tacometer is stuck at 1000rpm even when engine is shut off!

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Yukoner

Filing Flight Plan
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terryklassen
Plane is a 1946 cessna 140. Started it up tonight and everything worked good, when I when to pull the throttle back to allow the oil pressure to climb, the rpm kept climbing to 1500 and stayed there. I pulled the throttle all the way back and the tacometer still showed 1000rpm when it was idling. It still shows 1000rpm with the engine off completely. What could be causing this? What do I need to look at first?

Thanks for all your replies!
 
Welcome to POA.

I think it is a safe bet that it's the tach. A cable problem could cause a tach to be erratic or stay at zero. But if it stays at 1000 with the engine off - the problem is the tach itself.

If it's still the original tach, I think you got your money's worth out of it. :-)
 
thank you so much guys! is it a big job to replace the tach? Theres no mechanic out here. Can you point me to any useful resources on how to replace the tach ? Thanks again!
 
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Yukoner said:
thank you so much guys! is it a big job to replace the tach? Theres no mechanic out here. Can you point me to any useful resources on where to buy/ how to replace the tach ? Thanks again!
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During Prohibition, a Supreme Court justice was in a hotel down south and he asked the waiter if they had whiskey or if they obeyed the law. The waiter said that if the judge wanted law, they had the law, but if he wanted whiskey, they had that too.

The point being, any competent speedometer shop can repair your tach, but it is an unapproved repair and should cost less than $50. On the other hand, new tachs are available for not much more than $300 apiece AND you have to completely go through the math in your logbooks to change actual hours to the new zero hour tach time.

Your call. Whichever you do, go down and get some unapproved graphite cable lube and jam as much as you can into the old cable OR replace the cable.

Jim
 
Put a piece of aluminum foil between the cable and the tachometer (ask your wife if you can steal some aluminum foil from her kitchen), screw the cable again to the back of the tach indicator and test again.
If it works now, is because the cable has worn out, and need to buy a new stainless steel cable from the factory.
:wink2:

Note: if the problem persists, send the tachometer indicator to the instruments shop, probably the magnetic plates needs to be re-magnetized.
 
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WarriorPilot said:
Put a piece of aluminum foil between the cable and the tachometer (ask your wife if you can steal some aluminum foil from her kitchen), screw the cable again to the back of the tach indicator and test again.
If it works now, is because the cable has worn out, and need to buy a new stainless steel cable from the factory.
:wink2:

Note: if the problem persists, send the tachometer indicator to the instruments shop, probably the magnetic plates needs to be re-magnetized.
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I'm not understanding the aluminum foil bit. Are you saying to wrap the inner drive cable with the foil? But the problem isn't that the tach isn't moving, it is that the zero position is not zero but 1000 RPM.

And I don't quite understand how unmagnetized plates would all of a sudden quit working. RPM error, yes. Immediate failure with an offset zero, no.

Jim
 
just change the tach and cable :mad2:
is a plane no a kia optima :lol:
 
Comanche flyer said:
just change the tach and cable :mad2:
is a plane no a kia optima :lol:
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You remind me the local Chevy dealer. Replace the engine or buy a new car. Period. LOL

Do some troubleshooting first please..!! learn to troubleshoot things first. :wink2:
 
Does it make sense that if the cable is not turning and the tach is reading 1000 the tach is shot?

what has happened is, the needle drive has skipped gear tooth and moved the "0" to the 1000 mark.

get a new tach, have it set to the present (old tach) reading and replace it.
 
Tom-D said:
Does it make sense that if the cable is not turning and the tach is reading 1000 the tach is shot?

what has happened is, the needle drive has skipped gear tooth and moved the "0" to the 1000 mark.

get a new tach, have it set to the present (old tach) reading and replace it.
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Mine the drag cup started to rub, it is sitting on my shelf and I can park it where ever I like, 1000 is where it likes to settle.

EI electronic replaced it
 
WarriorPilot said:
You remind me the local Chevy dealer. Replace the engine or buy a new car. Period. LOL

Do some troubleshooting first please..!! learn to troubleshoot things first. :wink2:
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Troubleshoot what, what failure modes cause the tach to read something other than 0 when the engine is off that aren't internal tach failures?
 
Skylane81E said:
Troubleshoot what, what failure modes cause the tach to read something other than 0 when the engine is off that aren't internal tach failures?
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Sorry, you are thinking so much like a pilot, not a mechanic. Sure troubleshoot a 300 dollar tachometer, and at $75.00 per hour figure two hours to find out what is wrong, two hours to try to fix it and then find out you cannot fix it, and then two hours to put the new one in. My former avionic mechanic tried that one on for style when my G1000 transponder bit the dust. Tried to charge me for 32 hours of labor time when in the end I still had to buy a rebuilt transponder from Garmin for $700.00 and one hour of labor for removal and replacement and another hour to "certify" it. My AP was pretty sheepish when he realized what happened, and removed 30 hours of labor. Now a month later when yet another component(com1) of the G1000 dies guess where I am not going? This G1000 thingamajig is a great thing when it works, but it seems that computer component do not seem to last as long as their less computerized comrades. The hangar next to me is CAP, and it seems they are always replacing the components in the G1000, so I have come to the conclusion its not me.
 
Not much "troubleshooting" is necessary on a 65 year old 140 tach and cable.. The parts cost less than the mechanic.
 
Skylane81E said:
I'm thinking like a mechanic who likes repeat customers...
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Maybe I come from an old school, we used to troubleshoot and "swap" radios on DC-8s and 727s at DHL. There was not much time to do it, specially at 4 am in the morning with those cargo airplanes ready to fly. But definitely, I would like to save $100 or $200 bucks to my customers, that's why I love to troubleshoot first.
:dunno:
 
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My fuel flow gauge read 6gph when the engine is off - but it reads ok when its running - and when it first stops it goes down to zero. Whatever.
 
comanchepilot said:
My fuel flow gauge read 6gph when the engine is off - but it reads ok when its running - and when it first stops it goes down to zero. Whatever.
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Looks like you got some dirt in the "metering" screw, this screw goes inside the inlet port of the instrument (fuel port). Have your mechanic to visually cheek it for dirt, or you need to send the unit to a shop.
 
Get it fixed right away. Sitting at 1000 RPM just parked in the hangar is going to get you to TBO really, really fast!

:D
 
WarriorPilot said:
Maybe I come from an old school, we used to troubleshoot and "swap" radios on DC-8s and 727s at DHL. There was not much time to do it, specially at 4 am in the morning with those cargo airplanes ready to fly. But definitely, I would like to save $100 or $200 bucks to my customers, that's why I love to troubleshoot first.
:dunno:
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So troubleshoot away, what could be wrong with anything other than the OP's tach?

Ok?

Troubleshooting done, repair or replace tachometer:dunno:
 
Skylane81E said:
So troubleshoot away, what could be wrong with anything other than the OP's tach?

Ok?

Troubleshooting done, repair or replace tachometer:dunno:
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If it were my plane, I'd pull it into your hangar and say "I need you to install a new tach, mine's busted." I would probably laugh all the way to the next mechanic if you wanted to diagnose it any further than peeping in the window and looking at it.
 
I would go with new tach and cable and be done with it.
 
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