[AdamIsAdam]

Yes, I checked as instructed. I assume I should have 12 volts that way?

I'll check the fuses, the wires supplying the gauge, and also the wires under the seats since I recently had the seats out.

[AdamIsAdam]

So if I run a jumper wire from the positive of the battery to the sending unit, I should get a correct ohms (90 for my full tank) reading off the negative side of the sending unit, correct?

Sent from my EVO using Tapatalk

[Pepi]

Originally Posted by AdamIsAdam

So if I run a jumper wire from the positive of the battery to the sending unit, I should get a correct ohms (90 for my full tank) reading off the negative side of the sending unit, correct?

Sent from my EVO using Tapatalk

I personally wouldn't run power right from the battery to the sending unit. Like mentioned before, you need a print, to find out what supplies the power to the gauge, and trace it out. Maybe when you had the rear seats out your flat wire harness got stepped on or disturbed somehow.

[AdamIsAdam]

Originally Posted by Pepi

I personally wouldn't run power right from the battery to the sending unit. Like mentioned before, you need a print, to find out what supplies the power to the gauge, and trace it out. Maybe when you had the rear seats out your flat wire harness got stepped on or disturbed somehow.

That's exactly what I was thinking, which is why I was considering a jumper, to by-pass the flat wire. I'll do some trouble shooting over the next few days and see what I come up with.

Thanks for your input, everyone.

["QUICK-SILVER"]

Originally Posted by AdamIsAdam

So if I run a jumper wire from the positive of the battery to the sending unit, I should get a correct ohms (90 for my full tank) reading off the negative side of the sending unit, correct?

Sent from my EVO using Tapatalk

:

:jump ing:

NO NEVER EVER HOOK B+ TO SENDING UNIT WIRE.
The thinnest wire that would melt first is the rubbing block that the float arm rubs up and down own to vary ohm readings.
Might be like sticking one of those little wire lighters in a solid rocket motor.

Need to talk about your meter and how to use it.

Please do not work on your car untill someone talks to you.

Sending pm and e-mail if possible

[AdamIsAdam]

Thanks for the chat. I'll resume safe testing and report back later

[AdamIsAdam]

I'm getting 36 ohms and the tank has about 18 gallons out if 20. I'm seeing .8 volts the lead wire. Both readings were taken with the lead wire disconnected.

I'm going to now try and take volt readings at the gauge or elsewhere.

[Pepi]

If I am not mistaken, doesn't the gauge get the power with the rest of the gauges, and then just passes through basically looking for a ground. Maybe try checking at each connector point.

[AdamIsAdam]

ok, so here's where I'm at. First off, can anyone confirm if the `64 sending unit is a 90 ohm unit or 36? Q-S said early pontiacs were 36ohm, so maybe my 64 LeMans is also.

My tank was filled then I drove home, which should bring it to 18 gallons. I'm getting 36 ohms by disconnecting lead to sending unit and measuring across the unit (touching the lead plug and ground).

Next up is the gauge:
Volt reading into the back of the gauge is 11.8v. volts out is about 2.8v.
Then I pulled the back seat to check the flat wire connections since I had the seat out last week. While the connector from flat to regular wires had some corrosion, the volts were the same before and after the connector. (I cleaned them anyway).

More importantly, when I tried to ground the gauge, right at the gauge this time, and also at the flat wire behind the seat, I can't get it to read EMPTY, ever. It goes to FULL when the key is ON or ACC, and to EMPTY when OFF. But I can't seem to get the gauge to go to E by grounding it.

So does this mean my gauge is the problem?

PS I don't trust my gauge because I get varying readings. I'll buy an auto adjusting gauge soon. this is a sperry multi-metter, digital.

EDIT: I just noticed that Summit lists the 64 Bonneville sending unit as 0-30. I'm beginning to think my gauge is the problem.

[Greg Reid]

It sounds like you may be right. I'm not sure what the voltage should be 'coming out' of the gauge but I would think that it would be pretty close to 10 or 11 volts with 11.8 on the input side.
Again, I'm not sure about it because I haven't measured it. Also, the gauge itself will drop a little something due to internal resistance/impedance but I can't imagine it would drop anywhere near that much.
As for your sending unit, if you have information that some of them are 36 ohms and you are reading 36...that's probably not a coincidence. In fact, I bought one for my '66 once and it looked just like my original but it was somewhere in the neighborhood of 30 or 40 ohms...My old one was 90ohms and I had to return it for the correct one...so, there does seem to be more than one rating in mid sixties time period.

["QUICK-SILVER"]

I do think you need to try another meter. Numbers you're posting are not matching up with logic or what they should be.
Sending unit rheostat range is probably going to be 0-30 ohms or 0-35 ohms. Have seen both listed for sale for the "64" model...along with 0-90 and 10-90. The ones that go up to 90 are listed as fitting "64" to "67". The 30 and 35 are "64" only.

Numbers:
.8 VDC in the rear we'll come back to.
11.8 VDC going to the guage is what's to be expected.
Now then, the guage has three connection points: power supply (B+), ground (B-), and the one that goes to the sending unit.
2.8VDC??? what could that be We've got two points on the guage unaccounted for so far, ground and sending unit wire. Ground would ohm out at 0/zero ohms (ground to ground = 0) and sending unit wire. Now 2.8VDC sounds alright for the sending unit wire, BUT, it checked .8VDC at the back of the car.

So, numbers are not adding up. The voltage drop from front to rear, if it is the 2.8VDC, would definitely be a problem. But you stated that the guage wouldn't drop back to empty even when grounded up front. This sounds like two problems.

Try another meter and retest things. Besides the B+ on the back of the guage there is stud that has a voltage regulator (flat, wire wound, resistor) between it and B+.
Need the B+ reading, opposite end of regulator reading, and sending unit out reading. Sending unit coming out of the guage may be the same as regulator drop reading. And the sending unit out of the guage reading should be the same at the back of the car .

Enough for now
Clay

[AdamIsAdam]

Clay,
I couldn't agree more and have already planned a trip to Home Depot for a new gauge (I have gift certificates there, so I'll buy a nice, auto-adjusting gauge).

Just to be clear, I've got a two wire "harness" type plug at the back of the fuel gauge. I measured both wires. One is clearly the B+ in showing switched power of 11.8 VDC, the other goes to the sending unit, which I got 2.8VDC. I got about the same reading at the junction behind the seat on that wire.

What's troubling is the .8VDC at the gas tank. I examined the wire from the junction to the tank and it looks fine. I was tired and it was late, so I stopped short of putting a jumper wire at that section.

But I feel that I'm not getting accurate readings with my cheap Sperry Multimeter. So I'll buy a better meter and start again.

But, why the heck is my gauge not going to E when I ground the sending unit wire right at the gauge or when I grounded it at the junction behind the seat? Regardless of my readings, why won't it ground to "Empty"?

I'm not totally clear on this:

Besides the B+ on the back of the guage there is stud that has a voltage regulator (flat, wire wound, resistor) between it and B+.
Need the B+ reading, opposite end of regulator reading, and sending unit out reading. Sending unit coming out of the guage may be the same as regulator drop reading. And the sending unit out of the guage reading should be the same at the back of the car .

[Pepi]

Originally Posted by AdamIsAdam

Clay,
I couldn't agree more and have already planned a trip to Home Depot for a new gauge (I have gift certificates there, so I'll buy a nice, auto-adjusting gauge).

Just to be clear, I've got a two wire "harness" type plug at the back of the fuel gauge. I measured both wires. One is clearly the B+ in showing switched power of 11.8 VDC, the other goes to the sending unit, which I got 2.8VDC. I got about the same reading at the junction behind the seat on that wire.

What's troubling is the .8VDC at the gas tank. I examined the wire from the junction to the tank and it looks fine. I was tired and it was late, so I stopped short of putting a jumper wire at that section.

But I feel that I'm not getting accurate readings with my cheap Sperry Multimeter. So I'll buy a better meter and start again.

But, why the heck is my gauge not going to E when I ground the sending unit wire right at the gauge or when I grounded it at the junction behind the seat? Regardless of my readings, why won't it ground to "Empty"?

I'm not totally clear on this:

Are you sure you are getting a good ground for your meter back at the tank? Maybe try to run a wire all the way to the battery neg for your meter so you know you have a good ground, or the same place you are using when you check the junction behind the seat.

[AdamIsAdam]

I will do just that. I suspect you are correct. I cleaned up the grounding wire attachment point on the floor near the gas tank, but who knows if the floor itself is good ground.

[Old Goat 67]

This should help you visualize fuel circuit.
From the 67 shop manual, but the layout is the same.

[AdamIsAdam]

Thanks. I get it. I'm just having trouble getting decent, consistent readings. I do seem to be getting consistent OHM readings at the tank of about 34-36 with a nearly full tank. That seems correct for the 1964 sending unit.

I'm getting about 12v into the gauge. Where it gets fuzzy is the line out of the gauge to the tank. I can't get a good contact on it (while laying on my back, reaching up to the back of the gauge) to ground it out in an attempt to get that gauge to read E. But next attempt I think I'll get a wire on that post, then plug the harness back in on top of it, then ground that wire to get the gauge to read E. (The harness on the gauge is a L shaped plug, so both B+ and the output to the sending unit get plugged in as a pair.)

I bought a new multimeter and was getting very inconsistent volt readings out, but that's because I couldn't get a good probe connection I think. As you can read above, I was getting about 2.8v out before, but it was dropping to around .8v by the tank. If that is in fact the case, my problem should be along that wire.

So if I can run a jumper off the gauge, I will be able to do two things: check gauge by grounding it, and then run that wire directly to the sending unit which should then read accurately.

That's my game plan. I plan on executing it tomorrow evening.

[Old Goat 67]

Don't worry about getting a wire on the back of the gauge just yet, find that Tan wire going to the ribbon cable under the carpet and ground it there. I am assuming there is a ribbon cable on a 64, never worked on one there. If necessary, just disconnect it there and then ground the contact where the Tan wire is. Remember Tan, not Brown. Brown is for Park lights. And, other end of ribbon cable should accessible from inside of trunk, comes up from under back seat, connector there.

Ground it anywhere on that lead going back and the gauge should read empty if it is good. If it doesn't then you have to back up towards the gauge itself, maybe even having to disassemble the dash to get the gauge out. It could be tested when on the bench by duplicating Batt, ground and resistance to ground on the Tan lead connection to prove it's functionality.

If it does operate with that ground on that Tan lead, then you must prove continuity to the rear of the vehicle by either ohming it out front to rear or simply temporarily running a new wire from there to the sending unit for a test of operation.

You could always go by Radio Shack and pick up a couple of resistors to hook up in place of the sender to ground for testing. Say, 35 ohms for full, 20 for half full, ground for empty. I would think you probably have good body grounding if the rear lights work properly, as evident in your other thread.

Charles

[AdamIsAdam]

Right, I tried that and got strang results. See my post #112 above.

I got the same 2.8v at the flat wire-to-harness junction behind the rear seat. BUT, when I grounded that wire the gauge was still reading FULL! Which makes NO sense! That's why I am trying to ground at the gauge itself. It's also when I stopped and decided to buy a better multimeter and try again, but got sidetracked by the tail lights. Too many things at one time.

[Old Goat 67]

That voltage reading is a good way to troubleshoot, but my question is whether that is open circuit or not?

If circuit is open at sender, you should read 12V then. If connected to sender it indicates that sender is flowing current through it.

[Pepi]

Charles, I have never checked or tried, but will he get the full 12v after the gauge with that resistors in it? I can see the volts droping, I don't know by how much. Just trying to learn some stuff along the way also, and you seem very knowledgeable with a lot of this stuff.