Fast Gauges

S

Sparks

Brian Mattson
Now I know these X's are fast.......but my speedo reads 5 mph & the tach says 500 rpm when the car is sitting still in the garage with the motor off.

Is this normal, or has the PO been playing with the numbers?

Is the fix as simple as pulling the needles off and re-installing them correctly? Or some internal adjustment?

Brian, "in da UP"
 
Gauge fix

Sparks said:
Is the fix as simple as pulling the needles off and re-installing them correctly? Or some internal adjustment?
Brian, "in da UP"
Click to expand...

In a word, YES, but some say the accuracy will be affected at speed.
I don't think they're all too accurate anyway, but there is a way to adjust them both. The speedometer is the hardest to adjust outside of a bench setup. The tachometer has an adjustment pot inside.
Good luck!
 
my '81's tach reads 500 rpm at rest as well. I can't recall about the speedo but I know it was off by 5 mph at 65 clocking it with a stop watch and mile markers.
These are the kind of cute idiosyncrasies that make the car endearing (that's the BS I tell myself because I don't feel like dealing it).
It will be a cold boring week in Feb when I spend time on something like this. Your driving season in the U.P. is shorter than mine even. Nothing else to work on?

We should get some Michigan X heads to hit the St. Ignace car show some time.
 
Hi Jeff,

You're right, no real priority on a fix. I'm going to have a mechanic friend check my tach against his good shop stuff. I've heard that the speedos are normally fast, but the Sumitomo 185's have corrected that somewhat. A GPS run will answer that. I guess the real annoyance is that gauges that don't read zero at rest are an affront to the engineering mind! :shock:

Meanwhile, I've just been enjoying the now much faster X (besides camping & trout fishing during our short summer). I haven't done St. Ignace yet; our car "quality time" is spent at the vintage sports car races, particularly at RoadAmerica in Elkhart Lake.

Brian, "in da UP"
 
Guess what? It's a cold day in January. Even though the X is still in daily use!
I recently swapped out my 85mph speedo for a 140 and since the 85 housing was screw on and the 140 was sleeve fit, I moved the magnet and speed cup from the 140 over to the 85 just so I could use my old cables....

The 85 read high at highway speed. So does my 140. I did a needle move and now it's like this:
reads lower than actual speed below 50mph
is spot on at 50
reads high above that, to the point where it shows 90 when I'm doing 80.

I'm just curious as to how these would get calibrated if they are wrong in both directions. It would seem to me that re-magnatizing to make it stronger would make it read higher across the board, and making it weaker would make it read lower across the board.
Is the spring the thing to work with? If the spring tension is tweaked just right then it's rate could compensate a bit in whatever area you needed adjustment?
Just curious.
I know if I want it DONE I can send it to Chris's old outfit.
Maybe when I pull the cluster again to address the wacky volt meter I will swap the speed cups just to see what if anything that does. It's a hobby you understand.
I could do a series of careful measurements and make a face plate up to match the needle but that gauge would look really weird with a variable scale on it, very weird.
 
Gain and offset

Both the magnet and the spring determine the gain, or how many degrees the needle moves for a 1 mph change in speed. If the speedometer reads too low at low speeds and too high at high speeds, your gain is too high. Stiffer spring or weaker magnet would lower the gain. Once the gain is correct, the speedometer will show the same error (in mph) across all speeds. This error is the offset. You can calibrate offset by removing the needle from the shaft and reinstalling it in a different position (i.e. pointing to "0" when the car is not moving).
 
So the determining factors are magnet and spring, so when I moved those from the 140 over to my 85 speedo I was on the right track. The speed cups between the 140 and the 85 "looked" identical. Magnets were certainly different. The 85's was larger, more gain, more movement, smaller scale on the 85 so that makes sense. I'll play some, maybe bench test. I calibrated my Opel's speedo on the bench by timing the miles via the odometer and repositioning the needle. That was just needle position though. Sounds like fun.
 
Don't sweat the petty stuff...., pet the sweaty stuff...

That being said, at least with the late model speedos, the needle is what, 4mph wide?
 
I remember 5mph or so on my 85 but I have the needle WAY retarded on the 140 to even get it this close, it's set so far back that it was hard to get the cluster back together as the needle fouled on the trip meter reset knob. It points almost to 6 o'clock at rest now. I will pull it tonight and do extensive bench testing and fiddling.
 
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