do I need a voltage regulator?

[BerniniCaCO3]

Hey--

Retested my fuel sending unit tonight.

The hot coming from the fuel gauge DEFINITELY reads 8.04V STEADY. I tried setting my multimeter to "min/max" which takes a voltage range over 100ms of time. Nope, nothing, nada, just 8.04V steady. Granted I've never used that function before, so I might be doin it wrong; but it seems pretty self explanatory.

This hot is supplied from a voltage regulator that is supposed to put out a pulse width modulated 5V average.
Is 8.04V flat cause for concern and possible replacement of this regulator?
It's like a $30 part. Dunno how hard to get to.



I also did retry grounding out the hot, thereby simulating 0 ohms resistance. As 8ohms=full tank, the gauge needle should have swung to the far right, and it did not move a bit.
Granted, before I grounded out the hot, I had simulated infinite resistance, which -maybe- should have shown a super empty tank? Then again, maybe not, because 86ohms resistance (=empty tank) at least has some electricity flowing to move the needle with.
Not sure if this means a new gauge, which, would mean a new instrument cluster, from some where??

Or if it could still possibly be the sending unit?
I'd like to know why grounding the hot didn't do anything, and I'd like to know of that 8.04V is a problem requiring a voltage regulator.

[86VickyLX]

Hey--

Retested my fuel sending unit tonight.

The hot coming from the fuel gauge DEFINITELY reads 8.04V STEADY. I tried setting my multimeter to "min/max" which takes a voltage range over 100ms of time. Nope, nothing, nada, just 8.04V steady. Granted I've never used that function before, so I might be doin it wrong; but it seems pretty self explanatory.

This hot is supplied from a voltage regulator that is supposed to put out a pulse width modulated 5V average.
Is 8.04V flat cause for concern and possible replacement of this regulator?
It's like a $30 part. Dunno how hard to get to.



I also did retry grounding out the hot, thereby simulating 0 ohms resistance. As 8ohms=full tank, the gauge needle should have swung to the far right, and it did not move a bit.
Granted, before I grounded out the hot, I had simulated infinite resistance, which -maybe- should have shown a super empty tank? Then again, maybe not, because 86ohms resistance (=empty tank) at least has some electricity flowing to move the needle with.
Not sure if this means a new gauge, which, would mean a new instrument cluster, from some where??

Or if it could still possibly be the sending unit?
I'd like to know why grounding the hot didn't do anything, and I'd like to know of that 8.04V is a problem requiring a voltage regulator.

You have an anti slosh module in the gauge cluster. In order to check the operation of the gauge, you must turn the key off, and then back on.

[BerniniCaCO3]

would you mind explaining that more?
at which exact point do I need to cycle the key again to test it correctly?
what does anti-slosh mean?

thanks!

[GM_Guy]

Anti-slosh means that the circuit buffers what it reads from the sending unit so that the guage does not move when the fuel gets sloshed around when you are driving. It exists for the same reason idiot-guages exist--the general public is too stupid to realize that oil pressure fluctuates with rpm. That temperature can vary depending on conditions, etc.

Disconnect the low fuel warning module (just fyi to other readers, as I've noted some people complain it comes on at 1/4 tank: Its design spec is to come on around 1/4 tank remaining.) and see how the guage behaves. 5 wire module ABOVE the glove box opening as mentioned by someone else earlier.

Your basic fuel circuit in the box is cluster regulator, THROUGH fuel gauge TO the tank sender to ground. If you ground the yellow/white it should read full. Leave it connected for a while for the guage to "catch up" with the anti-slosh "feature". If it doesn't read full, disconnect the low fuel warning module and see what happens. I don't believe it affects the fuel readings overall, but it might, as its in parrallel with the fuel guage and may affect the guage reading. Anyway, if the needle goes full now, your low fuel module is messed up. (don't just throw it away if its bad. If you can't get another one, you may be able to repair the old, or have a friend into electronics tear into it to look for bad solder joints or cooked components).

While you got the tank sender disconnected for the above, take a resistance measurement across both prongs. When you go and fill up, take a resistance reading again. That will tell US if the sender itself is fubar.

Alex.

[BerniniCaCO3]

Filled 'er up just now and tested the resistance.
On Sunday I had 2 gallons maybe left. My c-man voltmeter at home read 80ohms, but, 20ohms just across the wires: so 80-20= 60 ohms resistance at pretty empty.

Today I filled it up, tested the resistance, and testing with my fluke voltmeter @work, which showed 0ohms across the wires, I had 190ohms resistance!!

With so much resistance, wouldn't that show below empty? As it is, when I start up the car, the gauge swings a little to the right, a little to the left, then settles right back at 3/4 full....

[86VickyLX]

No resistance is empty. High resistance is more fuel.

[BerniniCaCO3]

According to the 1989 manual (I've got a 1990-- I understand that it's the same car, but, if the fuel sender is one thing that changed 1989 to 1990, tell me!),
8ohms= full, 86 ohms=empty. So it's supposed to be low resistance full, not 190ohms full and 60 ohms near-empty. According to the manual, anyway.
However, it does seem to work, kindof, so maybe '90 is different after all.

I still don't know why grounding out the hot that went to the sending unit didn't register empty, then. In point of fact it did nothing.

GM guy, you seem to know about this system.
I yanked out the glovebox and could not for the life of me find that low fuel warning module. page 102 even has a 3D drawing showing exactly where it should be.
Under the passenger A pillar I did find a black wiring connector that seems to contain a Y/W wire, but it doesn't seem right... Voltage wasn't right.

[86VickyLX]

This is why it's a good idea to have your own car year's EVTM. Especially since they did change a bunch of shit from 1989-1990... Grounding what hot wire?

[GM_Guy]

My c-man voltmeter at home read 80ohms, but, 20ohms just across the wires: so 80-20= 60 ohms resistance at pretty empty.

'splain to me how you come to conclude the 80-20? Something ain't right.

WHERE precisily in/on the vehicle did you measure these resistance values. Yes, it makes a difference.
Get an evtm for your 1990. If things did indeed change at that time, it'll be helpful to you. And us. =-)

Alex.

[gadget73]

which wires are you reading 20 ohms through? 20 ohms from copper wire is a fair bit. Most copper wire in a car is less than 1 ohm per 100 feet. I know you don't have 2000 feet of wire in any circuit on that car.

[BerniniCaCO3]

Unplugging the 2-wire connector going to the sending unit at the fuel tank, one was @ground, the other had 8.04V. I grounded the 8.04V wire.
So if it is the other way around, then, 0ohms resistance should have shown empty. It did nothing at all.

I'd ended up with a 1991 mustang wiring manual; it does have the sending unit the other way around (20ohms empty, 160 full, or something like that).
Found a 1990 manual on ebay for $22. Better get it; you have to be right about 1989:1990 being different. I hadn't realized they changed anything, thought those were equivalent years.

[BerniniCaCO3]

That was just the shitty c-man voltmeter. The banana plug probes were firm in the sockets, but something was high resistance in the probe wires or in the voltmeter (by deduction).
The base resistance, probe shorted right to probe, was 20ohms. Double, triple, quadruple checked, 20ohms. So any resistance value I measured had to be 20ohms higher than reality.
I do the same thing with my fluke multimeter. Not always the same, but often it reads .3ohms probe shorted to probe, so I just subtract that from any value I read with it. An awful lot less than 20ohms, I'll grant you.

Anyway. I was measuring resistance directly across the sender leads. They're just two threaded rods sticking out, actually just about the diameter of banana plugs, and you can put alligator clips on each of them.
So, 190ohms with 17.5 gallons in it, 60ohms with maybe 2 gallons in it. Figuring, especially seeing that the 1991 mustang had a different sending unit, AND given that mine isn't completely screwy as it DOES work from empty to 3/4 tank, I'm thinking that my sending unit might be OK...

[BerniniCaCO3]

Hey, btw, when I have NO power going to the gauge, key off, it rests at around 3/4 full by default. Just gravity on the needle I guess, though, I'd expect 1/2 full...
Is that what all of you see, also?
Or does it suggest maybe something off with the gauge?

[sly]

guage should rest at the stop point south of E. If it does not rest there... something is screwed.

might be the resistor (looks like a bar on the back of the cluster) or a loose ground on the cluster connector. It may also be a loose connection on the anti-slosh module or anything between that and the sender.

[GM_Guy]

The guage has no power when the key is off, and should go all the way beyond empty. My gut reaction at this point is the fuel guage itself is faulty.

Alex.