PostPosted: Mon Mar 22, 2021 8:03 pm
by TTait
I've got to stick with the 4.3 liter engine for the foreseeable future, so no problem there. Upgrading the fuel pump some day to put in a stroker is the least of my worries - how to pay for all that would be a higher concern.

My connections at the top of the tank seem solid - I can just solder new wires on to the underside - but I like what you did there. What kind of rubber is the grommet? Is it fuel rated? Did you use any special wire inside the tank?

Can anyone describe how the sock/ filter screen is supposed to connect up to the pump assembly, is it somehow related to clamp B above?

Re: Fuel pump and sending unit

PostPosted: Mon Mar 22, 2021 10:05 pm
by gt350fme
I would think that you could carefully quickly solder a new wire to the underside pin before damage to rubber bulkhead.

Pics are copied from internet and the grommet is not fuel rated that I know of and may leak later we will see not leaking now don’t try at home, I may have to go with plan B later GOOD POINT!
I do have a fuel rated bulk head purchased from Amazon that has 3 holes for wires and nut that tightens from inside like $40 but needs a larger hole, will cross that bridge if first try leaks

Rubber piece B holds and insulates the pump to the end bracket and also secures over the filter, piece B that I needed is different than the one shown, mine has 2 ears and fits into said bracket, the one shown would not fit my bracket.

I will post a pic of my finished unit later for reference.

Re: Fuel pump and sending unit

PostPosted: Mon Mar 22, 2021 10:37 pm
by gt350fme
A bit modified, I used a stewart warner fuel sender but the rest is stock,

Re: Fuel pump and sending unit

PostPosted: Tue Mar 23, 2021 1:17 pm
by TTait
That helps a lot, thank you. It reminds me that if the shop is going to paint my tank for me, I should probably paint the top of the sender assembly too.

I'm hoping to stop by there today at the end of the day and plug in my rebuilt sender to see if the gas gauge suddenly springs to life - it should, it was getting 0 ohms before but one can never be sure. Either way I have an alternate sender and the new fuel pump arriving later this week.

On yours, I suspect the grommets will hold up ok if you don't keep the tank topped off too often. I think a leak is less of an issue than a spark. I'm not sure I ever believed the official story on TWA800, but I wouldn't want a spark in the fuel tank either way.

As I have to replace the positive lead going to my new fuel pump, I think my plan is going to be to go buy a foot or so of small diameter rubber/teflon fuel line at the auto parts store, and run the new wire inside that, cable tied to the main frame of the sending unit so nothing can move or come loose. That way if the wire insulation on any of this ever does degrade there is no chance for contact or a spark. I'm not prone to paranoia usually, but an extra dose of security might not hurt this time.

Re: Fuel pump and sending unit

PostPosted: Tue Mar 23, 2021 3:39 pm
by monzaaddict
carefully open the part where the rheostat is . it may just need to be cleaned. or even just spray electrical cleaner through an opening if there is one

Re: Fuel pump and sending unit

PostPosted: Wed Mar 24, 2021 6:29 pm
by TTait
The tabs holding it together were shot, the "wire" band coming down to it had an intermittant connection, it needed to be cleaned and the small spring loaded contact was half broken and needed to be soldered back together on one side.

Its now all cleaned up and repaired and I soldered the case back together - tested it out yesterday and it works like new. Once the new fuel pump comes in I'll take a few photos and add them here.

Thanks guys.


Re: Fuel pump and sending unit

PostPosted: Mon Jun 14, 2021 4:38 pm
by monzaz
I know old post...BUT good for reference.
I have a sending unit that read correct with ignition on BUT the minute you start the motor and vibration happens the gauge jumps all over the place.
Anyone with any ideas?
This tank had VERY VERY old fuel I just want to see if anyone else had this issue and fixed it first before tearing into it blind. :)
Thanks, Jim
JD Race

Re: Fuel pump and sending unit

PostPosted: Tue Jun 22, 2021 5:49 am
by TTait
I wonder if it actually the vibration, or if the signal is being corrupted electrically.

The float is on the end of an arm so even if the car had bad motor mounts causing the gas to be stirred around in the tank the float would barely notice the ripple.

I'd inspect the wiring for breaks to see if the vibration is causing an intermittent short to ground. The wires come off the top of the tank and then up into the trunk/hatch area inside the car. Look for damage to the wiring in this area first, particularly damage to the tan wire.

You should find a connector you can get to inside the trunk/hatch area. Assuming you are still running an in-tank fuel pump you need to leave it connected to run the car, but you can probably get a meter onto the tan and black connections with it still hooked up, If you no longer have an in tank pump you can disconnect it during diagnostics.

Hook up the meter and read the ohms value between black and tan. 90 ohms is a full tank, 0 ohms is empty. your reading shouldn't fluctuate much whether the car is off or on.

If it is fluctuating, turn the car off and disconnect the connector and read off the tank side of the connector only, if it is still fluctuating then the problem is with the sender or the wiring to the sender. If it was jumping around when connected but is steady when disconnected, the the problem is somewhere the wiring up to and including the gauge.

You can also pull off the instrument cluster bezel and lens and then the two screws holding the fuel gauge in place, pull it our a few inches and unplug the gauge from the harness and read the resistance from black to tan, or from black to the non-pink wire as the color may have changed by the time it gets to your gauge. Is it stable, or jumping? Does that change when the gauge is plugged in?

Next check the DC voltage at the gauge from pink to black. It will be 0 with the car off and 12.75 to 14.5 volts when the car is running. If that fluctuates quickly more than about a quarter of a volt then you may have Bad power or ground. If you saw fluctuations on any of the ohms reading before and also on the DC voltage at the gauge its likely the ground. Check this by keeping the red lead from you meter hooked up to pink and move the black lead to a good ground point on the body of the car, maybe the metal sill plate in the door or the parking brake or similar chassis ground point. If voltage is now steady then you have a bad ground. Read the ohms again with one lead on the tan (third color) wire and the other lead on your chassis ground and see if it is now stable.

If you are able to isolate the problem to a noisy ohms reading coming from the sender itself - best measured from the unplugged connector at the rear of the car - then your going to have to drop the tank and repair/rebuild/replace the sender.

That is doable - but there is more to discuss if you really have to do that.

Probably lots of typos - its late.

Re: Fuel pump and sending unit

PostPosted: Tue Jun 22, 2021 8:38 am
by monzaz
WOW.......... That is Awesome.... Thank you very very much. Will get testing.

Re: Fuel pump and sending unit

PostPosted: Sun Jul 04, 2021 1:51 pm
by TTait
Got her figured out?

Re: Fuel pump and sending unit

PostPosted: Sun Jul 04, 2021 4:10 pm
by monzaz
Have not had a chance to do this... Vacations racing etc. BUT I really do appreciate all the low down on the electrical evaluation and what to do. I will likely do it when fall comes around as we really want to drive it still for summer . Jim