First step, drop the tank and pull the sending unit. Process on doing so is covered elsewhere.
Now, check the resistance at the terminals connected to the Black and Purple wires. Or if yours is different just test the ground wire and whichever is connected to the float. Normal operation is 0ohms empty, 90ohms full.
Here is mine before the repair, with the float in the empty position I'm getting a reading of 40.6, nearly a half tank.
Remove these two Torx screws to get at the resistor, be careful not to lose the two springs:
Here is the resistor, you can see quite a bit of build up. Fortunately we are only really concerned with the leading edge, and the mount location.
Gently clean this leading edge, this is where the float mechanism slides against. I used a coarse sponge, very fine sandpaper may work as well.
Clean this plate as well, the spring needs to make good contact in this area:
Sand clean both ends of the upper spring, as well as the metal plate it contacts:
Re-assemble the resistor to the sending unit, ensuring the plate from the float is behind the resistor:
Re-test your sending unit in both the empty and full positions. I now get 7.6 empty and 107.6 full, this will work okay for now. I can adjust the gauge to compensate for the slightly high reading.
Re-install your sending unit into the tank and re-test. On mine, I had to bend the float arm up slightly, it appeared to be hitting the bottom. In fact, in the empty position it was below the bottom level of the pump.
Bending float arm:
Re-testing in tank:
My tank used to run out when the gauge read 1/2 , and filling the tank would result in the gauge pegging way off to the right. Now the gauge is nearly right-on. With the tank totally empty the needle is just above the empty mark, and filling the tank puts the needle just above full. This is the result of being high by 7.6ohms. Since its off by the same amount on both ends I'll just re-adjust the needle to be correct on empty.
------------------ www.FieroDomain.com "If any car is both the parade and the rain, it is the Fiero"
[This message has been edited by Xanth (edited 06-17-2011).]
Just curious how tuff the crud on the resistor and other parts was to get cleaned off?
I was thinking maybe "TARNEX" brand tarnish remover might work really well. I've used it on old corroded copper ground straps and various electrical connections and had them come out looking brand new. It's also plastic safe as far as I know. Works good on other metals too, except aluminum. Just a thought.....
Is that the 88 sending unit? It looks way different than my 87 sending unit. I did mine today. I had to pry a tab back to get to everything inside then is carefully used a stainless steal wire brush and scrubbed everything and went from empty at 43.2 ohms - 304.6 ohms full with multiple OL's along the swing to 0.9 ohms empty - 92.3 ohms full with a clean signal from top to bottom. Now I just got to get the tank back in. If I hadn't seen your post I wouldn't have thought about cleaning mine while replacing my tank.
Excellent write-up! Thanks for taking the time to document this info! I'll be saving this and will add it to my list of jobs to do. My gauge does read full after fill up but never gets lower than the 1/4 tank mark. I found this out the hard way a couple years back when I first got this car
Would this work on a sending unit that reads 1/2 tank when full, and just above empty when your coasting to a stop 1/2 mile before you get to the gas station?
Glad everyone likes the write-up so far, good to see its already helped a few people
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Originally posted by randye: Just curious how tuff the crud on the resistor and other parts was to get cleaned off?
On mine the build up was actually fairly easy to get off, but I was using a somewhat abrasive sponge to remove it. A chemical cleaner may do the trick if let to sit long enough.
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Originally posted by fresnofiero: Is that the 88 sending unit? It looks way different than my 87 sending unit.
Yep, 88. I'm kind of surprised it looks different than yours since the 87-88 used the same tank. I didn't realize there were variants in the sending units over the years.
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Originally posted by Jeckel: Would this work on a sending unit that reads 1/2 tank when full, and just above empty when your coasting to a stop 1/2 mile before you get to the gas station?
That actually sounds like someone re-adjusted the gauge itself to read closer to empty. Typically the crud build-up on the sending unit will result in increasing the resistance, which gives you a higher reading. A full tank will peg the gauge, and you'll run out of gas around 1/4 or 1/2 tank. I would guess your sending unit has the problem, but someone re-positioned the needle on the gauge.
You could try filling your tank, then testing the resistance at the connector for the pump/sending unit in the engine bay to see how accurate it is.
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Originally posted by 85LAMB: Can you please tell me what could cause the fuel gauge needle to go "nuts" jump up and down and do a 360 degree turn around ?
Before I cleaned my sending unit, moving it from the empty to full position and back would show huge jumps in resistance as the float connector moved over the resistor. Once I cleaned it the reading was much more smooth across the resistor. Possible thats your problem.
I just wanted to say thanks for the great write up.
Can you please tell me what could cause the fuel gauge needle to go "nuts" jump up and down and do a 360 degree turn around ?
I am hoping you might have an answer.
Thanks for you help again
I had one other thought, I believe the gauge is supposed to have some built-in damping function. So even if the signal is all over the place the gauge should only move slowly. This may actually be a problem with your gauge itself.
Xanth Great write up. I did the repair tonight. My reading before I cleaned the resister plate was 13 on empty and 100 on full. Before I pulled the resister plate I used a calipers to measure the distance from the plate to the bracket so I could get it back to where it was, because the screws on two of the units I have were not tight. When I put it back together I was down to 8 on empty to 100 on full. I then put the meter on and read the low side and turned the screw out and I was able to get a reading off 3. Then went to the high and turned the screw in and was able to get it to read 90. Maybe that is why the screws were not tight in the first place because they are used to adjust the high and the low. If you look at the float arm it swings on an arc so that would change the low and high readings. Longer arc or shorter arc. Will have to try it later on the other sending unit but it should be the same. Hope this Helps. Lynn
I believe you're right about the screws, I tried to do that on mine but the resistor was still far enough off I wasn't able to compensate for it with the screws alone.
I'll probably take the unit out again and see if I can get it closer. I originally thought the range was 0-100, so I stopped once I got 100ohms of range.
I also did this to my coupe, to repair a non working guage. I might add that I also cleaned the wiper on the float arm also before I re assembeled the sending unit. I just followed these same simple steps. I had a fit trying to find a new o-ring for the tank. I ended up getting a new one from the local GM dealer. It was very pricey as I remember.
I am going to pull my tank for De-Rusting (my favorite pass time). Even though everything was working with the pump and fuel gauge is there something I should do or check while the tank is out?
Xanth, Very Nice write-up :0 . Have you seen the 2 threads about increasing the capacity of the tank ? One involves re-locating the vent tube to a higher position on the tank (cutting required) and the other involves adding a bent piece of tubing to the vent tube INSIDE the tank effectively raising the tube. Maybe it's something you wanna try since you have the tank out.
I was looking through the Summit catalog and saw that Auto Meter makes a "programmable" gas gauge - anybody have any experince with anything like this ?
The fiero sending unit is 0-90 ohms. So your are about 17 ohms over full but you got really close to 0 on the bottom end.
I've done this before and adjusting the top end to 90 ohms is not too hard but you will never get zero ohms on the bottom end. The closest I ever got was 1.7 ohms...close enough for par though.
This should be a stickie. We should have a stickie section of good common repair threads like this. That way we don't have to pull hair trying to search. Plus to you
Xanth, Very Nice write-up :0 . Have you seen the 2 threads about increasing the capacity of the tank ? One involves re-locating the vent tube to a higher position on the tank (cutting required) and the other involves adding a bent piece of tubing to the vent tube INSIDE the tank effectively raising the tube. Maybe it's something you wanna try since you have the tank out.
I did read through at least one of those threads, looks tempting but it will probably be awhile before I attempt that.
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Originally posted by Alex4mula:
This should be a stickie. We should have a stickie section of good common repair threads like this. That way we don't have to pull hair trying to search. Plus to you
Once I have a moment I'll have this article available on my site along with the others
Excellent write-up! Thanks for taking the time to document this info! I'll be saving this and will add it to my list of jobs to do. My gauge does read full after fill up but never gets lower than the 1/4 tank mark. I found this out the hard way a couple years back when I first got this car
Heh, you aren't the only one...I was on my way to cash my paycheck and ran out of gas 3 miles away at 1/4 on the tank....
Looks like I will be doing this soon too, thanks for the write up WITH pics... All the other tech articles I have found online were sans pics, making it harder...
Thanks for a great write-up. The photo documentation is really great. As they say 1 picture is worth a thousand words. (And its especially helpful to those of us who are visual learners.)
Now I know how to fix the gas gauge on my favorite vehicle.
Thanks again, on a job well done.
~Bob
------------------ "Its nice to be important. Its more important to be nice."
I am going to remember you site (http://fierodomain.com/content/category/8/37/100/) for reference and check there first instead of coming here and asking "How do I do this and I want a real nice write up with lots of pic" Good job. A plus for your how-to articles.
------------------ 1987 Fiero GT built by my brother, merlot566jka, 3500 LX9 from 06 Malibu, WOT-TECH.com 1280 grind stage 3 cam, LS6 valve springs, 1227730 ECM conversion, Darrel Morse solid aluminum cradle mounts, Truleo headers modified to fit the 3500, 36# inectors, 70mm 4.3 throttle body adapted to 3500 intake, ported heads, upper and lower intakes, lightly polished, tcemotorsports.com crank trigger wheel, CenterForce dual friction clutch, Flowtech Afterburner muffler, 2.5" piping, cat deleted, EGR deleted, SinisterPerformance tuning, C6 Corvette exhaust tips. projected to be 35 MPG with a guesstimate of 250 hp to the wheels
Great "How to" working on these things isnt all cold air intakes and exhaust tips lol nice to see a "plain old fix whats not working" by testing and finding the real problem rather than replacing almost everything
------------------
87 Fiero GT 5sp with Vortec L35 4300 Turbocharged V6 Bully Stage 2 clutch Syclone intake manifold and engine management with Moates adapter and chip burner Air/water intercooler and Devil's Own progressive water/alky injection 50lb injectors, 3 bar map sensor, Walboro fuel pump and Jabasco Intercooler pump LM1 wideband on custom manifolds and 3" stainless exhaust system T31/T04B H3 turbo and a S10 caliper conversion. Murphy's Constant Matter will be damaged in direct proportion to its value Murphy's Law of Thermodynamics Things get worse under pressure. Arthur C. Clarke "Any significantly advanced technology is indistinguishable from magic"
Great stuff! Thanks for posting. I need to do this on the Formula. I ran out of gas last week at a 1/4 tank!
Cheers to you!
I went to do this on the '84 Fino and the sensor is different. It's a curved pickup inside a different housing. Thanks for still getting me to check the inside for corrosion. All is good.
With that type of housing I used JB weld and moved the outside cover when the metal tabs holding it inplace broke, while the JB wel was setting I adjusted the cover in relation to the rest of it and then clamped it in plavce when I got it to 2ohm and 96ish, not perfect but easy and it is much better than before.
With that type of housing I used JB weld and moved the outside cover when the metal tabs holding it inplace broke, while the JB wel was setting I adjusted the cover in relation to the rest of it and then clamped it in plavce when I got it to 2ohm and 96ish, not perfect but easy and it is much better than before.
All my tabs were intact so I just bent them back over the cover. I don't know if I'd trust JB Weld constantly sitting in gas. Does JB hold up in a tank? I'd hate to have it break up inside the tank.
