TFC 89

I was wondering if someone can tell me how the Range calc is done on an 89. My display shows between 129-135 miles remaining irregardless of how much gas is in the tank. The Tank level indicator seems to be working accurately. I do know that there is a cracked part on the tank level assembly. I noticed it when I was replacing the fuel pump, it's not the pulsator this part sits above that. So I really have 2 questions - how is the mileage range calculated and what might that part in the tank assembly be?
Thanks is advance

RollaMo-LT4

Well, the range is just a computer calculation based on the remaining fuel in the tank and what your current mpg is.

Say, your traveling down a flat road at a constant 70mph and the range indicates 200 miles.
That means: 200 miles based on the above conditions.

The range will drop off considerably if you come to a hill.

So really the range is "close", but also a constantly changing calculated number.
It does not really mean you have exactly that many miles until empty.

SunCr

Once it hits Reserve, it's mileage since Reserve. It isn't calculating how much is left. That's outlined in the Owner's Manual. Everything else is based on a serial data feed from the Fuel Level signal at the Dash back to the ECM, which then examines Pulse Width, Air Flow (MAF) and the TPS signal if I remembering all that's in it. If you have a constant Range regardless of what's showing on the Gage, you usually suspect something with the Serial Data line (or the ECM) which is spliced into the Diagnostic Link. Does it have a Check Engine Light illuminated with the key on? Can you ground the Link and flash a 12? Is MPG working?

JackDidley

Bringing this ttt. Never heard of this problem till I saw this thread. 2 days later, I hit the range switch on my 87 and it reads 24 miles. 1/2 tank of fuel. Fill it up and it reads 65 miles. All other dash functions are good. SunCr are you out there ??

SunCr

I'd start by zeroing everything out by hitting reset or clearing memory with a battery disconnect.

JackDidley

You are correct, as normal. I just got back from a drive and was going to post the fix. I always have the MPG set on instant. A few minutes ago, I flipped it to avg. It said, 3.5 . Hit the reset and all is back to normal. I probably have not reset that, in a year. Must have been a data overload.

RollaMo-LT4

That "instant" setting is almost worthless really.
It means exactly what it implies, at that instant in time.

A nano second later and you could have a different reading.
The average setting is much more useful. I usually reset my average when I fill the tank.
The first 10-15 miles after resetting it, you will notice it changing quite often.
The more miles you drive the more accurate it will become, that's why they call it average.

I've noticed some of the newer cars have done away with the instant setting and will only show average.

Too many people always thought they were getting way better mpg than they really were.

JackDidley

When I have it on instant, I get better mileage. I tend to use less throttle when I watch the numbers. My right foot needs all the help it can get.