typical fuel gauge/instrument cluster question

[tone]

So, I've been having a hell of a time figuring out what's going on with my fuel gauge .. when I got the car, the temp and fuel gauge were both inop. I assumed it was just the voltage regulator, but when I grounded both the gas and temp gauges, they went up as expected. I know it's common for the IVR to go bad, but I can't get any symptoms that are sure fire enough for me to take action on anything.

When I jumper the plug on the sender and the fuel line, the gauge slowly crawls up to about 3/4 full (before I lost patience - it might have went farther given more time) ... the sender was reading about 30 ohms with the *** end of the car jacked up (assuming it should have been lower, but 30 ohms should have still registered more than 1/2 a tank). At this point, I'm starting to believe it probably is the voltage regulator, but I'm stumped why the gauge operates when I jumper the sender ... is it possible that with a half-working regulator that 30 ohms is just too much? Any ideas or has anyone ever seen this before?

 

[67Dart273]

....... my fuel gauge .. when I got the car

"my gauge" and "the car" leaves a hell of a lot of room for guess work, but here's some general ramblings.

First I'm guessing from your post that you have "a car" with a separate, replaceable voltage limiter, and not built into the fuel gauge?

But for the rest of the story, ACTUALLY grounding the gauge at the fuel tank should result in the gauge PEGGING. Age, temperature, and condition of the gauge may determine how fast this happens.

So you could have a number of problems, just think about the PATH of the SIMPLE CIRCUIT

Starts out from the battery, eventually goes to the key switch, eventually to the voltage limiter AND IT'S CONNECTIONS, to and through the gauge, out the gauge down the sender wire all the way back to the rear, to the sender, it's connection, through the sender resistor, and (hopefully) to ground

THAT LEAVES A LOT OF PLACES for bad connections, rust, corrosion, frayed wires, AND

a gauge unit that because of age, etc, may be out of calibration.

So you just have to eliminate "stuff."

Ground the sender wire at the rear, and MAKE SURE that you have a good ground.

If the gauge wont' "full scale," go up under the dash and ground the sender connection RIGHT AT THE GAUGE

Any improvement? No?

Now, measure battery voltage at the limiter and be sure it's right at the same as battery voltage.

If the limiter is a "new replacement" electronic limiter, these normall output right at 5V DC. It's difficult to check an old original type, because they "pulse" like a flasher.

CLEAN the terminals. If you have a printed circuit dash, it could be an issue with the connector, and or the soldered pins onto the board traces.

Just because the limiter "was new" does not mean "was good."

If you've done all this, the gauge unit itself just might be out of whack.

IF ON THE other hand you do get the gauge to peg full scale while grounding the sender wire, then it's time to suspect:

that the sender is NOT grounded,

that the sender is hanging up/ stuck/ rusty / a cheap uncalibrated replacement/ or the float is bad, etc, etc

 

[tone]

Thanks for doing some guesswork - I probably wrongly assumed that early a-bodies were generally the same in that regard. It's a 64 plymouth barracuda.

I've pulled the cluster and bench tested it with a 12v supply - I get the same slow response on the gauges directly.. unfortunately, the 64 has the IVR in the actual fuel gauge. A test light shows the strobing and I'm getting strobing ~5v, but I don't really have a sense of what kind of regularity or rate I'd see out of a mechanical limiter. When you say "pegging", I'm going to assume that doesn't mean slowly crawling upward over the course of maybe 10 seconds? Thanks for the help.

 

[RedFish]

The needles should move smoothly to there max position

 

[tone]

Hi Redfish - how slowly? I realize it probably varies - but it does take a solid 10 seconds or more to climb up to the high end of the gauges. They're not particularly jumpy, though.

 

[RedFish]

Hi Redfish - how slowly? I realize it probably varies - but it does take a solid 10 seconds or more to climb up to the high end of the gauges. They're not particularly jumpy, though.

I've never actually timed the needle swing. Best guess is 3 seconds
The mechanical limiter would send full 12 V to the gauges during the warm up so the needle would blast off initially and slow as it gets closer to where it's going. My concern isn't how fast the needle moves. Accurate reporting is most important to me.

 

[tone]

Oh, no doubt - same here. I'm just trying to get a feel for whether a faulty IVR could be at fault here. I'm probably just going to find a trim pot and check its behavior on the bench with varying resistance. That said, if it doesn't respond with resistance, is it safe to assume that there wouldn't be much harm in installing a solid state regulator without disassembling the fuel gauge to remove the old mechanical?

 

[67Dart273]

is it safe to assume that there wouldn't be much harm in installing a solid state regulator without disassembling the fuel gauge to remove the old mechanical?

I have not worked on those "in gauge" units. I assume both the 12V input AND the 5V downstream is brought out of the gauge? (to jumper to temp gauge?) If so, I'd say yes

 

[RedFish]

Oh, no doubt - same here. I'm just trying to get a feel for whether a faulty IVR could be at fault here. I'm probably just going to find a trim pot and check its behavior on the bench with varying resistance. That said, if it doesn't respond with resistance, is it safe to assume that there wouldn't be much harm in installing a solid state regulator without disassembling the fuel gauge to remove the old mechanical?

The two regulators wont work together. The original mechanical limiter will need to be disabled. I completely remove them from those gauges I service.

 

[mpgFanatic]

is it safe to assume that there wouldn't be much harm in installing a solid state regulator without disassembling the fuel gauge to remove the old mechanical?

Only if it's completely dead, which is often not the case. If the needle still moves at all, it must be getting an input voltage from somewhere... and at 40+ years old, that input is usually not reliable, which is why you asked the question in the first place.

First, some theory. The old mechanical "regulator" is essentially a buzzer. It oscillates between supplying 0V and 12V, which averages out to about 5V or so. Because the gauge needles are bi-metal thermal units, they move slowly enough to damp out the vibrating input supply. (A switching signal is not really "old school" except in how it's created... nowadays, we see these kinds of signals created electronically, varying the duty cycle (the on/off ratio) to create pulse width modulation controls, etc.)

Adding a new electronic regulator is a fabulous idea, and results in renewed gauge reliability for probably another lifetime worth of driving. Here's a pictorial article that Daryl wrote about his 64 Valiant...
http://earlycuda.org/tech/gauge-convert.htm

Note that this type of surgery is complicated only on the older style, when the regulator is actually internal to the gauge. The newer style regulator is a separate little external metal canister that plugs into the back of the circuit board-- it's comparatively quite easy to perform surgery on that one. Here is a list of internal/external voltage regulator applications.

Those mechanical buzzer units have amazing longevity, but eventually any of them will die-- or partially die, or change behavior based on different temperatures or phase of the moon or what you ate for breakfast... which is the reason I wouldn't recommend trying to run two regulators in parallel. If you're going to go to the trouble of incorporating a reliable regulator to give you years of driving pleasure, it's certainly not a lot of extra work to disconnect the old one. The gauge will be much more predictable when it has only one (rock solid) source voltage.

- Erik

64 Valiant, 170 3-spd
82 Volvo wagon, 5.0 5-spd