Lark327

I posted a while ago about the gas gauge in my 1972 giving erratic readings. Well I replaced the sending unit in the tank with a new one and now it just stays in one place (about 1/8 tank). I purchased this car about 6 months ago and have rarely driven it. But now that I am paying attention to the gauges, I notice the following:
1) The voltage gauge just stays in the middle
2) The oil pressure gauge stays at 35 psi when the car is running regardless of rpm
3) the clock works fine.

The car is 100% stock.

Could this be an instrument cluster ground problem? If so can someone tell me exactly here to find the ground? I've read it is on the drivers door post, but I don't know where, or if I have to remove trim to find it. Thanks!

MelWff

remove the wire connected to the sender which provides the signal and ground it. If the gauge moves you have a bad sender.
your car should have an amp gauge does it stay in the middle even after starting the car?
does the oil pressure gauge go to zero when you turn it off?
what weight oil are you using?
how many miles on the engine?

7T1vette

You don't have a "voltmeter"; you have an ammeter. A voltmeter reads the voltage level in the main power line; the ammeter reads how much current 'charge' or 'drain' there is in the main power line.

When the engine is running and the charging circuit is working properly, the ammeter needle SHOULD be at ZERO on the scale.

I suggest that you Google some info on how to read/use an auto ammeter. It gives you lots of good info...if you know how to read it.

Lark327

Sorry for calling it volts. It is an ammeter and it stays zeroed. The oil pressure gauge goes to zero when the car is off. It has 60,000 miles and I use 10w 40 regular oil not synthetic. The ground at the tank is good

MelWff

i was not referring to the ground at the tank. I asked you to remove the wire that plugs into the sender coming from the gauge, not the sender ground wire, and ground the signal wire to tell if the sender is working or the gauge is not working.

Lark327

Peterbuilt

Yes!
With the key 'ON' unplug the wire on the sender.
The gauge should now read FULL.
Ground the sender wire and the gauge should go to EMPTY.

DUB

Was this a typo??

If the engine is running and the alternator is charging the needle should be on the '+' side of the gauge.

If you turn the engine OFF but the ignition key is still ON and pull your headlight switch..the ammeter needle should go to the '-' side of the gauge.

So with the engine running and the needle being at ZERO...does not make sense due to the ammeter shows the charging circuit. Or at least darn near all of the Corvettes with an ammeter show to be on the '+' side when the engine is running.

DUB

7T1vette

After starting the engine, the ammeter WILL show the system charging (needle in the + AMPS range). However, once the battery reaches full charge, the ammeter needle will be at or nearly at ZERO. The rate at which the needle returns from 'charging' to 'zero' after an engine start is indicative of battery condition (faster is better).

DUB

I guess that explains it.

I never drive a customers car so long that the ammeter does as you have described.

DUB

0Willcox Corvette

If the gauge is staying at 1/8th tank, I'd suspect there is an issue with the gauge. As you can see from the picture below, the different scenarios that can be wrong with either a gauge or a sending unit are listed and stuck on 1/8th of a tank isn't one of them.

Do as mentioned above, pull the ohms wire off the sender and don't let it touch anything. The gauge should go to full as shown in the picture below (missing ohms). Then ground it out and it should like the upper right picture (grounded ohms). If the gauge doesn't respond this way the issue is in the dash. If the gauge responds as prescribed above, then you have a sending unit issue.

Here is a manual that will help you figure out your issue.
http://repairs.willcoxcorvette.com/w...it-testing.pdf





Willcox

Lark327

OK been looking at this over the Holidays. Still having issue with the fuel gauge. As noted earlier I replaced the sending unit with a new one (from Zip Corvette). Gauge still doesn't work. At different times it has been at about 1/8 full or a little over 1/4. Right now it is sitting at about 1/8 full. I did not test the ohms on the new one before installing it, but now I have tested it in the car using the method described by Wilcox and the reading is 0. The multi meter reads 1 until I touch the leads to the ground and the sender wire the it goes to 0. The ground wire to unit is tested and is good. So I'm assuming I got a bad new sender? The old sender is out of the car and when I tested it the same way on my work bench I got the same result, a "0" reading regardless of where I moved the float arm to. Now I am questioning my multi meter, but it would make sense if the old sender was bad and I got a new sender that is bad. Any thoughts? I want to settle this sender issue before tearing into anything any further. Thanks.

jhudec

Did you test the gauge as described in #11 by Willcox?

Lark327

OK. I warned you guys I am not very good at electric. Big DUH on my part. I had the ohms set at 200K instead of 200 so of course the readings were zero. First I tested the old sender which tested fine. Then I tested the new one in the car which also tests fine, so now I know what it isn't. I'll move forward from here following the Wilcox instructions. Thanks!

7T1vette

Most likely a 'dirty' ground at the fuel tank. There is a small black ground wire that attaches to the frame. Remove it and check to see if the wire has frayed at the juntion to the terminal; fix it if the wire is weak at that spot. Then clean that connection spot on the frame down to bright metal, clean the terminal on the wire, reattach it, and cover it up with some Permatex Ultra Black RTV (so it won't corrode again). That will solve any grounding problems.

I'd bet that will solve your problems. If not, post again and we'll try to get your problem resolved.

P.S. If the fuel gauge is reading at all, your problem is NOT THE FUEL GAUGE OR THE WIRING IN THE GAUGE CLUSTER AREA.

Lark327

Well, the gauge does have a reading of 1/8, so if that means the problem is not the gauge then it has to be in the wiring since the sender checks out. I had already removed and cleaned that ground wire and I used a test light to check it. I attached one wire to the positive terminal of the battery with other wire of the test light to the terminal of the ground wire that connects to the fuel sending unit and it lit nice and bright. That would indicate a good ground right? My next step is going to be to check the ohms at the connector behind the kick panel.

Lark327

I have another thought/question. The Wilcox troubleshooting says that if the resistor is blown the gauge will read a little above 1/4 tank. It actually says it will only function up to that mark. So does that mean that if it is a blown resistor the gauge will operate between E for empty and a little above 1/4 for full? If so, that would make sense for me since the tank is about 1/2 full and the gauge reads about 1/8.

7T1vette

Possible. I am not a 'fan' of searching electrical faults with a "live" system. Any mistake would short-out a wire or a component....like a calibration resistor. Not saying that happened; but if that resistor has failed, it has seen excessive current and the resistor is capable of handling more than 5 amps.

Lark327

Yes I understand that. But this test was done after the gauge stopped working and I did it by pulling the ground wire off the sending unit and putting the probe to that disconnected wire with the other test light wire to the battery. Isn't this a good way to test for a good ground?

0Willcox Corvette

Just because the sender output test okay in a bench test doesn't mean its grounded properly or there isn't a wire issue...

So.. pull the ohms wire from the car and ground it with the key on, see where the dash unit goes... then let it free hang without touching anything and see where the gauge goes... if it pegs when not touching and goes to empty when grounded, the problem is in the rear of the car.

It's that simple... If you can eliminate the dash unit and all the wires from the rear of the sender forward... you're that much closer to solving the issue.

Willcox