Tach registers different rpm's sometimes for specified mph

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  #1  
Old 07-07-06, 02:24 PM
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Tach registers different rpm's sometimes for specified mph

I have a 91 Spirit that is doing this. One day it willshift out of 2nd at 50 mph doing 3000 rpm's. Then another day it wil shift at that speed at about 3300 rpm's.

Any opinions on this?
 
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Old 07-07-06, 02:37 PM
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Sound like sticky governor, is it 3 speed auto trans.?.
 
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Old 07-07-06, 03:55 PM
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Yes. But did you understand right? It isn't the shifting that is the issue. I am actually manually doing the shifting. It is the fact that one day, I will hit 50 mph and the tach will read 3000. Then on another day, when it says 50, the tach will read 3300. ??????????????
 
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Old 07-07-06, 04:10 PM
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may be due to the convertor clutch being engaged or not engaged if it is an automatic but dont really put much faith on the factory guages to be really accurate to begin with.
 
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Old 07-07-06, 04:54 PM
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Originally Posted by bejay
may be due to the convertor clutch being engaged or not engaged if it is an automatic but dont really put much faith on the factory guages to be really accurate to begin with.
But the gauge does work. It doesn't ocillate or do other weird things. It climbs and holds steady. It just unbelievably says different rpm's, sometimes, at the same given speed.

I have also noticed more subtle differences when just cruising at 60. I have seen the tach say 2300 (in 3rd gear now) one day and 2500 on another day.

Where do they get the electrical impulses from ,these days, for tachs. THAT may be the location of the problem. Back in the hot rod days I believe they got the pickup off the coil/distributor from I believe the electrical signals as the dwell broke.

Then I was just thinking and wondering if perhaps there isn't even a problem. Maybe the wind or uphill/downhill at the time has something to do with it? When I mentioned that 50 mph thing, I am not simply cruising. I am in a state of heavy acceleration when I notice this. Perhaps there is something else going on, where the reading is actually accurate...yet different at times..due to variables like what I mentioned? I will have to ponder on this some more.

When I was younger and actually had a hotrod with a tach, I maybe never thought of anything like this, nor noticed this, because actually I think of everything more now, than I did then.
 
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Old 07-07-06, 06:36 PM
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Is the problem with the tach only, or does the speedometer act up together with the tach?
 
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Old 07-07-06, 07:39 PM
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Originally Posted by DaVeBoy
Then I was just thinking and wondering if perhaps there isn't even a problem. Maybe the wind or uphill/downhill at the time has something to do with it? When I mentioned that 50 mph thing, I am not simply cruising. I am in a state of heavy acceleration when I notice this. Perhaps there is something else going on, where the reading is actually accurate...yet different at times..due to variables like what I mentioned? I will have to ponder on this some more.
.
An automatic will definately vayr the RPM for any given speed dependant upon the load on the engine to get/keep it at that speed.

Put your foot on the brake and hit the gas. The RPM's go up. correct?

Until the torque convertor locks up, the torque convertor will do what it is designed top do which is allow then engine to run at a higher RPM when there is a greater load. That is one of the benefits of an auto trans. There is no direct mechanical link through the TC intil the internal clutch (if it has a lock-up convertor) locks.

A TC uses a fluid coupling to do what it does.

Temp of the fluid will also make a diff. Cooler fluid is thicker. It will cause the trans to act differently. It may also vary the friction coeffieciant of the fluid as well based upon the temp.

The only fair and accurate test is a comparison with all the variables being identical.
 
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Old 07-07-06, 08:09 PM
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...and there's really no way to get all the variables identical in an uncontrolled environment.

Is this a problem or just something that's driving you batty? I really don't think you have anything to worry about...it's normal and natural. However, if it's killing you:

The tachometer gets its signal from the engine control module which comes in on the black connector to the instrument cluster at cavity B. You should have at least 1.0 volts present while the engine is running if you use a DVM on cavity B and ground. It comes out from cavity 43 on the engine control module.

The tach signal itself comes from the pickup in the distributor.
 

Last edited by davzack; 07-08-06 at 09:14 PM.
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Old 07-08-06, 04:14 PM
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Thanks for both of your replies. I am now better schooled. Now I have so much in my brain it will surely explode. [I borrowed this from a Dow engineer I was friends with years ago.]

Oh...and yes...it was simply just driving me batty...that's all it was.
 
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