Fuel Sending Unit - Short?

[stretchit2]

Hi All,

Recently I have been experiencing a short in my electrical system. This is a recent problem and I am wondering if it is possibly related to the installation of a new fuel sending unit.

My mechanic installed a unit purchased from Moss Motors when he reconditioned the gas tank.

I have had some other electrical problems related to an alternator conversion, but that manifested itself in the battery not charging properly and once we tried different model alternator ('79 MG Alternator)the problem went away.

Despite all my other electrical issues, I never experienced any problem with the system shorting out, until the fuel sending unit was installed, but the mechanic doesn't think this is the problem, rather he thinks it is in the Trafficator.

The short is intermittent and I haven't been able to replicate it at will and I was wondering if there is a way to check the fuel sending unit for a short?

Thanks,

Stretch

 

[Deleted member 8987]

A shorted sender will just give you a "full" tank indication. Mechanic is right on that part.

 

[Keoke]

Despite all my other electrical issues, I never experienced any problem with the system shorting out,---WHAT System????

 

[Swoz]

Hi Stretch,
You didn't indicate what your fuel gauge is doing when the so called "short" happens, but I'll assume you are getting either a pinned empty or full reading on your fuel gauge. Here's how the system works, which should help you determine what is happening. The tank sending unit is designed to work in conjunction with the fuel gauge such that the resistance values below give the indicated fuel level reading on the gauge. Each additional quarter of a tank is an additional 17 ohms.

o ohms ----- Empty (tank sending unit wire grounded)

17 ohms----- 1/4 tank

34 ohms----- 1/2 tank

51 ohms------ 3/4 tank

68 ohms------- full tank

Sooooooooo, you need to remove the sending unit and test that you are getting the right resistance when moving the float. The resistance should be tested between the sending unit terminal and the case body of the unit.

A hanging float is an empty tank, so that should be close to zero ohms. Please keep in mind that bringing the float all the way up will go way above the 68 ohms required to indicate a full tank. The float never gets that high in the tank. What is important to test for is a smooth transition from zero ohms at empty to 68 ohms as you bring up the float. Any bad contact along the way such that the reading goes to infinity ohms, will pin the needle to a full tank. And vice versa, any zero ohm reading or short in the sending unit will pin the needle to empty. I hope this helps.....Tony :wink:

 

[Deleted member 8987]

There isn't any indication in his post that it's a high peg or low peg gauge.
Having written service in my career, I usually try to pick out pertinent points:

"I never experienced any problem with the system shorting out, until the fuel sending unit was installed, but the mechanic doesn't think this is the problem, rather he thinks it is in the Trafficator.

The short is intermittent and I haven't been able to replicate it at will and I was wondering if there is a way to check the fuel sending unit for a short?"

So, I wonder, why would the mechanic think it was the trafficator?
What kind of "short" are we talking about?
Or, is it an "open"?

What was the indication of the "short"?
When and where did it happen?

 

[Johnny]

This may be a stretch, sorry about the pun, but I always add a ground wire from the sending unit to chassis ground when I replace the sending unit.

 

[stretchit2]

TOC

Here is what I am experiencing when the "short" manifest itself. The turn signals and brake lights and gas gauge quit working and the 50 amp fuse is blown. I haven't been observant enough to determine what precedes the blown fuse and haven't yet observed the actual event occur, it seems that the first time I notice something is afoot is when I go to start the car and notice that the fuel gauge reads empty.

Stretch

 

[stretchit2]

Johnny,

I appreciate the "stretch" pun.

Stretch :lol:

 

[Deleted member 8987]

First, there is absolutely, positively, no way the fuel sender could be blowing your fuses.
Period.
The sender is a resistive (variable) load on the gauge proper, which is fed by (probably) a voltage stabilizer from the fuseblock, probably with ignition switch thrown in for good measure.

So, for troubleshooting, remove sender unit from your calculations.

Next, disconnect the power to your turn signal flasher and/or trafficator.
Find the power feed, disconnect it, tape it up, and drive the car.
Use hand signals for safely and meeting the letter of the law.
If the fuse somehow does not blow, we have some idea where the problem might be.

If it DOES blow, we may have to look elsewhere in the loads on that fuse.

 

[Keoke]

Well Stretch that is a bit weird given that the 50 Amp fuse should be dedicated to the horn circuit.?????--Keoke

 

[Swoz]

Stretch,
If you're blowing a 50 amp fuse you have a short and it is NOT in your fuel tank sending unit or the wire going to it. A short in the sending unit or wire to the sending unit won't blow any fuse, it will just give an incorrect tank level reading.

You're problem lies in one of the circuits that leaves the A4 terminal from your fuse box.......Tony

 

[stretchit2]

Fellas,

Swoz's comment about the problem lies in one of the circuits that leaves the A4 terminal is compelling. I think those circuits would include the flasher unit/relay, wiper unit, which I never use (rain adverse), brake switch, heater motor switch and fuel gauge. I know that when the event occurs it definitely effects the directionals, the brake lights and the fuel gauge, and I hadn't thought to check the heater motor switch and/or the wiper unit, but they may be effected as well.

The one comment about the 50 amp being dedicated to the horn, could that indicate that the short may be related to the horn? I don't recall using the horn as I feel no one would hear it anyway, so I just try to keep out of everyone's way.

So okay, this weekend I will take TOC's advise and disconnet the power to the signal unit, tape it, etc. and see what I find out.

I will keep you posted.

From the Land of Darkness, yours

Stretch

 

[Deleted member 8987]

Ya know, I'd drive over and look at it, but last time I did that it was 1300 miles each way.
Sorry.

 

[Keoke]

The one comment about the 50 amp being dedicated to the horn, could that indicate that the short may be related to the horn? I don't recall using the horn.

Well yes Stretch 2 ,
But that circuit is so robust previous owners may have used it for other things too. In my case i get my switched power via relay for my driving lights off that same fuse.
Consequently, if the car's circuit has not been tampered with that 50 amp fuse is dedicated solely to the horn circuit.---

However,
The ground for the horn push is located in the trafficator which also runs inside the steering column tube??--Keoke

 

[Johnny]

A friend had a similar problem and found a short going to the blower fan motor. It was intermittent and difficult to locate.

 

[stretchit2]

I'm back with an update.

I have driven my healey 4 or 5 times since we last spoke. Despite all the sage advice I elected to go with the do nothing method............................ except with some critical information I failed to mention, as I didn't think it was related, well not exactly.

During the whole time I was experiencing the "short" and blown 50 amp fuse I had a continuing problem with my alternator not charging the battery. This had been going on since my mechanic converted my car to an alternator with a single wire, it had an built in voltage regulator, yet it was still wired through the Healey's original regulator. Long story why the mechanic did it that way, but it simply did not work.

So, after a few weeks of failures my guy installs a two wire alternator (79 MGB) and it seems to be charging the battery fine, and.......(wait for it).... since the new alternator it appears the "short" and assoicated blown fuses has gone away.


Does this make any since?

 

[Keoke]

oes this make any since?

Yep Stretch, SINCE the problem has gone away.---Keoke-- :laugh: