Fuel Sender Mod

When we made the trip to get my brother's Goldwing I filled my gas tank a few times. One thing I noticed is the fuel gauge would never go to the full mark even when the tank was completely full. When the tank showed nearly on empty I would only put about 4.5 gallons in its 6.6 gallon tank. I didn't like this. I prefer for a fuel guage to read 'full' when it's full and to be nearly empty when it reads 'empty'.

Motorcycles have relatively small gas tanks so they can't go as far as cars. One question in many motorcyclists' minds is "how much gas do I really have when the gauge says empty?" I decided to do a map of volume vs. sender resistance vs. fuel guage level to see how my bike actually performs.

Note: I am purposefully not going to discuss or show my setup for measuring this as to not encourage anyone else to do the same. Working with gasoline has the potential to be DANGEROUS and DEADLY.


When I started gathering information for this project I found out something I didn't know...the GL1800 has two fuel gauge senders. It has a standalone fuel gauge sender and one that is integrated with the fuel pump assembly. These senders are electrically in series and perform like one sender to accomodate the GL1800's uphill formed fuel tank.

I pumped the gas out of the tank one quart at a time. At every interval I measured the resistance of each sender separately. When the tank was empty (it holds 6.75 gallons, by the way) I poured a quart of gas back in at a time and noted the position of the fuel gauge. I then repeated the whole experiment for a second set of data. When finished I had the data below.

Gallons
in
tank
Sender
resistance
Fuel pump
sender
resistance
Total
resistance
6.75
2.6
10.9
13.5
6.50
2.6
10.9
13.5
6.25
2.6
10.9
13.5
6.00
11.7
10.9
22.6
5.75
16.3
10.9
27.2
5.50
22.9
10.9
33.8
5.25
26.0
10.9
36.9
5.00
29.0
10.9
39.9
4.75
31.7
10.9
42.6
4.50
34.5
10.9
45.4
4.25
36.6
10.9
47.5
4.00
38.8
10.9
49.7
3.75
40.8
10.9
51.7
3.50
43.5
10.9
54.4
Gallons
in
tank
Sender
resistance
Fuel pump
sender
resistance
Total
resistance
3.25
46.7
10.9
57.6
3.00
48.7
10.9
59.6
2.75
50.5
10.9
61.4
2.50
54.3
10.9
65.2
2.25
56.4
10.9
67.3
2.00
61.2
10.9
72.1
1.75
63.6
12.2
75.8
1.50
63.6
15.6
79.2
1.25
63.6
18.5
82.1
1.00
63.6
23.4
87.0
0.75
63.6
28.2
91.8
0.50
63.6
32.7
96.3
0.25
63.6
36.3
99.9
0.00
63.6
38.0
101.6


How gallons mapped to my gauge pre-mod.

According to the service manual the standalone fuel sender should measure ~2 ohms full and ~62 ohms empty, and the fuel pump sender should be ~2 ohms and ~29 ohms. Looking at the above chart the standalone sender was within spec but the fuel pump sender was about 8-9 ohms high throughout the range. If the proper resistance range had been low I could have added some resistance in series and solved the problem. No such luck. I decided to remove the fuel pump assembly and take a look at the sender to see where the additional resistance was coming from.


The fuel pump fuel sender assembly.

I measured the sender resistance with the arm at full level and empty level. The values were the same as the chart above. This told me the wiring and connectors weren't adding any unwanted resistance, and the discrepancy was in the wiper part itself.


Next I removed the float by lightly prying the float arm from under the two wiper teeth (above), then pulled the arm straight out. It's easy to do. Then I measured the resistance of the wiper. At the uppermost range of the wiper it read 0-1 ohms. This is where I wanted the wiper to be when the tank is full.

I rebent the float arm slightly (increased the angle/bend a few degrees at a time) to allow the wiper to be in this range when the float arm was resting in the "full" stop (as shown here). I put the whole assembly back in the gas tank to see what the reading would be when in the empty tank. 29 ohms...perfect!


How gallons mapped to my gauge pre-mod (blue) and post-mod (red).

I incrementally filled the tank again and mapped the gauge with the adjusted float arm. It is represented in the above chart (in red). The new gauge reading is more to my liking. When the low fuel light comes on I have a gallon or so left. Before it was almost 2 gallons. Also, now a half tank means a half tank.

From the resistance measuring I also found out the 'low fuel light' comes on at ~75 ohms and 'Empty' is ~85 ohms. The standalone fuel sender monitors the first 4.75 gallons and the fuel gauge sender the last two. I guess if you wanted to be really nerdy you could adjust the float and wiper points on both senders to customize the fuel gauge reading points. I think I'll just enjoy my new readings. It was an easy project as it only required the removal of the seat.

I am very pleased with the design of the Goldwing fuel tank. When you fill the tank to the filler neck the tank is full...there is no wasted space. Also, the fuel pump sits in a sump so it sucks out all the gas it can.

Note: I don't think an inaccurate fuel gauge is normal for Goldwings. I think mine was just a little tweaked.


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