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Iīm a newbie in this forum though I have been owning and restoring classic cars for the past 10 years. One month ago I bought the jewel of my small collection, a 70 Cutlass S. You can read the story of the car here: https://classicoldsmobile.com/forums...k-hardtop.html
The car is in an exceptional condition for a 47 year car; it drives and runs great. Everything works, even the clock or the A/C, except for a couple of small details I would like to solve to bring it back to its pristine condition.
One of them is the fuel gauge. Most of the time it shows more fuel level than correct. When I picked up the car I filled the tank with 68 liters. Total capacity is 76 liters so it should be close to empty but the gauge showed the tank was half full. When the tank is full the needle goes to the "O" of the "HOT" light, at the right of the gauge. When the tank level is lowering there is a time when the needle fluctuate; sometimes itīs near the "Full" mark and 2 minutes later itīs near the middle mark afterwards it goes back near the "Full" line again... Last time I filled the tank, the needle was near the middle but I needed 1/3 of the capacity of the tank to get it full.
The car spent more than 2 years with a very low level of gas in the tank. I donīt know if it may have damaged an old sender.
I would like to know your opinions about the origin of this malfunction. I think itīs the sender but maybe some of you have similar experiences or know about some typical problem on these cars before I take the tank down and change the sender... Not a pleasant job
By the moment I control how many kilometers I make since the last time I filled the tank, refilling when I make 260 Km / 163 miles max... And I have a 5 liter gas can in the trunk
Here you can see where is the needle when the tank is full. Instruments in Kilometers, as this is an "export" version
Thank you in advance, guys!
Last edited by Sportcoupe; May 10, 2017 at 05:01 PM.
Unfortunately, GM fuel gauges tend to be inaccurate in this way, though your gauge sounds like it is worse than most.
It is common for them to go well above F when full, and for the tank to be about ž full when it reads ―, and sometimes to run out of gas before the gauge reads E.
I would recommend cleaning all of the connections along the fuel gauge circuit, including the snap-on connection to the fuel sender on top of the fuel tank, and the ground from the fuel tank, as this might clear up the unsteadiness and wandering readings.
Good luck!
- Eric
Last edited by MDchanic; May 10, 2017 at 06:59 PM.
The picture of the needle looks like the typical loose connection / loose ground at the sending unit. Mine does that sometimes and the needle points to the "O" but if I reach under the bumper and wiggle the brown wire to the sending unit the gauge then reads correctly.
Here's a troubleshooting tip: When the gauge is pointing at the HOT light, unplug the brown wire at the connector in the trunk and ground the gauge side of the wire. If the needle goes to EMPTY this indicates there is an open connection at the tank sending unit.
I had this issue on my 70 Supreme and it was a loose connection at the sending unit. I dropped the tank and discovered that there's a push-on bullet style connector on the wire that goes onto a stud on the sending unit. I crimped the connector slightly so it fit tighter and the gauge worked correctly afterwards. I have the same intermittent issue on the 71 but I'm too lazy to drop the tank just to tighten that connector, so I reach under the bumper and wiggle the wire until the gauge starts working again.
A pegged gas gauge indicates an open circuit. Which could be a missing ground or open in the circuit or an open in the sending unit. As mentioned try cleaning the ground wire where it attaches to the car.
Unplug the sending unit wire by the gas filler neck (Brown wire black connector) and ground the gauge side with the key on. If it goes to E it is one of the above problems. If it doesnt go to E or move then its that wire or the gauge.
Worth dropping the tank and looking at everything in a 47 year old vehicle.
Many thanks to all of you for your info, guys, Iīll check as soon as possible what you say, Kenneth and droldsmorland. Last night I took a look to the brown wire to the tank and wiggled it a little but the needle is still over the Full mark (I only made about 63 miles after last time I filled the tank), in the middle between "Full" and "Hot". I had little time so I did not do the test of grounding the brown wire yet. Iīll do it and see what happens. Some days ago I bought a sender unit for some $29 taking the opportunity I had to order more parts for the car, so when I face this issue Iīll have it at hand if finally this problem comes from what oldcutlass points out.
Iīll keep you posted
Last edited by Sportcoupe; May 12, 2017 at 04:46 AM.
Yes, do come back and close the loop so we know how our advice was and if it fixed the problem. Nice cars by the way 69 Caddi is a sweet ride. A more door 3700 is something we never see over here. Is that a stick shift 318 car or the leaning tower of power 225 slant six??
Nice cars by the way 69 Caddi is a sweet ride. A more door 3700 is something we never see over here. Is that a stick shift 318 car or the leaning tower of power 225 slant six??
Yes, the caddi drives smooth and powerful. Although it had a hard life, It's reliable and its driving is a sweet experience. Sometimes I think how it would be if it was as well maintained and cared as the Cutlass. The only negative point is parking such a big car in a city like Madrid; driving it here is not too difficult.
The 3700GT is an A body designed in Detroit but never made in U.S.A.; only made in Spain and in Argentina (there itīs called Polara and Coronado). In Spain it received a European touch in finish and interior. Its engine is the 225 Slant six with a 2 BBL carb (165 HP SAE) and most of them had a 4 speed floorshift (A-833) but mine has the Torqueflite A-904 automatic tranny... A little less performance but smoother drive.
Yeah, itīs quite similar, specially the front half of the car. Itīs the same feeling I had first time I saw the 67-70 Rebel. After all, both were designed in the same period. That Rebel on the pic is a 69. 3700GT was introduced in 1971. Argentinian Dodge Polara was introduced in 1968.
Last edited by Sportcoupe; May 12, 2017 at 10:32 AM.
As I had already bought a new sender and in addition I had to do some more fixings to my Cutlass, I went with the car to a friendīs home. He has a splendid garage with a good equipment... Even elevator as he offered his help to solve the little problems of this car. In his opinion most probably the cause was a bad sender so he put the car on the elevator and lifted it some inches, just the necessary height to work under the car comfortably and lean the tank on the floor. Before changing the sender unit he tested the new one connecting ground and wire and it worked ok. The wires and connections to the old one seemed ok. The sender was changed and it now provides a good reading.
The old sender box connected to the float seemed to have lived much better days, as you can see on the pic below.
I thought that under 1/2 of the tank level the sender marked well but when I arrived at my friendīs home the tank had only 3 liters=0.8 gallons while the gauge read about 10-15 liters = 2.5 - 4 gallons... So they didnīt had to empty the tank
Thank you all for your help and support!
Tank lying on the floor
Old sender unit
Last edited by Sportcoupe; Jul 16, 2017 at 10:45 AM.
The case of the rheostat seems unusually corroded, which does help explain the problem, but note also that in many cases the inner parts can be cleaned up and the rheostat will work again. Obviously, replacing the sender, as you did, is the best choice, but for those in a pinch, there is sometimes another option.
The case of the rheostat seems unusually corroded, which does help explain the problem, but note also that in many cases the inner parts can be cleaned up and the rheostat will work again. Obviously, replacing the sender, as you did, is the best choice, but for those in a pinch, there is sometimes another option.
Glad you got it fixed!
- Eric
I think the cause of this corrosion is a tank half empty for long time. The former owner had many classic cars and used to put only the necessary amount of fuel in the tank to move the car from time to time.
Surely it could have been repaired. In fact, in one of my other cars, the '73 Dodge 3700GT, itīs the only option. fuel gauge is Jaeger, an European make, and the Dart sender does not work with it (though the tank is the same), so you can only repair that sender made in Spain and never refabricated.
Nevertheless, the old sender is with me... Maybe some day I open it and try to repair it.
Last edited by Sportcoupe; Jul 17, 2017 at 03:12 PM.
afterwards it goes back near the "Full" line again... Last time I filled the tank, the needle was near the middle but I needed 1/3 of the capacity of the tank to get it full.
while the gauge read about 10-15 liters = 2.5 - 4 gallons... So they didnīt had to empty the tank 
