I did a search and saw the docs speedo install thread in the archives, but was just wondering about some symptoms of a bad speedo cable. Since I bought the car in January the speedo had been inaccurate at higher speed, as in if you are doing over 70 it would just peg out at 85. Up to 70 it seemed accurate, today it started stopping at 75 and would not climb any more. I am assuming it is the cable, but was just wanting some opinions,

Thanks in advance,

Luke

 

More likely the speedo head. Only way the cable could cause that is if it's binding. If that's the case, pull the cable out of the sheath, clean and relube.

 

I guess that would mean I would need a new cluster? I am assuming thats where the speedo head is located, sorry never heard of that before. Has anyone done this repair before by chance?

 

Each gauge in the cluster can be replaced individually. Not a terribly difficult job, snaking the cluster out and disconnecting the wires/cable are the hardest part.

 

I wish my speedo will stop at 75mph

 

The speedo head has a meter movement(D'arsnal) in it just like a multi meter.

Essentially its a magnet with an electro magnet armature. The behaviour you identify is common for a meter movement that has been contaminated with some little bit, especially something that is attracted to a magnet like a small piece of rust.

Repair can be as simple as disasembly and a good cleaning. Compressed air can work, but care is needed. If the contamination is iron in origin it will try to stay on the magnet!

The meter movement is a very delecate instrument and can be easily further damaged.

 

Thanks for the replys, I will get to work pulling the cluster. I will just take my time and try not to break anything,

Thanks again

 

I used to repair speedometers at Kansas City Speedometer - they are now out of business. Metro Speedometer in Independence MO is operated by a former KC Speedo employee (816) 421-5996 . Any Who, here is how you diagnosis a mechanical speedo problem:

* If the odometer moves and the speedometer needle dosen't then the instrument is bad

* If the speedometer needle moves and the odometer doesn't then the instrument is bad.

* If neither the speedometer needle or the odometer move then the instrument is not being driven. The cable is broken or the gears in the trans tail-shaft are striped or missing

* If the speedometer needle moves but is inaccurate (fast or slow) and the odometer is accurate the istrument is bad. If the needle reads fast or pegs early then the speedometer speed cup/magnet are making contact or the speedometer magnet has become over-magnetized. If the needle is sluggish - reads slow - then the speedometer magnet has lost magnetism.

Service

The cable on an RX7 comes out from the bottom - most Asian cars do. The best lube is a light bearing grease. Mobile One or a light duty lithium would work great. NEVER use graphite because it has abrasive properties and will eventually ruin the speedometer magnet bushings.

The speedometer itself is not serviceable with new parts - none were available. An instrument in good condition can be cleaned, lubed and recalibrated - remagnetized - by a speedometer shop. The calibration process involves charging the instrument's magnet and then demagnetizing it until the instrument displays 60-62MPH at 1024 cable RPM.

Speedometers in SA and FB RX7's are mechanical and do not have anything in common with an electrical instrument. They do use magnetisim to move the needle but no electrical current. This type of movement was used from the very beginning of the car to the mid 1990's. They are very accurate when properly maintained.

Testing for calibration in the car

The best way to verify that your speedometer is being accurately driven in a Mazda is use the odometer. Lijke all other mechanical istruments, the odometer in a Mazda is gear driven and not prone to the same loss of accuracy that can occur to a mechical speedometer. We recommended making a 5 mile run on a relatively straight highway using mile posts. At the end of 5 miles the odometer should read 5 miles plus or minus 2 percent. If your car passes this test then you can check the speedometer using a stop-watch (time distance covered at a set speed) or by pacing with a car with a known accurate speedometer.

The above rules can be applied to an electronic speedometer as well. While they truly are electrical instruments - no different than a gas gauge - they have to be driven by a signal (instead of a cable) and have a separate drive for the odometer that is also driven by the same signal. To my knowledge there is no repair - just replace if they are bad.

As always check with an expert at a speedo shop.

 

Thanks Mustanghammer, I havent got started on it yet but that is very helpful, maybe once I get it out and find whats up it could be a possible archive canidate if this a common problem.

 

Excellent info. I have the problem where the speedo is not accurate until you get above 50 mph and then it bounces bit but is kind of on the money. Once you slow down it sticks at around 30 to 50 mph. I'm wondering if just cleaning it with compressed air gently would cure it.

 

Oh yeah, I forgot, archive this one.