Speedometer Reading to High and tach is bouncing

[Fraidknots]

Hi all.....

 

Two questions regarding my gauges on my 74 2002.

 

First Questions: My speedometer  is reading about 12 miles to high at about 80MPH.... so if reads 80, I am actually doing about 68MPH.  The error occurs at a lower scale but all though out all the lower speeds.  By the time it reads 30MPH, I am already off by about 5 MPH.  So my question is, how do I go about diagnosing the issue?  Or should I just replace the speedo and cable?

 

Second Question:  My tach "bounces" a lot through out the RPM ranges and doesn't really move about 4k RPMs even though the engine does.  Same question; how do I diagnose the core issue and is it even worth it, should I just swap out for a rebuilt tach?

 

Any and all advice would be welcomed.  Thanks all!

[Mark92131]

Your speedometer needs to match your differential ratio.  You can check it by by pulling the gauge cluster and the ratio will printed on the back of your speedo.  Then you can see if your differential gear ratio matches, by jacking up the back and turning the drive shaft with the car in neutral to count the number of turns the drive shaft makes to complete on revolution of the rear wheel.  For your stock 1974, the drive shaft should turn 3.64 times to one wheel revolution.  If the marking on your speedo doesn't match the differential gear ratio, it will read incorrectly, so you will either need to change the speedo or the differential.

 

The tach bouncing could be a simple grounding issue at the cluster.  When you have it out, search cluster grounding to see if this will fix your issue.

 

Mark92131

 

 

[Fraidknots]

Thank you Mark... seems very straight forward.

 

I continue to be impressed with the knowledge in this forum!!!

[AustrianVespaGuy]

You can do a hack pseudo fix for the speedometer by pulling off the needle, twisting it a little bit counter-clockwise, and sticking it back on again so that the little clock spring in there is wound up 'tighter' than before for a given speed.  This will screw up the linearity of course, but since yours is more than 15% off already I think you could get it a good bit closer in your normal cruising range than it is now without needing to buy a new part or sending yours off for professional calibration, though those are of course both options too.

 

The tach certainly sounds like an electrical issue; in addition to grounds, when's the last time you changed your points and condenser and checked your coil?

Edited October 29, 2018 by AustrianVespaGuy

[Fraidknots]

The ignition system is all new.  Switched to electronic ignition with new points and coil.

[worzella]

80 really doing 68... tell your insurance company and they might give you a 5% discount ?

 

But back to facts... I just pulled my pod and fixed my wonky speedo that was jumping all over. When installed it registering about 6 higher than actual. I plan to just live with it. Not sure my car can even reach 80!

 

Interesting detail about diff ratio. I need to see if I took a pix of the back of my cluster when it was out to compare

 

Randy

[Simeon]

42 minutes ago, Fraidknots said:

The ignition system is all new.  Switched to electronic ignition with new points and coil.

 

If that is the case, you might need a tacho adapter. What ignition do you have?

 

I would also check each connection carefully. It sounds like you could still  have an intermittent connection somewhere 

[Mike Self]

If your car still has its 3.64 differential (standard for all 2002s except CA 76s) there should be a stamping on the back of the speedometer housing that says W=1.28.  If yours says differently and you have the original diff, that's your problem--speedo doesn't match diff.  

 

If the speedometer not only reads high but the needle swings wildly once you're up to 30 mph or so, drop me a PM.  I did a column on how to fix that problem.

 

cheers

mike

['76mintgrün'02]

47 minutes ago, mike said:

3.64 differential (standard for all 2002s except CA '76s)

 

3.64 standard for all 2002s, including CA 76s... the 49 state '76 got the 3.90 differential.

 

There was a factory glitch regarding the speedometer gearing installed on some '76 cars and Conserv describes that here, along with some speedometer numbering information.

 

https://www.bmw2002faq.com/forums/topic/183033-found-speedo/?do=findComment&comment=1126960

 

Colin gives this longer list of speedometer ID numbers in the link below the list:

 

3.45:1 diff -- W=1.24 speedo

3.64:1 diff -- W=1.297 speedo (W=1.30 is close enough)

3.90:1 diff -- W=1.39 speedo ('76 49-State 2002)

3.91:1 diff -- W=1.393 speedo (E21 320is LSD)

4.10:1 diff -- W=1.4 speedo (1.461 calculated)

4.11:1 diff -- W=1.4 speedo (1.465 calculated)

4.38:1 diff -- W=1.559 speedo (calculated)

 

https://www.bmw2002faq.com/forums/topic/70732-speedometer-markings/?do=findComment&comment=465943

 

I'm just curious, what size wheels/tires are you running?

[Guy Cocquyt]

A miss matched gauge to drive train assumes one or the other were changed since it rolled out of the plant in Bavaria. Did you make a recent change to either?
A mismatched wheel/tire diameter could cause a variance at the needle too.

The tach issue is a poor ground. I added a wire from one of the nuts on the back of the cluster to a screw behind the cluster on the firewall.



Sent from my STH100-1 using Tapatalk

[Conserv]

6 hours ago, '76mintgrün'02 said:

 

...Colin gives this longer list of speedometer ID numbers in the link below the list:

 

3.45:1 diff -- W=1.24 speedo

3.64:1 diff -- W=1.297 speedo (W=1.30 is close enough)

3.90:1 diff -- W=1.39 speedo ('76 49-State 2002)

3.91:1 diff -- W=1.393 speedo (E21 320is LSD)

4.10:1 diff -- W=1.4 speedo (1.461 calculated)

4.11:1 diff -- W=1.4 speedo (1.465 calculated)

4.38:1 diff -- W=1.559 speedo (calculated)

 

https://www.bmw2002faq.com/forums/topic/70732-speedometer-markings/?do=findComment&comment=465943

 

 

As with most things, there’s never a definitive list!

 

I’ve seen quite a few W=1.28 speedometers (as Mike mentions above) and I’ve never seen a W=1.39 speedometer (my 49-state ‘76, like every other 49-state ‘76 I’ve seen thus far, has a W=1.393 speedometer). I wouldn’t be shocked to discover more variations, but I also don’t have an explanation for why these theoretically-inaccurate gearings exist!

 

Regards,

 

Steve

 

[Fraidknots]

thank you all.  I am going to check the ratio and make sure the tach is grounded.

[ingramlee]

Mine is about 9 mph off. I just live with it around town. Whenever I'm on a longer drive on the highway I use a mph app that I downloaded on my phone, hey, I always have my phone!

 

https://itunes.apple.com/us/app/speedometer-speed-box-app/id307529605?mt=8

[Fraidknots]

Thanks ingramlee ....  I live with it as well and use the app also.  I actually enjoy learning from this forum more than I care about some of these small issues.  Thanks to all for sharing!!

 

[M3M3]

And note what minty asked....What size tires?   Any deviation from stock size will mess up the Speedo.  Speedo is only accurate for a specific speedo gearing, diff ratio AND tire size combination.