speedometer shows 1.6 times the real speed

[enochbell]

I still don't buy it.  The speedo needle doesn't care if it is in a metric car or not.  It still sweeps through the same (approximately) degrees of arc regardless of whether the dial face reads mph or kph.  It responds to shaft speed in a linear fashion.  I don't believe the issue here is a simple "wrong gear" problem.

The fact that the error was 1.6 is, I believe, coincidental.  Now the odo readout is an entirely different matter.

But I am just using logic here, and would sure like to know the real explanation.  

Can anybody help?

Best,
g

'64 230sl, fully sorted out...ooops, spoke too soon

[Ricardo]

Here is a photo of the worm gear that drives the magnet for the hall effect needle...note that there are three different gears here, though not all from a 113 speedo and I assume there are others.
The metal one was in my car when I got it and the speedometer was erratic. I found the nylon gear was badly chewed and I managed to cut and slide it on it's shaft to a different part of the worm and the speedo then was steady enough to get me through the saftey check and licenced, but it read too high, I've found these other gears and more at scrap yards and I now have a speedo that is nice and steady and approx. 20% too high, so there has to be an assortment of these depending on rear end ratio....
my opinion, though I'm not an engineer

Download Attachment: speedometer gear close up.JPG
41.26 KB

[enochbell]

Ricardo,

Thanks for the pic of the speedo guts.  Is the worm gear driving or is it driven by the shaft?  I believe it's purpose is to drive the odometer gears, not the speedo needle/spindle?

Sorry to the board if I am getting boring with this, but it has me puzzled and I generally don't like to stay puzzled.

Best,

g

'64 230sl, fully sorted out...ooops, spoke too soon

[Benz Dr.]

Mr. tooth;
no one seems to understand how these things work least of all anyone who has never ben inside of a speedo gauge. The neede moves in direct relationship to input - the wrong input will give a higher or lower speed than what the car is actually traveling. The speedo is set to the rear axel ratio and there is usually a number stamped on the back housing of the guage indicating what the ratio is. Miss match the speedo to the rear axel and it can be way off. The hall effect on the needle is not that great and minor adjustments can be made to bring it into line with real speed.
The input shaft of the speedo runs the set of gears that run the odometer and trip counter so this is a hard mechanical drive. The needle on the other hand, is run by a magnetic flywheel and perhaps the magnet is different for higher gear ratios. All I know is if you put a 3.27 axel in a 280SL that had 3.92 gearing it will show a 20% difference on the gauge. When the speedo shows 60 MPH you will be really going about 75 MPH. In this case the speedo is too slow because the speedo measures drive shaft speed not rear axel speed.
The amount of adjustment inside the gauge is really for small fine tuning so if you want to use the same gauge ( there is no 3.27 for a 280SL ) the gears and the magnet or spring tension on the guage has to be changed. The cable and gears at the trans really have no bearing on this from what I can see.
This is why the gauge is set to the rear axel ratio. What everyone seems to want to know, is not how they work, but rather how one gauge is different from the other and how those differences relate to function.
 I was never good at math but a KM is .62 miles. I multiply the number of KM by .62 to come up with miles which I guess is the same as dividing miles by 1.6 to come up with KM. However, a 60% difference in guage function would mean that instead of showing 80 KPH the gauge is showing 128 KPH? Is this what Roman saw? If this is the case, this IS the difference between KM and miles and it would seem they got it backwards somehow.
If the change would have been made using a standard metric guage none of this would of happened since the change is as simple as instaling it. The gauge is set up and makes the conversin for you as long as the axel ratios are the same as the gauge.



Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[Ricardo]

Hi Greg, Dan
I must admit I have made an error here and upon reflecting on this and opening up another parts speedo, I have come to a different puzzling conclusion. Greg you are right, the pictures I showed were of the gear that drives the odo and not the speedo, and I was forgetting that when I pulled mine apart to get a steady working speedo, that the repairs I made then, weren't about the cross shaft with the nylon gears. What I discovered then was that the spinning magnet, which is directly connected to the speedo cable, plugging into the end of it, was able to move in and out from wear and this caused the speedo to fluctuate. I later repaired and replaced the nylon gear to get my odo working...memory isn't what it used to be....
I'm thinking now that the difference in speedo's for different rear ratio's may be changed by the length of the shaft to which the magnet is attached, causing the magnet to be in a slightly different position within the cup that connects to the needle itself. This cup has a sloped shoulder internally and perhaps the position of the magnet relative to the slope may influence the actual reading....??
Last night I drove the car at a steady 3000 rpm and my reading was 116 kph, (should be 105 or so with a 3.69 rear ratio and 205/70 tires) so it can't be simply k's or mph, there must be something internal to the speedo which gives the different readings depending on rear ratio and since the cable is directly connected to the spinning magnet...where else can the difference be made up???