Adjusting/calibrating the DMCH Fuel Pump/Sender Combo?

[jangell]

I've had the fuel pump/sender combo installed in my car for a few years now, but it's only in the last week that the car has actually been driving and had a full tank of gas. However, I've noticed two issues:

- With a full tank of gas (it spilled out of the filler a bit), it reads mostly full, but not completely full. Like, 15/16th full. You can see a decent gap between the the full mark and the need.

- The low fuel light never comes on. I found this out when I went for a test drive and the car cut out. Took me about 10 minutes of tinkering before I realized my fuel pressure gauge read zero. I just thought the fuel gauge was reading low, but not actually empty (if it wasn't on empty, it was really really close, but the light wasn't on, so I still had some fuel, right?).

Now, the low fuel light bulb might just be burned out. I haven't gotten around to taking the fuel pump access cover off again to test it yet (re: it's cold out). Before, the fuel light was the only thing that worked reliably, as the old sender had was rather... non-linear.

So, is there some way to calibrate or adjust the fuel sender? And what are the chances that the sender isn't triggering the light vs it being burned out? I really should just answer that last one myself by testing the bulb, though.

Thanks!

-- Joe

[David T]

Seems to be a common complaint. TADT (They All Do That).

[dmcnc]

What direction is your float arm facing? If it's facing the sides, it could give you mid tank readings but jam along the extremes and not give you either a full or an empty reading as the arm wouldn't hit both ends.
Separately, if your gauge makes it to the quarter tank and below, you should see the fuel light come on with the Houston logic board. If your needle mark makes it close to the E line and you don't get a light (and your wiring and bulb are in good order) then I would assume your logic board is bad. The DMCH logic board turns the fuel light at about the quarter tank mark (maybe just below) from what I've seen.

[DMCMW Dave]

I've used this test:

With the tank near empty, remove the dummy plug where the sender used to be. You'll see the float arm through the hole. Push the arm all the way to the bottom of the tank and hold it there. (use something like a paint stirring stick). Start the car or jump the RPM relay to run the pump (pump has to be running for the gauge to function as the adapter box is powered at the pump +). The gauge should read now empty and the low fuel light should be on.

Now use a hook tool to lift the float all the way to the top of the tank. Shut off and restart the engine, or wait a minute or two for the reading to come up. There is a pretty long time delay to damp the gauge from tank slosh. It should read all the way full. If you are patient you can lift the float to several positions and note the reading on the gauge.

If it is off one way or the other, you can bend the arm a bit to compensate. Before you do, make sure that the arm is seated all the way and snapped in place on the rheostat on the side of the pump assembly. I'm not sure about now, but for a while the arms were shipping detached from the pump and it's easy to get the arm installed incorrectly.

I'd suggest that you test the dash bulb and circuit, it's pretty rare for the sender to work but not turn the light on at 1/8 tank. Just unplug the pump assembly, ground the light wire (light-green/orange to black at the 3-wire harness), and turn on the ignition. The car does not need to be running. If the light doesn't come on check the connections in the left front fender area, at the dash cluster, and the bulb.

[bfloyd]

I have the same fuel pump / sender combo and while mine has never given me any issues, it always turns the light on at 1/4 of a tank. Never been an issue, but it just reminds me I need to go get gas within the next 30 minutes of driving.

[steve]

I have the same fuel pump / sender combo and while mine has never given me any issues, it always turns the light on at 1/4 of a tank. Never been an issue, but it just reminds me I need to go get gas within the next 30 minutes of driving.

Same for me but it will never show a full tank always just south of full.:8ball:

[jangell]

I filled the tank again today, and the gas pump shut off every few gallons (it's been so long since I drove this car that I forgot that was common) and overfilled a little each time. I was at ~1/4 tank on the gauge, and after I put in 10.5 gallons, the gauge read completely full. So it looks like I jumped the gun worrying about calibration.

I'll try to look at the fuel light issue this weekend. I'll be changing the trans fluid and doing some muffler tweaks, so while I want for the muffler too cool down will be a good time to do the fuel light tests.

Thanks!

-- Joe

[NightRider]

I haven't filled my tank since I got the pump/sender installed, but I have had the tank almost empty, and the low fuel light did come on as expected. I don't drive my car much so I rarely fill the tank.

[jangell]

I confirmed that the fuel light doesn’t work when I ground the loght green/orange wire, so it’s probably the bulb.

Of course, I put everything back together and now the fuel gauge points straight up. Any idea what would cause that? Or will it be obvious when I take the one off again in 5 minutes? :)

— Joe

[jangell]

Apparently the answer was “push the connectors together a little tighter and try again”, because now it works.

— Joe

[jangell]

An update on the low fuel light: the bulb was burned out. I had to take the binnacle off when my coolant temperature gauge suddenly started pointing straight down for no good reason, and that became a priority to fix due to how paranoid I am about the engine temperature. I only found the "hit the binnacle really hard to fix it" article after I got it all apart, of course, but at least I could confirm the bad bulb and replace it. I haven't run the fuel tank down enough to test the fuel light with anything but a PowerProbe yet, but I'm sure it'll be fine now. And the temp gauge is working, too. :)

Thanks!

-- Joe

[David T]

You start getting these kinds of issues if you don't use the car.

[jangell]

Yeah, being off the road for 5.5 years during my engine swap certainly didn't help. But now I'm driving it as much as possible.

Thanks again for the help

-- Joe