Speedometer Magnet(ism)

[willisit]

Hi all,


In all my years around DeLoreans, I've not come across this one.
So, first question is; anyone know any cross references or how the inner workings of the speedo work?

Here's the issue. My speedo needle doth not move. The mileage increases (both barrels) so before anyone mentions it, it's not angle drive or cable related . To add, I have the speedo out and have spun it up with a drill and, nada.

The speedo itself is a metal speed-cup/magnet affair. The cup is attached (with spring) to the needle. If I spin the cup, the needle moves.
The cable-end has the magnet on it. No matter how I spin that, it doesn't drag the cup around (but the mileage increases so I know it's doing something right via the worm drives).

The question is, before I start dismantling it and realise I can't buy a new one (really), is it likely I just need to remagnetise the magnet? That is, if I can. Otherwise, is there something else that should be in this that might be missing? (it really is just a cup and a metal rectangle that spins within it).


Cheers!

[WHO1DMC]

How free is the magnet? It might need a little lubricant. The magnet has to spin pretty fast. The magnet has to create a eddy current inside the speedcup witch drages it along with the magnet. It's rare they lose their magnetism. They can lose their magnetism from heat because there not really that strong. I would not try to remagenitise the magnet but try to find a similar speedometer for a magnet. Than you would have to recalibrate the speedometer.
Worse case you have to buy a new one or contact a vendor to see what they might have. They might have a junk one with a good magnet.




Dave B.

[DMC-Ron]

Dan,
Usually a weakening magnet will not suddenly cause the speedo needle to not move. All speedo magnets slowly lose their magnetism over time. This usually results in the speedometer reading under-speed as they age. But it is gradual and the error is minor. At the same time, the speedometer needle spring looses its tension over time. A weak spring will cause a over-speed indication. Often these seem to cancel each other out.

Your speedo symptoms are unusual. It sounds like the magnetic disk is turning with the speedo cable (you've verified the magnet is spinning), but the aluminum drive cup (and therefore the speed needle) is not rotating. First, note that it takes reasonable RPM for the magnet to "induce" the drive cup to rotate (so make sure you are giving it a proper spin). Regarding the magnetic strength, you can check your magnet with a paper clip to test it. Unfortunately, judging the magnetic strength is subjective and you are testing the back side of the magnet, but it should grab the paperclip similar to the magnets you find on a electrician's level. The air gap between the disk with the magnet and the aluminum drive cup makes a difference. They should be quite close to touching. If the gap has somehow doubled, the force imparted to the drive cup is 1/4. There are still shops around the county that specialize in repairing these older speedometers. They can not only get them working, but they can calibrate them as well.

One noted repair shop is West Valley that specializes in vintage car speedo repair:
https://www.westvalleyinstruments.com/
There may be other recommended shops if you search.

Ron

[MrChocky]

The speedo is a GM part. Here's another guy that rebuilds them:

https://www.facebook.com/ricksrestorationservice

[ssdelorean]

... and if you need another recommendation, I have delt with Bob's up in Michigan when recalibrating my DMC speedo.
https://www.bobsspeedometer.com/

[David T]

Never saw or heard of this with the magnet still attached. Normally this occurs because the magnet has fallen off but it is not common in Deloreans. I did have one car that kept damaging the angle drive and the speedometer cable. I suspected a problem in the speedometer head. When I opened it I found a plastic post had broken off with the clip still attached to it and because the clip was steel it was stuck on the magnet creating a LOT of drag which was what caused the damage to the angle drive and the cable. The best option, if you don't see anything obvious to fix, is to send the head out to a speedo shop where they can fix it and calibrate it. Also consider if you want to modify it to a 170 MPH as long as you are sending it out. All you need is the decal from DMCH. Bob's Speedometer is a good shop and I have used them in the past. They don't have the fastest turn-around times and they are not inexpensive but they do good work. Maybe it would be faster and cheaper to just get a 170 MPH head from DMCH and put your odometer into it so you still have the correct mileage.

[willisit]

Thanks all.

Interesting; so even with the magnetism on it's way out, it should still read. I attached a drill to the cable and it span the magnet side up to 60+mph and the needle just bobbed near the pin. However, I can manually turn the cup and the spring returns the needle to where it should be. It's all "free" and easy to move. However, no matter the rate the magnet spins, the cup just sits there. It's really really odd!
I have found videos of old MG/Corvette parts under reading and then being remagnetised, which is the reason I mention it. That said, over 3 DeLoreans, I've never had to get this intimate with the workings of a speedo so I don't know if there is something missing (i.e. a piece of plastic/metal/etc.).

I have many pictures but they are likely mostly useless unless someone knows what they're looking at.

I'm in the UK (shoulda mentioned that!) so thanks for the recommendations. I have found a UK-based person that will look at it, but they want ?200 to do just that; once couriering/parts/etc come in I'm likely at "new speedometer" territory, I figured I could at least try myself but now I'm less sure.