gas gauge

[MrWagoneer]

was going to post my question here...
http://www.ifsja.org/ubb/ultimatebb.php?ubb=get_topic;f=3;t=007829;p=3#0000 52

but decided to make a thread anyways.

yes, my gas gauge doesnt work (surprise surprise) :]

according to the the test described in the manual i must replace my sending unit. no, i dont have the access plate under the back seat of my 75 wagoneer. in fact my sending unit is further forward on top of my gas tank, bout half way along.

i purchased a gas sending unit (and instrument cluster)from the wreckers.
it seems to test fine, 110 ohm at full and 10 ohm resistance at low gas position.

the manual describes that if you short the sending unit wire to ground and you get full scale deflection on gas gauge then to replace the sender unit.

i figure, ok, let me try simply hooking the sender unit i just purchased while outside of the vehicle and i can play with the level of the float to see the effect on my gas gauge.

so i wire it up and same thing, no gauge movement.

if i apply 12v across the gas gauge, i get full scale deflection.

all other gauges work fine.

my question to anyone that may have experienced this ...(or even if you have'nt :])

when i apply a 12v light bulb tester to either contact of the gas gauge (to check for power), i observe a periodic flashing of the light?? (but the frequency is not constant)

similar to my indicator lights...(but they are constant frequency)
i check to listen if my indicator flashing unit is ticking away while checking my gas gauge (incase there is a crossed wire), nope, it only ticks when i use the indicators.

i could have sworn i read somewhere on this forum that your gauges are 'updated' every so often, does this fact account for the periodic flash?

[Tourettes]

sounds like the sending unit.
the flashing might be caused by the Constant Voltage Regulator(a bimetalic strip wrapped in wire, opens and closes as needed).
A shot in the dark here, check the prongs coming out of the back of the cluster, some have had problems with them braking.

[MrWagoneer]

thanks Tourettes, bimetallic was the word i was looking for.
i knew i had read it on this forum b4...obvious abuse of my privileges when i was young resulting in memory loss in my old age :]

doing a search on that word brought up more intersting reading.

also fixed the fuel gauge problem. turns out it was the old (origonal) brass float on the sending unit. a small hole ensured it would fill with gas and never rise to the occassion.

in other words, it didnt perform as a float, more link like a sink :]

incidentally (for anyone else that may come across this brain teaser), the 2 sending units i pulled out from the wagoneers at the wreckers had me puzzled.

at full position the resistance was 110 ohm.
at empty they were 10 ohms.

what i could'nt figure out was, how was the guage supposed to a indicate full tank when the sending unit was offering max resistance in the full position 110 ohm, ie. logic states, impeding the current flow thru the gauge will result in minimum deflection of the gauge needle.

conversly, how was the gauge supposed to indicate empty when the sending unit was offering minimum resitance in the empty position 10 ohm, ie. allowing maximum current flow thru the gauge will result in maximum deflection.

welp it turns out after pulling my origonal sending unit out that it shows 10 ohm in the full position and 110 ohm in the empty position....this made sense.
which is contrary to the Hayes manual.

[joe]

Originally posted by MrWagoneer:
which is contrary to the Hayes manual.Gee...big surprise there. :D

[Wart Hawg]

The only problem I've ever had with my gas gauge is the fact that the needle keeps wanting to point at the "E" at the end of every week. :D

[MrWagoneer]

i hear ya Wart Hawg, i do 151 odd miles/day.

i can pass everything on the road...cept the gas station.

[Kukur]

Ok, what is the 'norm' mpg you guys are getting?

Running the 2brl on a 360. I just had it rebuilt and the her first long haul (100 miles) trip she squeaked out 11 mpg.

Not bad?

[Kukur]

One more question,

The engine in my Wag went out last Feb. the gas gauge worked at that time.

It sat until about a month ago when I got the new engine in it and now the gas gauge doesn't work.

Would this tend to be more of a stuck float rather than an electrical issue?

[LRRH]

kakur...this may be the ground wire that runs from the sending unit to the frame. usually it's a thin wire that can rust easily (if left sitting) and if it's not hooked up right then your guage will read empty all the time....then again....my FSJ gas tanks read empty all the time anyway even though they both work!!!

on the 86 I can get 11-15mpg depending on the situation and on the 70 i get about the same, but less if I have too much fun with the gas pedal.

86=360 2bbl
70=350 4bbl

[Kukur]

LRRH,

I sure hope it is something that simple. I know I had some ground probs with one of the rear tail lights because of that.

Is the sending unit is located on the top of the tank?

I won't complain about the 11 mpg in the winter (warm it up for 5 minutes) and a new engine.

Thanks!

[MrWagoneer]

10 - 12 mpg...401 with 4 barrel carb.
if you do a search on the forum, there is a great thread with many people submitting their various milage.

check what LRRH said, apparently it is the most common cause of malfunctioning gas levels.

Kukur, if you have that access hole under you driver side back seat, then lucky you. i wasnt so fortunate. it may be rivetted in or maybe held in with 4 screws. that circular plate gives you direct access above your sending unit in the gas tank. if nogo with the access plate then you need to go under and reach the sending unit electrical line coming from the gas gauge, not the ground line if you have one (i dont).

disconnect your battery.
first test would be to connect that sending unit to gas gauge line to a good ground on your chassis.

connect the battery, turn the key to the on position. if you see your gas gauge going up (to full), then you have determined that you gauge is working as it should.

the electrical circuit (at least in my 75 wagoneer) is simply a postive voltage feed to one terminal of your gas gauge, then out from its other terminal to the gas tank sending unit with the rheostat, then out from the other end of the rheostat to ground...a simple series circuit.

if you conduct the first test described above you are effectively eliminating the sending unit rheostat and simply shorting the gas gauge to ground, and if your gauge is working correctly, it should read full scale deflection (full tank)since you have grounded one terminal (0 volts on the sending unit terminal) and you have power to the other terminal (approximately 5 volts coming from the 'constant voltage regulator').

if this is the case then you need to take out the sending unit and inspect it.

if no defelction on the gauge then you need to eliminate the possibility that your electrical line from gas gauge to sending unit is bad.

you need to access the back of your instrument cluster and ground the gas gauge to sending unit terminal. if now you get full scale deflection, then you have a problem with the electrical line to your sending unit.

if no deflection, then your gauge is suspect.

to check the gauge, determine if there is power coming to the gas gauge terminal (with a 12V testing bulb or VOM). test for power at this terminal (not the terminal that goes to the sending unit), you may see a strobing/flashing effect on the bulb or vom, this is the cvr :] and not unusual.

if there is power there then you have a malfunctioning gauge.

if no power then you need to find out why there is no power getting to you gas gauge. usually no power menas that all your other instruments wont be getting power as well, so if the other instruments are working you can be pretty sure there is power going to you gas gauge, they all share a common point for power and it seems to be coming from the 3rd terminal on the temp gauge.

[Kukur]

Mr W,

I don't think she has the access panel, but I will try to crawl under her at lunch and see if I can alteast try the ground test.

Thanks for the detailed checklist. Even I can't mess those steps up.