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Discussion Starter · #1 ·
About to take the dive again and scoop up another 78 Eldorado. The speedometer is not working on it tho. Probably just a speedometer cable. Are those connected to the odometer? B/c the mileage shows 40k
 

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1984 Eldorado Biarritz, 1983 Sedan deVille
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If the cable is broke, the odometer will not spin.
 

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Is there something like a small gear box connected with the cruise control? I don't have a 1978 but I thought I recall reading something like that. I will try to find the reference but from memory I recall something like a plastic speedometer gear in the box and the speedometer cable running from the box.

Scott
 

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Discussion Starter · #4 · (Edited)
Brilliant! I found the cable online for $10 but will check that cruise control box as well.
is the speedometer connection to the odometer as well or are they independent?
Thanks 🙏 y’all
 

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My understanding is the drive for the speedometer/odometer (same cable) comes from the cruise control box. There same cable enables the function for both speedometer and odometer. If the cruise "box" is the issue you may not even need a new cable. Important because normally there is at least a somewhat difficult to reach spring clip which normally keeps the speedometer cable end connected to the instrument cluster. It's typically a spring clip fastened on one side an so it creates an angle when "free". When you push it the spring is perpendicular to the cable end so the cable passes through; release and it won't let the speedometer end back out. So if it's the cruise box only you shouldn't need to work on the dash/cluster end at all as I know.

Scott
 

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Discussion Starter · #6 ·
My understanding is the drive for the speedometer/odometer (same cable) comes from the cruise control box. There same cable enables the function for both speedometer and odometer. If the cruise "box" is the issue you may not even need a new cable. Important because normally there is at least a somewhat difficult to reach spring clip which normally keeps the speedometer cable end connected to the instrument cluster. It's typically a spring clip fastened on one side an so it creates an angle when "free". When you push it the spring is perpendicular to the cable end so the cable passes through; release and it won't let the speedometer end back out. So if it's the cruise box only you shouldn't need to work on the dash/cluster end at all as I know.

Scott
Beauty! I’ll have to check that out. So what we are saying is if the Speedo cable is inop. The odometer is not turning over, correct?
 

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1978 Cadillac Eldorado Biarritz
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Speedometer cable goes directly from cluster to transmission. The cruise control is electronic and separate from speedometer cable.
 

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Something German S550, 69 Eldorado, 68 Impala, If I tell you the rest, well
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Disconnect the cable from the transmission and spin the internal cable by hand, if you're by yourself use your phone to record if the speedo moves if you can't see the speedometer. If it does its the pinion gear (in the transmission that's no good(could be both)unscrew the casing where the cable connects and it slides out. If it is the cable it runs through the firewall to back of speedo from the transmission. When you connect a new cable spray some silicone down the cable.

Several years ago I was able to get the gears from the dealer for my impala when I went larger/fatter meat in the rear. They might have the gears if you need. For my Eldorado I got the gear from the classic men's mall.

Yes the speedo and odometer are connected.
 

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Hey all! My speedometer is kaput. I sent some silicone down the cable; nothing. The car sat for years so maybe bunk? Idk. Anyhow can I add an LED speedometer to it? Or is there something I’m missing in the diagnosis? Trying to pass MD inspection 🧐
595241


The pic is looking into where the speedometer cable enters the transmission
 

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2011 CTS4 Coupe, 2014 ELR, 2018 XT5 AWD
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If the odometer is working there is no problem with the cable. The speedometer itself has probably failed.
 

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I think you had a similar thread where I pasted a link. That thing I called a "box" initially is that thing you pictured. See if you can find that other thread with the link I copied as it was discussed there. It was your thread I'm thinking about correct?

Edit: I just realized the post title says OD does work. Just to tie the other thread together here's a link to your somewhat related thread. Post 7 has the other link I was referring to. Since your OD does work now that info isn't as relevant as I initially thought.

I had my speedometer apart once to fix the trip odometer. There's something like a ring/pick-up driven by the speedometer cable. The cable directly drives the odometer but the rotation of the cable indirectly causes the speedometer needle to move. Unfortunately I don't know much about the mechanism. I only noticed it because I was trying to fix the trip odometer which turned out to be a gear snapped in half due to heat and age.

Scott
 

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2002 F55 STS, 2014 Explorer XLT, F-150s
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Similar threads on the same car/problem have been merged.
 

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Something German S550, 69 Eldorado, 68 Impala, If I tell you the rest, well
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The speedometer needle is there correct, I wonder if its intact? If you're are comfortable take the speedometer apart to make sure all the gears are intact.
 

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Something German S550, 69 Eldorado, 68 Impala, If I tell you the rest, well
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To add there is a magnet in where the cable connects. The magnet spins the needle shaft. It might be messed up also.
 

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1978 Cadillac Eldorado Biarritz
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When I had to lube the speedometer cable on mine I didn’t get it snapped in all the way when hooking it back up. So just make sure it’s connected correctly on the back of the cluster.
 

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Discussion Starter · #18 ·
So curious if I can just update the speedometer with a digital aftermarket one and bring the Eldorado into the 21st century?
 

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I personally like the old school sweeper much better. Recall in the 80's everything started to go to digital... Not so much now. That said if anyone makes a digital replacement, with a factory like fit, it would probably be Dakota Digital. I've never looked. If your cable is simply not snapped in all of the way which someone suggested that's the easiest solution. It's probably something simply if the odometer is still working. If you take the dash apart label the screws and take photos of the order everything comes apart as well as fastener positions.

I replaced all of the burned out bulbs when I did my '79 instrument cluster and I fixed the trip odometer snapped gear. Now it's a thing of beauty :)
 

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Something German S550, 69 Eldorado, 68 Impala, If I tell you the rest, well
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So curious if I can just update the speedometer with a digital aftermarket one and bring the Eldorado into the 21st century?
If you can find a speedometer that will fit and the you will need a sending unit for the trans.
 
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