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Tach Trouble

Maintenance/Repairs
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#1

Hey guys. I have a 1983 Mazda RX-7 with a pile of issues. Today’s issue is my tachometer is failing intermittently. Sometimes it works perfectly, sometimes it sits in one spot without moving, sometimes it very slowly reacts to changes in revs, sometimes it’s both sluggish and off target. Seems to vary day to day.

Usually if it’s working when the engine starts, it’ll keep working for the rest of the drive, and vice versa: if it’s malfunctioning when it starts it stays messed up.

Any thoughts?

All I’ve done so far is test the lead and trailing ignition coils, as suggested by someone else. Lead is 1.5 ohms primary, 10.13k secondary. Trailing is 1.3 ohms primary and 7.46k secondary.

#2

I hate to sound overly simplistic but the fact that it sometimes sits in one spot without moving suggests that the gauge itself may be sticking.

#3

In the case of diagnosing automotive problems I don’t think there is such thing as overly simplistic! Always start with the most basic and move from there.

However, I’m not sure about it sticking. When it’s lagging/slow it’s very very smooth. I would think if it was actually sticking it would stay in one place or maybe snap from moving normally to being stuck?

#4

It connects to the igniter and ignition coil 1. Could be a connector just needs reseating.

#5

If it’s too expensive to fix…ignore it. I find a tach to be a totally worthless instrument unless you’re trying to diagnose a problem…then just hook up a portable tach.

#6

Thanks for the diagrams knfenimore. I’ve tried wiggling and cleaning all of the contacts I can find, but I’ll keep up my effort.

Mike, the engine has very few miles since rebuild and a mandatory break-in schedule so I need to keep it under 4,500 rpm. Rotaries are well known for fooling the driver, Mazda put an RPM buzzer in their for a reason. If it were a family sedan with an auto tranny or something I’d agree with you, but in a little high-revving sports car a tach is important!

In any case, having a big broken gauge staring me in the face every second I’m in the car drives me nuts haha

#7

While I’m inclined to think the following is not the cause of this problem I’ll throw it out there since I’ve seen a number of times on other makes of cars. Flaky tachometer operation can sometimes be caused by worn distributor shaft bushings. That’s easy enough to check.

Just pop the distributor cap loose and see how much sideways wiggle the shaft and rotor have. A very tiny bit is acceptable; if it’s very noticeable that may be the source of this problem. Just some food for thought anyway.

#8
f it were a family sedan with an auto tranny or something I'd agree with you, but in a little high-revving sports car a tach is important!

I’ve owned high-revving sports cars before…You learn to shift by sound and feel. Tach really only helps if your on the track racing.

#9

If it is a gauge problem, have you shopped around for a replacement gauge cluster? I’m sure one could be had on ebay or such. Do you use an RX7 or rotary forum? If not, I highly suggest finding one. I have an '88 Supra, and find my Supra forums indispensable in keeping my ride working. You could also buy a cheap add-on tach to see if the problem is in the signal wire to the dash gauges. If a reliable tach is important, you should be able to find some solutions. BTW, @MikeinNH may not understand us wanting to keep our tachs working. His opinion, but feel free to ignore.

#10

ok4450, worth a shot, will try that out first thing when I’m back with the car.

Mike, I should have mentioned the engine has about 100 miles on it since the rebuild. 60 of those were 5 years ago, the remaining 40 of were short test drives with 2 different exhausts, of which 30 were with a faulty tach and most were drowned out by sounds from a bad ring and pinion. You can understand that I have no learned reference for what the frequency or resonance of 4,500rpm sounds like in this case.

More importantly, the thread is about fixing something that’s broken, not whether or not you want it in your own car. Thank you for your thoughts, but if you don’t have any suggestions for getting a tachometer working, please leave the discussion.

Busted, I actually have another cluster, saving trying that out until I’ve gone through more accessible diagnostics (especially because the spare cluster has its own issues). However, the cheap tach solution sounds a lot less troublesome than pulling apart my dash tof figure out if it’s the gauge itself. Will look into that. As far as communities, I am in fact part of a thriving RX7 forum, but nobody there seems to have experienced my issue, so that’s why I came to car talk, wondering if this might be a rare but not RX-7 specific issue. I

#11

I’m not much of a Mazda guy although I used to work for a Mazda/various other lines dealer. My co-worker was the Mazda whiz and during the course of a day mechanics often exchange verbal notes about problems; sometimes with polite language… :slight_smile:

A flaky tachometer or worn distributor is not something I’ve personally seen on an RX-7 nor have I heard any mention of it from my co-worker but I have seen it on a few other Asian makes of cars including Nissan, Honda, and Subaru. That’s why I threw out the suggestion about a worn shaft.

I’m inclined to think the problem may be the cluster itself but weeding out a worn dist. shaft is so easy that should be done first.