My 87 k-5 blazer had the same problem on mine the sending unit in the tank
had fallen apart. Once you have an open circuit it shows full. In my case
all I could buy was a complete unit with fule pum and it listed at $400 87
was a crossover year and different from 86 or 88 instead I bought a $20
generic unit from J.C Whitney and modified the actual sending unit to fit
where the original one had been. Its not ver acurate across the scale but I
do know when I'm getting close to empty.
I'm finally trying to troubleshoot the always-full fuel gauge on my '91 T-15
Jimmy (I'm tired of trying to track down that front end clunk (NOT t-bar
mounts)). Digital cluster, it will occasionally (very rarely these days)
drop down one bar when I'm low on gas, altough it hasn't done that in months
and it was fairly low recently when I lent it to my brother (who refuses to
fill a tank even when you have to rely on the odometer to track the level).
So I figure it's either a bad sending unit or a short/break somewhere down
the line. I tried unplugging the sender and pump at the tank (easy on the
s-series) and turning on the ignition and it still read full.
Questions. If the line from the gauge were broken (open circuit), would that
be read as a full or empty tank? What sort of resistance should my VOM read
at the tank with a full tank of gas? Low tank? Could a bad solder joint in
the gauge cluster cause such a problem (a common occurance for dead gauges)?