[blewbyu]

In trying to find the problem with a 1970 GTO fuel gauge I have replaced the tank sending unit. I'm sure that is not the problem. Under the dash I am having trouble following the sending wire. I know it goes to the printed circuit board behind the gauges. Does any one have a drawing of this board? Some way to follow the traces? A close up pic would even help. The service manual just shows the sending unit going to the board.

I am working under the dash without the cluster out of the car that's what makes it a little difficult. Any help would be appreciated,

Thanks in advance,
Dave

["QUICK-SILVER"]

Can't say myself but there's only three wires behind the guage that make it work; ign power, ground, and sending unit. A continuity/ohm test from cluster plug to guage lugs should tell you which is which.

If you'll post what kind of problem you're having someone may be able to help.

[blewbyu]

The fuel gauge reads 1/2 full when ignition is in run, but the engine isn't started. As soon as the engine is started the gauge goes past full ( to the right). I won't get a chance to work on the problem untill next week because of work. Is the plug to the printed curcuit board for the gauges at the bottom of the cluster? How hard is it to get to it? Are there any other connections under the carpet?

Dave

["QUICK-SILVER"]

for most unusual fuel guage problem

Originally Posted by blewbyu

The fuel gauge reads 1/2 full when ignition is in run, but the engine isn't started. As soon as the engine is started the gauge goes past full ( to the right). I won't get a chance to work on the problem untill next week because of work. Is the plug to the printed curcuit board for the gauges at the bottom of the cluster? How hard is it to get to it? Are there any other connections under the carpet?

The plug-in is in the back of the cluster. Several here have spoke about repairing clusters in car, but not me. Not in many years anyhow. I'm no longer able to crawl up in a dash like a snake. The only connections related to the cluster and fuel guage are the front and rear flat wire harness connectors. The flat wire body harness is what runs from front floor board to trunk.

Your symptoms are most unusual to say the least. If you're willing to answer some questions and do some ohm meter testing we MIGHT be able to save doing a bunch of unnecessary work.

Need to know if there's been any modifications to the car that has to do with wiring. Like standard to rally cluster, voltage regulator from external to internal, and most any other thing related to dash, charging system, or wiring.
Does the fuel guage respond at all to different fuel levels.
Auto to manual transmission or vice versa.
Any other guages or dash lights acting up or not working.
Electric fuel pump added.
Stereo equipment in the trunk.

First test I'd like you to do: un-plug alternator, crank car, and see what fuel guage does. If it acts normal, doesn't move up, hook alternator back up and test charging system voltage.

I've got fuel guage in hand and can replicate any malfunction EXCEPT what yours is doing.
This one might get tricky.

[blewbyu]

I have ohmed out from the cluster plug under the dash to the gas tank and I read the ohms fine (30.6 ohms) without the gauge cluster board hooked up. By the drawings in the manual one side of the fuel gauge has 12v on it and the other side goes to the tank float for a resistance and in this case it's 30.6 ohms. This is the way I'm thinking at least. Correct me if I'm wrong with the way I'm thinking. I'll try disconnecting the alternator tomorrow and write back. I just don't see why this would change anything?

I would believe that I have a problem with the circiut board on the back of the cluster. Maybe a short or open circuit. Can you please explain what the alternator disconnected would do to alleviate the fuel gauge problem?

Thanks Quick_Silver for your help,
Dave

[blewbyu]

I just remembered to answer that the 70 GTO is all stock. The wiring is in nice condition and it has an automatic transmission. There are no modifications that I know of. I have just pulled this car out of a 10 year hibernation and I'm getting it ready for the street.

Any help is always appreciated,
Dave

["QUICK-SILVER"]

Your testing sounds right. If the guage was working, you should have 1/3rd tank.
An open on the sender wire or lost ground on tank is what normally sends the fuel hand full off the scale.
Losing power or ground at the guage doesn't make the guage go full. Something could be wrong with the printed circuit, it's the difference between KOEO (key on engine off) and KOER (key on engine running) that doesn't make sense.
My question is what changes when it's running, the alternator putting out is the only thing thats changed. Just possibly higher than normal voltage is messing with the guage.
Or maybe the gen light wire is shorted into fuel guage wires on the back of the cluster.

All the test is for is ruleing out things that change with engine running. If nothing else is acting up like oil and temp, charging system is about all that's left.

The guage does have a voltage regulator on the back, probably/should be 45 ohms between (+) post and sending unit post. A short there could possibly throw the guage. Any added voltage, beyond guage regulator, could be a possibillity.

My technique may seem odd to you. deal is I can't out wrench them any more so I have to out think them. Can't see it and dont know all the details, so have to try and out think it best I can. It's one of the reasons I play 20 questions, the person asking usually ends up telling me what is wrong.

[blewbyu]

I thank you Quick-Silver for asking these questions. It is making me think a little more in a different direction. I know that this is a simple circuit for the fuel gauge.

I work a night shift, so that's why I have to test the disconnected alternator tomorrow afternoon. I'll let you know what I find.

Thanks Again,
Dave

[blewbyu]

Okay, I disconnected the alternator. I turned the ignition to run and the gauge went to about the 1/3 mark. It looks to be working right. When I went to the start position the fuel gauge went to full as the starter was cranking.

Does this mean that the voltage regulator for the gauges has a problem and probably not regulating the voltage down to 12v? Is this voltage regulator behind or in the gauge?

Dave

["QUICK-SILVER"]

Originally Posted by blewbyu

When I went to the start position the fuel gauge went to full as the starter was cranking.

Does this mean that the voltage regulator for the gauges has a problem and probably not regulating the voltage down to 12v? Is this voltage regulator behind or in the gauge?

Details? Guage went to full while cranking with aternator un-plugged, then what? engine start, guage stay on/past full with engine running and alternator not charging?

Regulator's are normally on the back of the guage, between guage and printed circuit.
IF there was no voltage increase, the regulator shouldn't be the problem. Voltage should've been the same or less going from KOEO to KOERunning.

Guage pegging while cranking, sounds like a BAD ground problem. Battery to block, block to firewall, etc.. All need checked with the ohm meter, metal to metal, beside cable ends, for little to no resistance. None better than a little.

Fuel sender wire is probably the weakest current flow on the car. With a bad ground, any current flow stronger could mess up the guage reading.

Do some checking and post back.

[blewbyu]

I never started the engine. I just had the engine crank until the gauge went full with the alternator disconneted. I would believe that my grounds are good since everything else works including the stock radio. I will check the grounds, because I have had some weird stuff happen on other cars because of ground.

Is there a voltage regulator on the back of the fuel gauge? Because the battery puts out about 12v and the alternator puts out about 14.5v? I have read up that Ford and Dodge use them.

Dave

["QUICK-SILVER"]

Lots of things, like guages, act strange with the starter engaged/cranking. Because of the strong high amp current flow before it gets back to the battery. Current flow doesn't stop at the starter. Ampers are used up by the starter but the current continues through the block and ground cables back to the negative post on the battery.
-------------------------------------
As mentioned earlier, yes there is a regultor (45 ohm resistor) on the back of the fuel guage. It should be BETWEEN the guage and the printed circuit. It's a flat rectangle shaped strip with fine wire wrapped around it. It's bolted down with the (+) battery lug and the sender lug on the back of the guage.
------------------------------------
You've got to crank/start the engine with the alternator unplugged to rule out a high voltage problem or possible short in the printed circuit from the alt/gen light.
-------------------------------
Once upon a time little story.(True though)
Lots of cars that had no signs or symptoms of a bad ground or electrical problem because everything was good and worked like it was suppose to. These cars all of a sudden had shifter cables, throttle cables, etc., sticking for no apparent reason. Once inspected it was found they had MELTED because of bad ground wires elsewhere on the car.
Moral to the story; just because something works doesn't mean everything is OK.
----------------------------
You've got to be more open minded about this car and problems it may have. It's old and who knows what other people have done to it, and don't rule out critters chewing on wires for the mineral content. We all want things to be good but you wont KNOW till you actually test something.
---------------
FWIW something told me you hadn't started the engine to verify alternator charging (our only known varible) wasn't effecting the fuel guage.

And don't leave out details.

[blewbyu]

Today I have been in the garage thinking about this problem. First I went to the truck and openned the sending unit plug. I hooked a lead to the side that goes to the gauge to verify that I do have the correct wire. I verified that I have contenuity on this wire using the ohm meter.

I cleaned the grounds from battery to engine and two connections from engine to body.
Turned the key to start and the gauge went to under 1/2 tank. Then I started the engine and the gauge went to passsed full.

I disconnected the Alternator and started the car. The gauge went to past full with the tank sending unit wire grounded.

I looked in the manual at the wiring connections and I might have stumble on something!
I have only had this car for about 3 weeks and it has had this problem since I got it. Where I ohmed out the tank sending unit correlates to the "Pontiac printed circuit board with auxillary gages" by the service manual. I stuck my head into the dash as far as I could to see the circuit board.
I believe I have the "Tempest and Grand Prix printed circuit with gage". I have the pictures of the circuit board in the service manual. Could this be true? The wiring is different on the plug that goes into the dash. The only gauge that I see move other than the fuel gauge is the oil pressure and I'm not to sure that it is reading correctly. By looking at the ends of the plug they seem the same, but the wires are in different locations on the plug.

I need to find a better test to see if this is correct. I'm still looking at the service manual and wondering.

Dave

[blewbyu]

I have looked again and the plug connections match the receptacle. But the wires look to be in the wrong places? If the diagram in the service manual is correct. I am looking up the colors of the wires now.

Dave

[blewbyu]

The colors of the wires going into the plug are right. But I see saw a trace lifting up on the board when I checked the colors. I still feel the wiring is to blame.

I don't read anything off the lugs that are attached to the circuit board for the fuel gauge, but that might be just anodization on the metal.

I guess the next thing to do is trace all thie wires I can. Does this sound right? Or is there something I am missing?

Dave

[blewbyu]

The dash lights work, it seems like the oil pressure works, So those are probably fine. Tomorrow I am going to have a friend come over and turn on the ignition as I'm under the dash reading voltages. Does this sound okay? I know the the tank sending unit wire is in the wrong spot. I just wish I could get to the plug a little easier.

Dave

[blewbyu]

After looking at the pictrue of the circuit board in the book, I think that I have the wiring for the Tempest without the auxiliary Gauges. The I have the gauges and circuit board on the car. By looking at the electrical schmatics only two wires need to be moved. The first is fuel gauge sending wire (30) and switch it with the generator telltale waring (25). These might be the only wires that need to be switched.

I'll have to check this out tomorrow. Wire 25 and 30 are both tan or brn in color.

I see the light at the end of the tunnel! Hopefully

Dave

[blewbyu]

Between the circuit board without the aux. and the one with aux. you have 2 wires switched by looking in the service manual at the board connectrions. But the service manual shows on the schematic that there wired the same. Which is right? It's telling me two different things on Page 12-11 and a few pages into 12-65 when you look at these boards.

It looks like there is a mistake in the schematics?

Dave

["QUICK-SILVER"]

I do not have a service manual to look at, so can't help there.
Cluster plug-in wire locations is different between the possible cluster/guage packages for that car, and guage swaps are quite common.

Any new news?

[blewbyu]

The problem with the fuel gauge has been solved! My suspicion was right. The wiring harness installed was for a non gauge Tempest. I have a GTO with the auxiliary gauges. The only difference between the two was the fuel gauge and the generator telltale in switch position. It took quite a bit of time switching these wires from under the dash without pulling the steering wheel.

I will have to remove the circuit board someday when I have the time and repair the high beam indicator trace. But the fuel gauge is working. The new gas tank float is stuck, but I still have the old one. I have proven that the gauge works with no problems.

Thank You for all the help Quick_Silver,
Dave