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June 22, 2013

Worthwhile Workarounds: A Replacement Fuel Gauge for A 1996 Dodge Caravan

I rarely have the time that I’d like to have, to allocate to repairing things in the thorough way that I would prefer to. Here’s one such case, with a quick workaround that, well, just works. For several months, the fuel gauge on my 1996 Plymouth Grand Voyager (pretty much the same vehicle as Dodge Caravan and Chrysler Town and Country) had been working intermittently, and finally 2 weeks ago, it stopped working altogether. Per this picture,

Fuel Gauge

dead fuel gauge

the gauge read empty, and the low fuel light was on. This time, unlike a previous problem, the fuel sensor (located in the fuel tank) was working correctly. Here’s a relatively quick workaround I did that will suffice until I find the time (ha) to do the proper repair:

First, I could check that the sensor was OK. In this vehicle model, the sensor is simply a rheostat (variable resistor) with a float attached on an arm, suspended inside the tank. From what I could find out from some tests, the sensor is “read” by applying a reference voltage across a fixed resistor in series with the rheostat, and then measuring the voltage drop across the rheostat. The reference voltage is sourced from the Body Control Module (BCM) that is located under the dash, and then communicated to the instrument cluster and displayed there via the fuel gauge. The sensor voltage ranges from about 1 volt (full tank) to 6.7 volts on empty, but the curve of voltage against fuel level (or rheostat resistance) is far from linear. One can work out fairly readily that it is essentially an x/(1+x) relationship. 1/2 tank corresponds to about 5 volts across the sensor.

Note that the Haynes manuals or similar are pretty helpful for locating components in the dashboard area and elsewhere:

1996-2002
2003-2007
2008-2012

Note that if you buy either manual by clicking on the appropriate image, I get a small commission. Even better, if you really want to help support a starving author, you can buy my new book Dog Friends. Dogs often ride in vans :-) . Click on the image to the right for details and previews. And please look out for my forthcoming book about repair!

In my case, I was able to locate the inboard wiring to the fuel sensor, within a wiring bundle coming down from the dash on the passenger side, beside the floor console.
See the picture below of the bundle after I’ve connected in a blue tap wire and taped it back up (sorry, forgot to take a picture when I had it open).
The wire to look at in my case was dark blue in colour, matching that from the sensor, and by checking continuity, I could verify that the wiring to that point was OK. Furthermore, the sensing voltage was present on that wire, so I knew that the BCM supply voltage was OK. I knew therefore that the problem likely resided between the BCM and the instrument cluster, or in the cluster itself. The instrument clusters in this vehicle model are notorious for cold solder joints and other issues, and so I had previously taken the cluster apart to reheat some suspect joints and look for other issues. On reassembly after that, the fuel gauge still did not work. Some day I hope to have more time to figure out why, but lacking time and lacking a schematic for the cluster, I went with the following workaround instead:

I happened to have a spare “Aratester” multimeter lying around that my friend Hugh Templeton had given to me when he moved away to New Zealand a while ago. The old “ohms-per-volt” kind with no active parts and no power required to measure voltage other than via the input signal itself. Luckily for me, that meter had a decent 0-12V range, which covered what I needed. I used a felt tip pen to mark off full, empty, and 1/2 full marks at 1, 6.7, and 5V levels respectively. I attached the meter leads to my tap off the wire bundle and to a good ground under my dash. Presto, a new, substitute fuel gauge. I have the meter in a bin at the bottom of the dash console. Far from perfect; can’t read it in the dark without an interior light on, but it suffices for now. Actually, that old Aratester (made in 1961, apparently) with its rounded corners looks kind of cool there:

Maybe one of these years when I’m less busy, I’ll dig into what is probably a problem in or near the instrument cluster, but for now, this is a worthwhile workaround. I hope someone else finds it useful, and if anyone has any suggestions or a schematic for the instrument cluster, I’d appreciate hearing from you. Happy repairing (and working-around!).

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