A couple of electrical issues in a '68 - Chevelle Tech
Electrical & Wiring Troubleshooting electrical problems.

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post #1 of 13 (permalink) Old Jun 5th, 20, 9:56 PM Thread Starter
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Question A couple of electrical issues in a '68

Hi all,


My car has me completely frustrated right now, so looking for advice, tips or anything that will keep me from putting a slug into it LOL.



It has started with the fuel gauge. It constantly reads full. I grounded it out, and it goes to empty. With the key on, I put a test light on the connector in the truck, and the bulb barely lights. Because of it going to empty when grounded, my fears are that the float is stuck in the tank. Is there anything else I should check? If I have to drop the tank...does somebody have a writeup on it?



Next problem. My drivers side turn signal bulb burnt out. I figured that while I had it out, I would switch over to LED bulbs. Now, the passenger side turn signal doesn't work with the LED in it. Here is the kicker...it works with a standard bulb. When I put on the hazards, they blink, but the turn signal won't work with the LED. The drivers side, with the LED, works. WTF is going on???


Thanks all!!!


Dave

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post #2 of 13 (permalink) Old Jun 5th, 20, 10:09 PM
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Re: A couple of electrical issues in a '68

Quote:
Originally Posted by Dave Hintz View Post
Hi all,


My car has me completely frustrated right now, so looking for advice, tips or anything that will keep me from putting a slug into it LOL.



It has started with the fuel gauge. It constantly reads full. I grounded it out, and it goes to empty. With the key on, I put a test light on the connector in the truck, and the bulb barely lights. Because of it going to empty when grounded, my fears are that the float is stuck in the tank. Is there anything else I should check? If I have to drop the tank...does somebody have a writeup on it?



Next problem. My drivers side turn signal bulb burnt out. I figured that while I had it out, I would switch over to LED bulbs. Now, the passenger side turn signal doesn't work with the LED in it. Here is the kicker...it works with a standard bulb. When I put on the hazards, they blink, but the turn signal won't work with the LED. The drivers side, with the LED, works. WTF is going on???


Thanks all!!!


Dave
ok, first if you grounded out the wire at the tank and everything works good. It the gauge goes full to empty as you ground or open the wire all is fine. If it does that then the sending unit is defective. No biggie to change. Empty the tank and unbolt the two bolts toward the rear end.Watch out for vents, fuel lines and wires and disconnect as you drop the tank. Second LED lights require much less power so it requires a digital flasher for the blinkers and flashers to work. So change both, one on the fuse box and one attached to the dash brace under the dash if you changed all bulbs minus the headlights.
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post #3 of 13 (permalink) Old Jun 9th, 20, 5:54 PM Thread Starter
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Re: A couple of electrical issues in a '68

Switching to digital flashers didn't work. Still no passenger side LED turn signal.

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post #4 of 13 (permalink) Old Jun 9th, 20, 7:33 PM
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Re: A couple of electrical issues in a '68

https://www.chevelles.com/techref/ftecref18.html

Perhaps this may help.

.
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post #5 of 13 (permalink) Old Jun 9th, 20, 7:40 PM
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Re: A couple of electrical issues in a '68

Also on the flasher and led lights, the don't have enough resistance to make the o.e system work. I know led light have been out a while but there are still too many different brands and variables to give you a solid answer. I'd say just stick with the old style lights unless you want to go down a rabbit hole. It can be done though.

.
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post #6 of 13 (permalink) Old Jun 9th, 20, 8:53 PM
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Re: A couple of electrical issues in a '68

Dave,
If the one side led is working ok then the led's probably are not the problem. I have chased a few over the years and it almost always turns out to be a ground issue. The standard bulb pulls more current than the led and will sometimes overcome a bad ground or resistance somewhere in the circuit. Check all of your power and ground connections for that side.

Bill Koustenis
Advanced Automotive Machine
Waldorf Md


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post #7 of 13 (permalink) Old Jun 11th, 20, 5:16 PM Thread Starter
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Re: A couple of electrical issues in a '68

I finally got them to work. I needed to put in the digital flasher with the ground pig-tail. Once I did that...I had turn signals, nice and steady and crisp. Thanks everybody for your help..its appreciated. Onto the next project.
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post #8 of 13 (permalink) Old Jun 11th, 20, 5:29 PM
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Re: A couple of electrical issues in a '68

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Originally Posted by Dave Hintz View Post
Hi all,

It has started with the fuel gauge.

Dave
Quote:
Originally Posted by Dave Hintz View Post
Onto the next project.
So the fuel gauge still does not work ?.

Be careful of the advice of testing the gauge by not having the gauges sender wire touching anything (ungrounded) and then grounding it. One can get a false positive result.

I too used to do the two tests but then found out what is really needed are additional tests than just those two. The reason is the two tests can get the gauge to read empty or full BUT when hooked to a sender that reads from 90 ohms down to 0 ohms the gauge may not read properly due to a shunt issue.

Eventually I'll put more together but look at this video and he talks about that shunt resistor and if it is not the right value or missing then the gauge will not rear right with a 90 ohm or less signal.


Ideally if you can do like I did testing the gauge in my 68 using resistors.


Jim

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post #9 of 13 (permalink) Old Jun 11th, 20, 6:02 PM
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Re: A couple of electrical issues in a '68

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Originally Posted by Dave Hintz View Post
If I have to drop the tank...does somebody have a writeup on it?
https://www.chevelles.com/forums/14212-post2.html

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post #10 of 13 (permalink) Old Jun 12th, 20, 11:13 AM
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Re: A couple of electrical issues in a '68

So before you go dropping the tank. Look at the ground wire that goes from the sending unit to the frame. It is a single black or brown wire with a ring terminal on the end of it. There should be a 5/16" head sheet metal screw securing it to the frame. If the ground wire has corroded off, the gauge wont work. If you have to drop the tank...:

Get some help because you'll need it if you decide to do what is written below.

1. Remove ALL or as much of the gas as possible. Balancing the tank with gas in it is difficult at best.
2. Disconnect the ground wire.
3. Disconnect any and all vent hoses and fuel hoses.
4. Slowly lower the tank and then remove the sending unit wire from the sending unit. Simply pry it up, GENTELY.
5. Remove the tank from under the car. Using a punch, tap the locking ring on the sending unit counterclockwise. You will have to use all three
tabs, cause the ring will shift in the hole when you tap it.
6. Carefully lift out the sending unit. The rheostat is under the "D" shaped cover. But, before removing the cover, TEST IT! It should read
INFINITE with the float arm all the way up (FULL) and 90 ohms with the arm all the way down (EMPTY). It should read about 45 Ohms at the
half way point. If it reads, "Infinite or open" with the arm up or down, then its bad.

Re-assembly is the opposite from the above. Easy, peezy. Just time consuming.

Chris
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post #11 of 13 (permalink) Old Jun 14th, 20, 4:26 PM
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Re: A couple of electrical issues in a '68

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Originally Posted by Jayhawk500 View Post
6. Carefully lift out the sending unit. The rheostat is under the "D" shaped cover. But, before removing the cover, TEST IT! It should read
INFINITE with the float arm all the way up (FULL) and 90 ohms with the arm all the way down (EMPTY). It should read about 45 Ohms at the
half way point. If it reads, "Infinite or open" with the arm up or down, then its bad.
How you stated with the float arm all the way up should not read infinite or no connection but should read around 90 ohms. Then when the float arm is all the way down like it would be in the tank with no gas then the resistance should be right around 0 ohms.


If you have the sending unit out of the tank and positioned like it would be in the tank, then when the float is all the way down, then the resistance should read 0 ohms. This would be the position if the tank was empty and the float and arm at the bottom of the tank. If the float arm then is then lifted 1/4 of the way up over it's travel full travel, then the resistance should read about 20-25 ohm. If it is then lifted halfway up, then the resistance should be around 45 ohms, and then when about 3/4 up, around 65-70 ohms, and then when fully up should be around 90 ohms.

Below is an album of drawings I did back in 2009 showing the fuel gauge operation with matched parts for the 0-90 ohm system as well as the 0-30 ohm system and then what happens if there is a mismatch or lack up ground off of the sending unit.

https://www.flickr.com/photos/121766...57647365261351

Jim

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post #12 of 13 (permalink) Old Jun 14th, 20, 6:14 PM
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Re: A couple of electrical issues in a '68

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Originally Posted by Jim Streib View Post
How you stated with the float arm all the way up should not read infinite or no connection but should read around 90 ohms. Then when the float arm is all the way down like it would be in the tank with no gas then the resistance should be right around 0 ohms.


If you have the sending unit out of the tank and positioned like it would be in the tank, then when the float is all the way down, then the resistance should read 0 ohms. This would be the position if the tank was empty and the float and arm at the bottom of the tank. If the float arm then is then lifted 1/4 of the way up over it's travel full travel, then the resistance should read about 20-25 ohm. If it is then lifted halfway up, then the resistance should be around 45 ohms, and then when about 3/4 up, around 65-70 ohms, and then when fully up should be around 90 ohms.

Below is an album of drawings I did back in 2009 showing the fuel gauge operation with matched parts for the 0-90 ohm system as well as the 0-30 ohm system and then what happens if there is a mismatch or lack up ground off of the sending unit.

https://www.flickr.com/photos/121766...57647365261351

Jim
Sorry Jim, I had a 50/50 chance. I knew it was one way or the other. My apologies.

Chris
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post #13 of 13 (permalink) Old Jun 14th, 20, 11:01 PM
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Re: A couple of electrical issues in a '68

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Sorry Jim, I had a 50/50 chance. I knew it was one way or the other. My apologies.
Hey, no apology necessary . We just need to keep things straight and correct should anyone else come across this information later. There is way too much good info out there but also information that is not right.

Jim

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