I don't have the digital dash but I suspect replacing the dash would not solve the problem. When there are issues with fuel level reading normally all connections should be checked and the sending unit tested after that. I would also be careful in the assumption that you can actually find a replacement sending unit. Your car is old enough that you cannot just "throw" parts at a problem because in some cases those parts will be very hard to find.

Scott

 

Probably the sending unit.
I think maybe the Cadillac King in CA might have the digital sending unit. An analog unit won't work with a digital dash.

Cadillac King, Cars and Parts

Dismantler acessories alante engine electrical electric part filter cadillac used searchable parts database search find series 62 sixty two 70 seventy calais 6e 6400 6900 brougham brogham biarritz coupe convertible

cadillacking.org

.

 

It is likely the sending unit. Unplug the harness at the tank to verify, the gauge should read either “F” or “- -“ with the harness unplugged.

I was recommended this place to have a digital sending unit rebuilt by someone who had it done:

John Wolf & Co. Inc.| Antique Car Gauge Repair in Willoughby, OH

John Wolf & Co. Inc. provides Antique Automobile and Aircraft Instruments in Willoughby, OH. And also Boats, Gas Tank Sending Units, Custom Instrumentation, Temperature Gauges, and much extra. Visit us for more.

antiqueinstrument.com

Here’s my chronicle of fixing this thing. I would have used the above place had it not been for serendipitously finding a sending unit. But they are next to impossible to come by.

GM experimented with digital gauges on a lot of different models in the 80s. This car has a digital cluster and I bought the car in spite of it. I never understood why they mix-matched the center stack fuel/hvac/radio amber with the speedometer green. Anyway, over the past 3 years that I've had it, I've grown to like it, and while I won't say I would have ordered one with it, I do appreciate it. What I did not appreciate was the lack of a fuel gauge! This seems to be a problem that is apparent on cars of this era with digital displays, and I suspect the same inaccuracy is true on the standard analog units but is just harder to notice. The problem usually shows itself when filling up. Instead of displaying "F" in the gallons remaining display, you get " - - " indicating infinite resistance. Then as you begin to burn up fuel, it drops from " - - " to "F" then "18" "17" and so forth, even though you're at least a few gallons ahead of what it is indicating. Again, I think the same is true of analog but it's just not as noticeable if the gauge pegs a little past full (the equivalent of it reading " - - " ) and since there's no real way to reconcile the gallons used display with the gallons you have in the tank, no one realizes there's a problem.

This type of failure points to a fuel tank sending unit problem. No big deal, as sending units are readily available from the aftermarket for the 79-85 Eldorado...if it has an analog dash! As luck would have it all models with a digital gauge option, had a different sending unit in the tank than did the ones with a standard cluster. This is because the standard sending unit displays full earlier than is really true-in other words it maxes out before the gas in the tank has maxed out. This would defeat the purpose of the digital gauge which is to give you I guess a better idea of exactly how much gas is in the tank. The difference between analog/digital sending units on a '79-'85 Eldorado/Seville TBI motor is in the float arm which has different bends in it to more accurately reflect the fuel level in the tank. Both analog and digital are standard 0 <--> ~95 ohm senders. Digital sending units are very hard to come by, and more reading about them can be done on this thread: Fuel sender for 82-85 Eldorado and Seville

Some vendors do offer rebuilt units but after you factor in shipping, tax, and possibly not being able to get your core deposit back, it makes a lot more sense to just clear the trip mileage when you fill up!. Thankfully after stalking ebay for the better part of the last 3 years, I was finally able to grab a new one that wasn't outrageous.

I also bought a fuel pump that had the older style connectors on it-I don't really trust my crimping skills on electrical components to 21 gallons of gas...and even then I still don't get how this thing doesn't blow up from the electrical current in there

I had to burn up as much fuel as I could now to get the tank light enough...I had just filled up the night before I found the sending unit for sale, just my luck! Another thing the HT4100 takes forever to do is burn up fuel…

The tanks on these cars are just like anything GM made, 2 hinged straps. 14mm bolts at the front get removed and the tank begins to pivot down. As I don’t have a lift I found it easier to just take out the 13mm strap hangers and straps altogether to just lower the tank with my jack and plywood. This was done with the rear wheels on ramps.

I disconnected these wires to shut down the fuel pump and cranked the engine for a bit to relieve any pressure that was still in the system.

Then I disconnected these ancient hoses and dropped the tank. They had shrunk so much I was able to rotate them to be able to get to the screws on the clamps. Amazing that they didn't leak any fuel. There was a build sheet on the top of the tank-that makes the 4th one I've found on this car so far.

I had tried to burn up as much gas as I could but I was too timid to run it completely out of gas. Getting the tank down was one thing, but maneuvering it back up would have been a challenge with fuel still in it. I bought this DC transformer years ago and its been great for oddball tests of things. I connected it to the tank and pumped out the remaining fuel (~2.5 gallons) You can see where the build sheet was!

I removed the lock ring with a screwdriver and mallet and pulled the sender

The only difference I could see between the original and replacement senders was the lack of this little round thing on the fuel pump positive wire. Not sure of it's purpose.

I was really happy with how the inside of the tank looked-like new. I didn't grab a picture and stuffed a rag in the hole as soon as I could because of the fumes. It would make your eyes water.

I put the new sending unit together in the same way that the old one was.
And installed it into the tank, along with new fuel hoses that were probably way overkill for this application. I ended up purchasing Gates Barrier fuel injection pressure rated hoses in 3/8, 5/16, and 1/4 inch sizes. You'll need to buy about 3 feet of each to be on the safe side. The tank went back up the same way it came down, build sheet and all, and viola!
[

The old sending unit would never read empty. The tank was as dry as I could get it so I was hopeful that this would be okay. I put the gas I took out into another car and put exactly 3 gallons into the can at the station. I wanted to know just how accurate this thing was, or if this sending unit even worked at all. The answer was very! I drove around from gas station to gas station adding a few gallons here and there. Everything seems to add up!

I am so happy to finally have this thing fixed. Another thing off the checklist. This car is definitely better off now than it was a few years ago. A lot more still needs to be done, but its getting shorter and shorter. To anyone who has a digital gauge, I have yet to be able to find a vendor that makes them new. However, I was recommended to this service by someone who had a digital sending unit for one of these cars rebuilt by them with success, so you may want to contact them if you want to fix yours. I was about to when this unit popped up for sale.
John Wolf & Co. Inc.| Antique Car Gauge Repair in Willoughby, OH
Hope this helps anyone who faces this problem. It seems to be a common thing with these digital dashboards in Regals, Rivieras, Sevilles, and Toronados. For those who don’t want to drop the tank (which is not a bad job at all) there is no hatch to access the sending unit. I stumbled across this video of someone who had cut a piece of the floor of his trunk out to get to the sending unit on his Eldorado. It seems like it’s more trouble than it’s worth, but it gives you an idea of what you’re looking at.

And I unfortunately ran out of storage on my camera towards the end of this project, hence the thinning out of the pictures. Sorry guys!

 

Thank you all for your response and your helpful information.

 

You can test the wiring from the fuel sender to the dash and also the dash gas gauge display by disconnecting the 6 pin wiring plug at the gas tank filler neck just behind the rear bumper. As Smokus indicated when you unplug it the digital display should show "--" with the ignition on. If you ground the pink wire in this harness on the side of the wiring that goes into the trunk and turn on the ignition you should get an "E" displayed on the gauge. When you remove the ground the gauge should count back up to "F" and then "--" with the ignition on. Just be careful not to ground any of the other pins in this connector as you can damage the level sensor for the air shocks by doing so.
My digital dash fuel gauge on the 81 Eldo was displaying "--" all the time and I did a lot of investigation into repairing it and found out that the fuel sender that is shown on several websites that says it is for the non-digital dash cars will work in cars with the digital dash. I was able to determine that the resistance range for the sender was the same for both digital dash cars and standard fuel gauge cars just the length of the float arm was different. I believe that they used a longer float arm on the digital dash cars to get a more accurate fuel level reading and also to provide that reading as an accurate input to the Fuel Range display computer. I installed a new Spectra FG15B fuel sender in mine and while the gauge is not completely accurate it is pretty good and much better than not knowing how much fuel remains and possibly running out of gas. You need to drop the fuel tank to remove the fuel sender/fuel pump assembly from the tank but you can replace either of them separately. When I changed mine I replaced the fuel pump at the same time just to be sure for the future. One issue that does come up with the in tank fuel pump is deterioration of the short piece of rubber hose that connects the pump to the metal pipe on the fuel sender assembly so by replacing my pump I also changed the hose out to a new one as it usually comes with the pump. Hope this helps and if you need more info please don't hesitate to send me a PM. You might also want to read this previous Forum topic.

'85 Eldorado Biarritz sending unit help

I have a 1985 Eldorado Biarritz and i just put a new gas tank on it and my digital fuel gauge doesn't work, so i went to look for a new sending unit and it will cost me about $150 or so. but with everyone i have found it says without digital gauge. Any idea where or how i can find one with the...

www.cadillacforums.com

Dave

 

Dave,
I've been doing some work on two sending units for two "other" (not my Eldorado) and it's not that hard to remove the arm. I have been carefully prying up on the clinch tabs holding the variable resistor assembly. I'm mentioning it because you could probably put your old arm in the new sender for the mechanical dash. I know it doesn't sound critical but I thought I would mention it. I have one apart in my shop and I could take a couple of photos. That assembly could be a little different from sending unit to sending unit I guess but most or that vintage seem to look similar.

Scott