speedometer/tachometer problem
 

I re-activated the SiriusXM radio in my 1993 FD and now there is a problem. Sirius had to send a signal from the satellite to activate the subscription on the car radio. Now the tachometer does not show rpm but the speedometer shows 20 mph with the engine idling, but the car standing still. When I push the accelerator to increase engine rpm the tach does not move but the speedometer increases mph with the car standing still. The car has the aPexi aftermarket computer. Is it possible the signal sent by Sirius changed something on the aPexi so that rpm signal goes to the speedometer? I did not change any wires or connections on the car. Only recently put in a new battery. The problem started immediately after Sirius activated the radio. The Mazda shop manual is no help. Any suggestions?

speedo/tach problem
 

One quick note. I disconnected the car battery to see if that would re-set the tach and speedo on the car's computer. I know there are sending units for each of these meters, but thought that somehow the car's computer was using the wrong signal. That did not work........would have been way too easy a fix.

Check if any of the harness grounds in the area of the radio were detached.

Did you mess with any of the radio's wiring? The SiriusXM activation process has nothing to do with your new speedo/tach problems - it's merely a weird coincidence, UNLESS you did something with the radio's wiring that may have inadvertently buggered up the dash cluster wiring.

As my post stated, I did not change any wiring.

I am checking grounds. Have not found any bad ones. The tach and speedo worked immediately before Sirium activated the radio. As I was checking the pre-set radio stations about 60 seconds after Serius activated the radio I noticed the tach and speedo were not working right.

OK....I just took the car out on the road and drove at highway speed...50 mph....for about 5 minutes. The tach started working and the speedo appeared to be working. However, when I stop the car the speedo still reads about 20 mph with the engine idling and the car standing still. When I start to move the car the speed initially goes back to zero and then reads what appears to be an accurate speed, but goes back to reading 20 mph with the cars standing still. There is some signal going to the speedo that is not coming from its sending unit.

FD cluster suddenly going wonky is very common and we all have experienced that. Bad grounds are often the culprit. Mike Gange will refurbish your unit for you for maybe US$200 (before shipping costs). Study carefully how to remove to avoid damage.

I had some cluster weirdness on and off for about a year, but for the last 2 years or so, it has been working perfectly.

For a while RPM read too high after start-up and then went back to normal after running for 15 minutes or so. I was thinking a cluster rework was going to be necessary, but since the issues went away on their own, I'll wait until it misbehaves again.

Could be sensitive to temperature and/or moisture. My tac ticks at, or below, freezing. But the tick goes away once the interior warms up.

When mike refurbishes he will also replace capacitors (etc) he expects will fail sooner than later.

Thanks for the contact. Peter Farrel used to have a FD shop in Manassa near where I live, but moved to somewhere in MD. It's hard to find someone to work on these cars. Even when they were new there was no good repair service. The Mazda dealer was the worst place to take the car. I spent thousands of $$ at the dealer in Hickory, NC before going to RotorSports Racing. If I can't find a bad ground I will have to use your contact. I'm getting it ready to sell. Can't really get a top asking price if the gages do not work.

I found a more detailed diagnosis section in the shop manual for the speedometer. The speedometer sending unit located on the transmission is a small electric generator. It has a gear to the generator that turns by linking to a gear in the transmission and generates A/C current from the small generator to actuate the speedometer. By removing the sending unit and turning the gear slowly you can get a voltage/amperage reading to determine if the unit is damaged. I'll have the technician working on the car run this test. Both the speedo and the tach go thru the CPU. With the car idling, but standing still the speedo sending unit is sending no A/C voltage or amperage to the speedometer. But I suspect the speedometer is getting a low amperage D/C signal that makes the meter give a low level mph. When the car moves, the sending unit sends an A/C signal that makes the speedo actually read properly. Old fashioned speedometers with a cable were easier to fix.

Car is stored in a heated garage, so moisture and temperature issues should be minimized.

No signs of mice, I suppose.

Living near woods and other open spaces, there are continually signs of mice. To minimize their threat, I have spring mouse traps and multiple glue traps everywhere they might have access. I also try to keep door, etc., gaps minimized, but they still find ways in. The last time I had a mouse nest or debris in a car was ~2010 in my 2005 Focus, but luckily I found it before there was any damage.

No mice. I have a mouse problem in my farm truck which is parked most of the time. I found the mice do not like the smell of moth balls.

There are many lessons learned about fixing the instrument cluster in this old thread
https://www.rx7club.com/3rd-generati...-599220/page5/
It is in the FAQ.
It focuses mostly on the speedometer at the beginning but lays out the common problems applicable to the tach as well.


Or - just send it to Michael Gagne :)