OK. Here is my plea....I knew what I was getting in to when I purchased my 1970 Corvette Convertible. I understood that it was a 53 year old car and that it would need some repairs. There are several signs that the PO had done some things....spliced wires, cut wires, extra wires, and sometimes what appears to be extra wires that have since been cut. The outside, to me is great looking!! I just need the underneath to be right and that is how this all started. Here are my 2 issues that I cannot get right no matter how many articles I read, how many threads I read, how many videos I watch, it still will not work right. It has reached the point where it is no longer fun, mostly because I cannot get to a point where I can step back and say "job well done!" It just leads to more frustration and more of me not understanding. I really only have 2 issues that I can not let go of. It is not even like these 2 things will keep me from driving it, it just annoys me and deep inside I can't silence the voice that says it must be fixed correctly! Here is what I have and if any of you can help.....It is greatly appreciated.

The 2 main problems at this point are my fuel gauge and my temperature gauge.

First, for the fuel gauge. I have power to the sending unit. The sending unit has the correct amount of ohms resistance when tested with multimeter (black on ground and red on power to sending unit), The gauge itself has power getting to it. So, my thoughts were that it was the resistor (it wasn't...it is getting almost 90 ohms when tested) on the back of the gauge or maybe the gauge itself. So, I purchased a new gauge that doesn't need an external resistor. Fuel gauge still on E and does not budge. It looked like tonight that it might have moved 1/16 of an inch or so from way below E to E but that was it.

The other headache is my temperature sending unit. I have run every test that I can find online and something doesn't add up to me. I checked the resistor and it checked out (90 ohms). I disconnected the wire to the sending unit and grounded it out and the gauge shot over to a point way to the right. Reconnected the wiring to the sending unit, turned on the key and it went even further to the right. The Wilcox video says that this means "you have lost your ground". I do not wish to sound ignorant but not sure what that means exactly. It was previously stuck on 100, but through testing I found that it was missing the fiber washer on the 3 o'clock post. I could not find a fiber washer, is it bad if I used a nylon washer? Again, after watching Wilcox and many other videos and articles, it does not appear to be reacting the way it should.

Any ideas to any of these issues? I am on the brink of just saying screw it! And getting the whole wiring harness replaced. At least that should stop some of these issues that I think I am having due to some alternative wiring practices. I don't want to pay for a wiring harness because that is a lot of money that could be spent elsewhere on the car, but at least I could stop chasing these goblins!

Thank you for any help, advice, etc.

 

You said "I lost my ground" and maybe you did because the gauge cluster has to be grounded.



A new harness is not the answer, let's just fix what needs to be fixed.

 

That is a great picture!! I appreciate that. That is exactly what I have except instead of a fiber washer I used nylon.

I also added an external ground wire (extra wire) and put it under one of the screws that holds the back plate to the bezel and connected the other end to part of the metal under the dash. This did not change anything.

Thank you!

 

Note the ground stud on the left of the gauge.


How the factory ran the ground wire.



You did a good job of testing, let's see who else has a suggestion.

 

Have you run a Temporary Wire from the heat sender to the gauge? You want to ensure that the original wire (in the harness) from the sender to the gauge is not damaged, ie. grounded, high resistance. I am not very well versed on gas gauge issues, but again, it simple to run a nice 16/18 gauge Temporary Wire from the float pickup to the gauge. I have seen where the 50 year old cables are corroded inside the old hard plastic covering. Don’t get me started on trying to solder these old wires, they must be hit with emery cloth to remove the corrosion, then cleaned with mineral spirits. Good Luck. Oh, have you read about all the issues with New Restoration Gauges?

 

A simple "Resistance Box" helps make the troubleshooting a lot easier when working with gauges. Just dial in the value and see what the gauge is doing or should be doing. Also good for bench testing gauges.

 

Great photos of the back of the dash......still love this forum after 11 years......I have a 72' apart right now for a dead short and these always help......
Thanks guys

Jebby

 

Dumb question. Have you looked at the float in the tank to see if it still floats? Can you carefully move it with a wire coat hanger? Jerry

 

Is the temperature sending unit original that you can tell? Has it ever been out? It should not have any nylon teflon wrap or other insulation on the body. Just a thought as one more thing I have heard of that confounds diagnosis.

 

Not a dumb question at all @Tampa Jerry That is exactly what ran across my mind last night! I started thinking what if I am doing all of these troubleshooting steps and the float or something inside is broken!! I am unable to see the float inside the tank to even try and get a hanger on it.

 

Hey @ignatz , the sending unit is not original. The PO decided he did not need a sending unit and removed it and put a plug in its place. I have replaced it with one from Corvette Central and another from Mid America due to conflicting votes of confidence on aftermarket sending units.

 

Ordered one today, @Mr D. !! I will give this a go tomorrow when it arrives and I get off work! Thank you!

 

I just did this on my 73 yesterday. I took a coat hanger and bent an "L" at the end and fished around in the tank for the float. With my DMM connected to the sending unit and setting next to the fuel tank opening I was able to fish around until I saw O Ohms in the down position and 90 Ohms in the up position. Took a few tries but I was able to at least see 0 & 90 telling me the sender is good. The Resistance Box worked great; I was able to dial in 22.5, 45, 67.5 and 90 and the fuel gauge readings were 1/4, 1/2. 3/4 & F. You need to disconnect the Black & Tan wire at the tank sender and connect them to the box.

 

Willcox used to sell an adjustable resistor that replaced the 90 ohm one on the temp gauge. They sell now on ebay amazon and corvette america or topflight....cant remember which... if you get your motor hot and get the temp with a infrared gun you can dial the gauge in to get it accurate with mismatched senders.

As for the gas gauge since you are positive the cluster is grounded i would ohm out the wire. If its an aftermarket gauge you can buy a calibration box that goes inline to set up any sender to any meter...they work

 

Has the temp sender unit been replaced lately?

Another type of "loss of GRD" is on the other end from the dash.
The temp sending unit whether it's mounted in the Intake or head has to have an excellent GRD. That is done through the threads of the unit.
Some people apply too much thread sealant tape or too much thread sealant compound.
You only need to cover the last few threads of the sender so as to keep the coolant from weeping.

 

Check your grounds. Check the main birdcage to frame ground. Also make sure the birdcage to wiring harness ground is AOK.

Your 70 might be different but the ground for the dash wiring and everything terminated on a the drivers side A pillar with a 1/4 sheet metal screw. There was no way on my brothers 68 it was staying tight. We drilled out the spot and installed a 1/4” rivnut to ground everything to.

We also ended up installing another birdcage to frame ground up near where the front most body mount is located.

Checking for continuity in a circuit is a good preliminary step, but just because there is continuity doesn’t mean it can hold an electrical load.

 

Update: Man!! I really love this forum!! Thank you all so much!! So many ideas....I WILL test them all until I find one that works. I decided to take some advice and work on one problem at a time and since everything I heard was that the sending units for the coolant are not reliable, I decided to start with the fuel gauge.

So, I saw a video about bench testing the fuel gauge. So, I made really long jumper wires and hooked them up from battery to fuel gauge directly to the posts as directed. This was supposed to make the gauge sweep towards full (clockwise) as there is no resistance. I FINALLY got the gauge to move. However, the video I watched said positive lead to 12 volt post on fuel gauge, which for ours it is the bottom post according to the pic above. So, I then put the ground on the top post. The gauge ran backwards (counterclockwise). If I switched it, and put the positive lead on top post and negative on bottom it moved clockwise. While I am happy that the gauge is moving, it seems to be moving in the wrong direction. I then hooked the gauge up to the connector from wiring harness and it ran backwards all the way down to E.

I then decided to test the sending unit and followed the directions I found on one of the Wilcox videos. These tests did not do exactly what they were supposed to but everything kept making me think ground. So, now I am going to ask a dumb question and I am not sure if it has anything to do with all of this or not, but. In the pic above of the gauges. The post located at 9o'clock has only a nut on it and no labels for wires. Mine is missing the nut and always has been since I got the car. For some reason I was messing with a ground jumper wire trying to make sure that the bezel indeed had a good ground. I eventually connected the ground wire to this post and everything worked as it should. Even went to the back and moved the float and this made the gauge move with it. Is there supposed to be a wire on this post or does the nut that is on it (that mine is missing) serve a purpose of helping with a connection? I put a nut on it and it did not do anything. When key is turned on, gauge moves to E. Put a ground on that post and it moves to the correct location. Connected ground wire to several other places on bezel and it did not do anything except when placed on that specific post.

Just thought I would give an update, ask about this, and mainly say thank you all so much!!

 

Hey @HeadsU.P. , yes the sender was replaced recently. The PO had a solid plug in the place where the sender goes. The sending unit wire was then spliced to some random wire that was added to the engine bay. My guess was that it was a ground to bypass the sending unit, keeping the thermostat open?

I put in the sending unit. I did not use any tape or sealant. I reconnected the wire as it was supposed to be and this has led me to here.

 

Good job of troubleshooting.
In the picture I posted if you're saying you are missing a nut of the 9 o'clock lug then install a nut because that will ground the gauge to the tin can that holds it.
Borrow a nut from another gauge.

 

Well, Fuel gauge is fixed!! Now working on the temperature sending unit. I am going to have to take a break on this one. I have the problem narrowed down I think. However, when I cranked up the car to see if the temp gauge would actually work, I noticed an oil leak. I just changed the oil. It has not been changed in some time (car is new to me), the oil filter was made by Penske for Kmart Industries. I am not sure about where you all live but I think the last Kmart in Texas closed almost 10 years ago. The oil was very black and pretty thick. Now that it is clean oil, when I cranked it oil leaked in several places. So, I am going to have to deal with this first!

I took notes and I have all of your tips for the temp sending unit, I really appreciate all of your help and advice!!!