Tach goes crazy
03-04-2018, 01:03 PM
#1
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Location: Chattanooga, Tennessee
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Tach goes crazy
At WOT my factory tachometer immediately pegs out to 7000. You can ease into it and it will come up ok with the RPM's but once the foot is on the floor, it jumps to 7000. Anyone know what's going on? Thanks.
01-06-2019, 01:21 PM
#5
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Re: Tach goes crazy
Definitely something I haven’t seen before. Wonder if it’s some sort of high voltage interference on the wire from the PCM to the tach.
01-06-2019, 01:40 PM
#6
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Re: Tach goes crazy
Yes, that is correct. If you just ease into it, the dash tach will go up normally. But once you mat the throttle, it jumps up. Log showed TPS at 100% but rpm's looked normal--just climbing gradually. Now my speedometer is doing the same thing. I guess it's just the wiring going to the dash gauges. Very odd though. Tracking down that wiring would be a pain, so I guess as long as it isn't affecting anything performance wise, I'll leave it alone. Don't really watch the gauges at WOT anyway. It would be interesting to know where that voltage/signal is coming from, though. I was just wondering if anyone else had experienced this and knew what it was.
01-06-2019, 05:16 PM
#7
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Re: Tach goes crazy
I notice that the speedometer and tach have a separate power input to the cluster. If nothing else is being affected, perhaps that connector is intermittent.
01-06-2019, 08:58 PM
#9
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Re: Tach goes crazy
They appear to be but I really don't know much about the internals of the gauge cluster. If the power to the cluster is intermittent, that might cause the electronics to make extra pulses. But I doubt that is the case. I suspect the problem is internal to the cluster and those gauges are losing whichever voltage (12v or ground) keeps the needles down. The natural tendency is to think that the ground keeps the needles from pegging positive, but I have assumed wrong before. Electrical engineers are crazy bastards, don't assume they did the logical thing ;-)
Last edited by GaryDoug; 01-06-2019 at 09:00 PM.
01-06-2019, 11:15 PM
#10
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Re: Tach goes crazy
You reminded me of a problem I ran into maybe 40-50 years ago. Every time I nailed it, one of the gauges would go full deflection. Turned out each gauge had two bolts holding it to the gauge panel and the nuts had vibrated loose. Under heavy accel the gauge would pull slightly away from the panel and lose the ground. Forgot all about that.... but obviously the design of the panel was way different “back in the day”. Can’t even remember what car it was I had the problem with.
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