more on my fuel gauge... - Team Camaro Tech
Electrical & Wiring Troubleshooting electrical.

Reply
 
LinkBack Thread Tools Display Modes
post #1 of 17 (permalink) Old Aug 11th, 00, 08:06 AM Thread Starter
Senior Tech
 
Join Date: Nov 1999
Location: brandon, ms. usa
Posts: 278
Unhappy

my fuel gauge reads over full...(please see post in troubleshooting). anyway, i hooked the new gauge up and it did the same thing. shots to way past full. tried switching the wires, but still the same thing. also ground the new gauge to a different location. looks like it may be in the wiring or a bad sending unit. any suggestions? would the sending unit send the gauge to the way past full mark?

------------------
To see a pic of my car, click here... pages.prodigy.net/mtbmtb/rf396ss.jpg
mike brown is offline  
Sponsored Links
Advertisement
 
post #2 of 17 (permalink) Old Aug 11th, 00, 09:00 AM
Senior Tech
Carl
 
Join Date: Apr 1999
Location: Newbury Park, CA
Posts: 2,711
Post

How about disconnecting the sending unit wire from the tank and gauge and testing for a short? Not sure where the connection at the tank for a '69 is, but maybe accesable from the license plate?

Perhaps try unplugging the rear wiring harness from the connector plug near the fuse block. Does the guage still bury itself? If so then perhaps there is a short between the plug and the gauge.

I believe all GM fuel gauges operate at 0-90 ohms. If the tank is 1/2 full and the resistance is at one end of the scale, I'd say the sending unit is bad. You will have to disconnect the sending unit at the tank to test. Connect one end of the meter to ground, the other to the sending unit wire. You cannot test with the gauge or other wiring connected. If there is a short between the tank and gauge this will show zero resistance and mess up your readings.

I've never tried doing this test on a sending unit while it was mounted in the car. My gut feeling is that it would be safe if you were using a small digital meter. Better to be safe and confirm instead of having a fireball.

------------------
Click here to see see my car and hear 5-speeds. http://www.geocities.com/casanoc
CarlC is offline  
post #3 of 17 (permalink) Old Aug 11th, 00, 10:55 AM
Senior Tech
Steve
 
mccorry's Avatar
 
Join Date: Apr 1999
Location: Grove City, Ohio, USA
Posts: 1,663
Garage
Wink

A pegged full gauge relates to infinate resistance(bad ground or broken wire) or no ground. I would really put a meter on the wire from the gauge to the plug in the trunk to make sure it has continuity. If it does, then check out the wire from the plug to the sending unit.

------------------
Steve McCorry - Central Ohio Camaro Club
'69 Camaro RS/SS-350 - Daytona Yellow
'95 Camaro Z-28 - Sebring Silver
click here to see my car ---> www.mindspring.com/~mccorry(updated 7/1/00)
mccorry is offline  
 
post #4 of 17 (permalink) Old Aug 11th, 00, 01:19 PM
Senior Tech
Carl
 
Join Date: Apr 1999
Location: Newbury Park, CA
Posts: 2,711
Post

Good point mccory. If the circuit is open then the gauge would bury itself.

What about disconnecting the plug in the trunk and using a 10 cent 50 ohm or so resistor grounded on one end and connected to the plug with a jumper wire on the other? If the wiring from there forward was OK the gauge should read something besides full deflection. If the gauge still buried itself then there is a problem with the wiring from the plug forward.

I bet it's a bad connection somewhere, either in the trunk, dash, or tank plug. Those connector pins will push out and you can't tell until you take a real close look.That's where continuity checkers are a big help.

------------------
Click here to see see my car and hear 5-speeds. http://www.geocities.com/casanoc
CarlC is offline  
post #5 of 17 (permalink) Old Aug 11th, 00, 01:22 PM
Senior Tech
Carl
 
Join Date: Apr 1999
Location: Newbury Park, CA
Posts: 2,711
Post

Gee, I guess running a jumper wire from the sending unit to the gauge would answer a lot of questions. Be sure to disconnect the other sending unit wire from the gauge.


------------------
Click here to see see my car and hear 5-speeds. http://www.geocities.com/casanoc
CarlC is offline  
post #6 of 17 (permalink) Old Aug 11th, 00, 02:57 PM Thread Starter
Senior Tech
 
Join Date: Nov 1999
Location: brandon, ms. usa
Posts: 278
Post

guess i'll have to wait until next weekend to try some of this. if i unplug the wire at the trunk and hook up a long wire from the sending unit to the gauge and it works, then i know i have a wiring problem between the gauge and the trunk. this ? may make me look stupid, but i figured i better ask...what i'm wondering is what exactly are the 3 lugs for? i know the bottom middle is ground, but what do the other 2 lugs do? one comes from the sending unit to the gauge, is the other some sort of power supply and does it have 2 wires attatched to it because the power jumpers over to something else?
i'll check everything for continuity and see what i can find. thanks for the suggestions.
mike brown is offline  
post #7 of 17 (permalink) Old Aug 11th, 00, 06:18 PM
Senior Tech
Carl
 
Join Date: Apr 1999
Location: Newbury Park, CA
Posts: 2,711
Post

Mike,

Yes, run the long wire to troubleshoot the main wire run to the gauge. Disconnect the tan wire from the back of the gauge and replace with the long jumper.

I know almost nothing about how analog meters (or digital) work. My guess is the gauge needs a referance voltage. The differance between the referance (12v) and whatever the sending unit supplies is what the meter reads (deflection of the needle). But I'm probably somewhere in Egypt on this. Shawn may be able to help us on this one.

According to the Console Wiring Diagram:

Looking at the back of the gauge:

The LH lug should have a 20ga tan wire (fuel tank) and a 20ga light-green for the low fuel indicator.

The RH lug should have a 20ga pink for + power and 20ga red for the low fuel indicator. There may be two pink wirescoming from this lug or two wires from the same ring connector because the temp guage is also powered by this wire. This may be what you are seeing.

If you have one of the aftermarket LFI's the lg and r wires may be different colors. You should disconnect them for initial troubleshooting. Add them back in after everything else is solved.

------------------
Click here to see see my car and hear 5-speeds. http://www.geocities.com/casanoc
CarlC is offline  
post #8 of 17 (permalink) Old Aug 11th, 00, 07:35 PM
Senior Tech
Carl
 
Join Date: Apr 1999
Location: Newbury Park, CA
Posts: 2,711
Post

If you have an auto trans there may be three pink wires. The third is for the backup lights. I was looking at a manual trans diagram.

So the pink feeds + power to the fuel gauge, temp gauge, and backup lights on automatic cars.

Sorry for the extra post.

------------------
Click here to see see my car and hear 5-speeds. http://www.geocities.com/casanoc
CarlC is offline  
post #9 of 17 (permalink) Old Aug 12th, 00, 06:26 AM
Moderator
David Pozzi
 
davidpozzi's Avatar
 
Join Date: Nov 1999
Location: Central California, USA
Posts: 10,100
Post

I believe the fuel guage circut is similar to an ohm meter where the 12V goes thru a resistor and steps down the voltage to the sender.

The voltage coming out of the fuel guage going to the sender is probably less than one volt.
On other later gm cars the lead to the sender is sometimes a solid aluminum wire (this was a mid 70's gm fullsize) and subject to corrosion damage, and or increased resistance.

I haven't had to work on my Camaros guages, so I don't know much about trouble shooting them. The factory manual I have says nothing about it.
David

------------------
Check my web page for First Gen Camaro suspension info:
David's Homepage
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer



[This message has been edited by davidpozzi (edited 08-13-2000).]
davidpozzi is offline  
post #10 of 17 (permalink) Old Aug 12th, 00, 06:02 PM
Senior Tech
Scott
 
Join Date: Nov 1999
Location: Arroyo Grande/SLO, ca usa
Posts: 598
Post

The reohstat in the sending unit is going bad. I have had this happen twice. No matter which of my two extra fuel gauges I put in, it still reads 125% when full and 25% when empty. D&R has some on sale for $49.
bonecrusher67conv is offline  
post #11 of 17 (permalink) Old Aug 12th, 00, 06:07 PM Thread Starter
Senior Tech
 
Join Date: Nov 1999
Location: brandon, ms. usa
Posts: 278
Arrow

the gauge just pegs out full and never moves no matter how much gas is in the tank. this makes me think it's the wiring over the sending unit.
mike brown is offline  
post #12 of 17 (permalink) Old Aug 13th, 00, 07:51 AM
Senior Tech
Steve
 
mccorry's Avatar
 
Join Date: Apr 1999
Location: Grove City, Ohio, USA
Posts: 1,663
Garage
Wink

Think about it for a second...the sending unit is no more that a contact (connected to a float) that sweeps over a resistance coil. The guage looks at resistance to ground (through the sending unit) in the range of 0-90 ohms.

A pegged full guage is an indication of either NO ground (infinite resistance) or VERY high (>90 ohms) resistance.

I would first check your wiring from the guage to the trunk. Look at continuity and resistance (should be very low).

IF it checks out, put the meter across the sending unit (brown wire leading to the unit(in the trunk) and a chassis ground). It should read somewhere between 0-90 ohms (maybe a bit higher).

If you have some HUGE resistance, then your contact has worn out in the sending unit and its time to drop the tank. I actually disassembled mine and refurbished the contact. It isn't too hard and much cheaper

Good luck!

------------------
Steve McCorry - Central Ohio Camaro Club
'69 Camaro RS/SS-350 - Daytona Yellow
'95 Camaro Z-28 - Sebring Silver
click here to see my car ---> www.mindspring.com/~mccorry(updated 7/1/00)
mccorry is offline  
post #13 of 17 (permalink) Old Aug 13th, 00, 07:52 PM
Moderator
David Pozzi
 
davidpozzi's Avatar
 
Join Date: Nov 1999
Location: Central California, USA
Posts: 10,100
Post

Mike,
I believe a high reading fuel gauge indicates a short to ground.

The reason I think it's that way is I added a extra fuel tank to my GM motorhome and used a ford tank and sender and the gauge would not go above 1/4 full.

When the tank was half full the gauge read empty.
I found out later the ford sender has more resistance than a GM which is 0 to 90 ohms.

I think using a variable resistor to test the gauge is a good way to troubleshoot it.

Or if you disconnect the tank lead at the gauge, it should read empty.
David

[This message has been edited by davidpozzi (edited 08-13-2000).]
davidpozzi is offline  
post #14 of 17 (permalink) Old Aug 15th, 00, 01:32 PM
Senior Tech
Steve
 
mccorry's Avatar
 
Join Date: Apr 1999
Location: Grove City, Ohio, USA
Posts: 1,663
Garage
Post

David....I may be wrong...but I think you have it reversed. I know that on my '69, without a ground, the gauge pegged full.

Thats why it took so long for me to find the problem...gauge read full ALL the time. Finally ended up being the contact in the sending unit like I mentioned above.

------------------
Steve McCorry - Central Ohio Camaro Club
'69 Camaro RS/SS-350 - Daytona Yellow
'95 Camaro Z-28 - Sebring Silver
click here to see my car ---> www.mindspring.com/~mccorry(updated 7/1/00)
mccorry is offline  
post #15 of 17 (permalink) Old Aug 15th, 00, 07:30 PM
Senior Tech
Scott
 
Join Date: Nov 1999
Location: Arroyo Grande/SLO, ca usa
Posts: 598
Post

New sending unit for a 67 out of the box:
94 ohms at full position
48 ohms at center
0.5 ohms at empty

Zero resistance or short equate to empty. Open circuit (which could be a broken resistor wire in the sending unit) means full+.

Which component is subjected to the nastiest environment? The resistor wire emersed in fuel and is rubbed daily for years, a frame wire exposed to mother nature, or a gauge in your pristine dash. I'll go with door number one.

bonecrusher67conv is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Team Camaro Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address.
NOTE we receive a lot of registrations with bad email addresses. IF you do not receive your confirmation email you will not be able to post. contact support and we will try and help.
Be sure you enter a valid email address and check your spam folder as well.



Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome