fbekholden.com

Got a real head scratcher trying to install/calibrate a Mustang sender to the FB/EK gauge.
--Edit its FM101 not LM101

I’m using a FM101 as per Drew's build. This allows you to calibrate a different sender to match your gauge. In this case a 10/73 ohm sender to a 30/0 ohm gauge . I bought 2 of this units and BOTH are doing the same thing so I am either unlucky or missing something.

Let me start off with the things I have tried on the bench and confirmed with a meter.

Earths are all correct and tied together between FM101, sender and gauge.

Sender reads 11 ohms full and 74 empty and sweeps through that range without dead spots when you move the float.

Gauge reads full to empty when I attach a 30ohm pot in place of the sender. Sweeps through that range smoothly

With sender and FM101 attached, I follow the provided instructions. Done it a few (20+) times now and having also tried to do it incorrectly it’s pretty clear that there is only one way that this unit will program.

Set full on FM101 with sender at 11 ohms - gauge reads F and meter reads 30 ohms. Then the fun starts. Try to set half, with the sender in the middle of its range, and I can only dial the FM101 down to 22.3 ohms before it runs out of range on the trim pot. Setting E is not possible as the trim pot is out of range.

Finish programming and test and the output of the FM101 sits at 30 no matter where you move the sender to.

Maybe the FM101 is confused by the inverse ohms? Proved this is not the case by doing the same test with the 30 ohm pot that I have already used to check the gauge. Same deal. Calibrates 30 ohms fine but will not go below 22 ohms. Static at 30 when set to run.

Both units - same issue. Unfortunately its been so long since I got these that a refund is not an option.

Has anyone other than Drew had success with these? Not sure I want to keep throwing money at this solution.

My alternative mechanical solution to the problem is to strip the sender, flip the resistor and put a 47 ohm resistor in parallel to it between the gauge and ground. That would theoretically end up providing the 30-0 ohm range, but I doubt the sweep of the float would fully cover it. Here are the internals. If I cut a slot in the plastic I should be able to slip it over the float arm arse about. Then I need to cut a square out at the opposite end of the metal holder so I can re assemble. Might work and easier than trying to remove the rivet on the resistor strip and re riviting with the resistor strip flipped (there is a hole already provided). Alternative to the alternative is to drop even more coin on a GM style sender and gut it for the pot but they are around $180. Seems like a waste.

Or I just use the sender as is with the 47ohm in parallel and have it read just below half way on the gauge for full and F when empty. I really hate that idea though.

I have never used one so I am no help whatsoever, have you spoken to an instrument fitter to see if they could rewind and modify the sender unit ?
Tanks Inc. do one of these converters as well - have no experience with them but may be an option ?

I got mine to work by reversing the feed and earth on the resistor strip in the sender as mine was reading backwards. Can’t remember the exact details as it was about 8 years ago, but works as well as any old Holden fuel gauge. It took me a couple of senders to get it to work.

Well had success. Not with the FM101 but with some brute force to the sender.

First up cut a notch in the plastic resistor holder so I could flip it 180 degrees and get 0 ohms for empty and 75 full.

Unfortunately the plastic housing does not fit back on due to an indent in the metal backing plate that the shaft passes through. So plan B, take the base of the plastic housing off and flip it to the other side. Tape applied to guide me.
Used a MAP torch to heat a blade out of a Stanley knife red hot, which I held clamped in the vice. Took 3 goes at getting through it. Many toxic fumes inhaled.
Did not plan on being so close to the resistor wire but got away with it. Circled is the area around 10 ohms which is wound tightly together. This will mean than the gauge will jump a bit around this point.
Cut a new slot for the reversed resistor to exit the metal holder.
Original plan was to fuse the top of the plastic housing back together with the soldering iron. But when I dropped it in to test everything seems snug so decided to avoid a repeated dose of toxic fumes.

Adjustment time.

Clamped it shut and tested. Full empty is reading 3 ohms so need to bend the arm a bit. First take a picture to reference the correct float position when empty.
Need to bend this part of the arm in a counter clockwise direction.
So I know that it is counter clockwise only by bending it clockwise first. But soon corrected it.
Now reset the float as it is too high
Probably gone too far but next job will be to try it in the tank.
So with empty at 0 ohm, full is reading 65 ohms. Out with the parallel resistor calculator and I need 55 ohms in parallel to get the gauge to read correctly full. I have a 47 and 10 ohm in series to make 57 and then tied that between ground and the gauge input.

Success! Well apart from the little bump when the sender slides over that cluster of resistor wire a 10 ohms.

But I’m calling that a win. Frustrated that I had to go this way as I hate wasting time and money on a product that just does not work, but the end result is the same and less electronic gremlins to catch me out in the future.

Yer a bloody genius Scotty. Toxic fumes, you bewdy.


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Geez Scott……….. no doubt you thought that should have been a straight forward job at the start but…..no Sir

Well done on preserving to get the result.

Stephen

Good work mate

Rocket surgery mate!


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Thanks everyone. For reference, this was the sender I used - C5ZZ-9275-C 64-7 FUEL SENDER 5/16 TUBE SS Got a pretty good price from https://harrysclassicsmustangparts.com. ... uctId=2859

With the sender installed in the tank the float was hitting both the top and bottom of the tank. Not great. Made the decision to have it sit at 0 ohms about five mm off the bottom of the tank so there would be a bit of reserve in there when the tank reads empty. Now when it hits the top it reads 55 ohms. Swapped out my parallel resistor for 67ohm (47+20) to get full to read correctly on the gauge.

As I write this I'm not sure I like the idea of the float slapping away at the top of the tank. Either I mod the end stop (weld a tab at the top of the range) or bend the arm to lower float, and bend to re adjust the float sitting on the bottom of the tank to get 0 ohm. No question that the tank is empty if it is reading empty on the gauge with this set up. Welding the tab to lower the float is ideal, but if the unit is stainless I'll need to put it on the to do list as I have no stainless mig wire or tig gas.

Or am I over thinking this? There is no baffle in these tanks so it will tap the top as the fuel sloshes. But will this be violent enough over time to damage the brass float/top of tank?

Personally I would weld a stop to the sender both top and bottom, would have the empty mark where there is still around an inch of fuel in the tank as well