Boats, Balls and BCMs


The Man found reason to buy one of the worst designed vehicles on the road IMO.  A vehicle so soccer-mom, it actually looks kind of like a soccer ball.  This Dodge Caravan ran ok, but it had 2 issues:  A torn intake boot, and a gas gauge that thought it was a windshield wiper.  It would read full, then read empty and ding the gas light on.  Then, without hitting a bump or banging on anything, it would read full again.  It didn’t matter how much gas was in the tank.  Drive it down town to pick up parts or a customer, it would ding back and forth at random.

The first thing I found was the bottom of the gas tank looked like the bottom of a cheap wine bottle from the damage of some sort of extra-curricular driving.  I made a mistake.  I assumed that this damage was the cause, and dropped the tank. Normally this process involves a gas tank that is invariable full.  However, since The Man had been driving it, the tank was nothing but fumes.   I did find a send unit that was worn out, but reshaping the tank and replacing the float and resistor fixed nothing.

I plugged in a Snap-on “Soul-less” scan tool and read the BCM.  It showed the proper voltage drop for the amount of fuel, and it was unchanging.  On a test drive, the gauge wavered only once, but the fuel level reported by the BCM did not change.  Since I have no clue how the data line to the instrument cluster interprets into a fuel gauge reading, The next trip was to “Ripp A-Part” to find another cluster. I found a full cluster, and a board. Some hard up mechanic had pulled out all of the light bulbs and stuffed them in his pants to sell for food for his starving children.  Didn’t help any, the other gas gauge did the same thing.  Only thing left to replace was the BCM.  Bummer.  It was a lot of work moving all of those light bulbs back and forth. I called around, and found a new BCM and the labor to program and install it at the dealer (to get their sorry arses to warranty everything) ran about 800 bucks.  12 year old “miles way over” soccer van ain’t worth 800 bucks, and The Man wouldn’t pay half that to fix something he wasn‘t getting paid to.

Then there was the AiTN network.  I get a set of links from other mechanics every day, that need help, and sometimes i offer my half asked opinion.  It was about time i tried out this site to see if it was worth upgrading to the pay account.  I was disappointed.  I only received 6 responses, and they were all things i had already tried.  O well, maybe next time.

Then I got an idea.  Since this is California, it was marked with an compact-fluorescent bulb over my head, so it wasn’t a very good idea, but it was cheaper than a new BCM.

I called up the local cartoon sponsored parts house, and asked about a fuel gauge for a boat.  Reading the directions, I found that I could get another gauge from the supplier that had a hi reading for an empty tank and a low reading for a full one, similar to my stock send unit.  The resistances were not the same, but that could be fixed.


I tore apart the yard supplied cluster and the boat gauge and found everything to be close enough mechanically to work.

gas hak1
A bit of silicone sealer held the new guts in the same screw holes as the old one. (arrows) I stuck a piece of phone wire insulation in the stock needle hole so it fit snug on the shaft.  Then I just needed 3 new wires out the back.

gas hak
I made a series-parallel resistor drop circuit with one variable resistor for fine tuning (upper arrow). This allowed the different load to effect meter movement. If I did the math right, it will read empty accurately, but full will go up a bit past the F. I disconnected the lead to the BCM and shunted it with a 470 ohm resistor to keep it happy with a “half tank” all the time. (lower arrow).

Its all zipped up right now, but The Man is too cheap to  put more than 5 gallons of leftover fuel into it.  It reads 1/8th of a tank.   When the phone rings and he’s stuck half way to the OC cussing about what a lousy job I did, I will have to adjust it.

2 Responses to “Boats, Balls and BCMs”

  1. The annoying little things are always the biggest pain in the ass to fix…

  2. It looks like you have a degree in anglo-american enginering. I used to use a different ethinc discription but was told that was a no-no. With Barack righback in office now we will see more political correctness spreading around.

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