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 Post subject: Tachometer Woes
PostPosted: Mon Aug 27, 2012 3:19 pm 
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Joined: Fri Jul 13, 2012 10:14 pm
Posts: 100
Location: Kentucky
Hello all, I am finally trying to diagnose the problem with the tachometer on my 1974 SR5 Corolla. Unfortunately, both of the shop manuals I have just skip over the tachometer entirely and assume that the car is equipped with just a speedo and multi gauge. I understand somewhat the theory of how an electronic tach works, but I cannot wrap my head around diagnosis of my actual system. The issue is that when I turn the key to the "on" position (not starting the car), the tach pegs at 8k. It remains that way once started and running. This seems to be indicative of a very simple problem in the electrics, but I do not know what that problem is. Does anyone have any suggestions?


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 Post subject: Re: Tachometer Woes
PostPosted: Tue Aug 28, 2012 12:07 pm 
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Joined: Fri Feb 15, 2008 11:16 pm
Posts: 791
Location: Saratoga N.Y.
Im no help but I think my 77 te51 had this problem also??

If the cluster is out of the dash,check the back of it,the plastic grid wiring could be damaged and bad ground somewhere??

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 Post subject: Re: Tachometer Woes
PostPosted: Tue Aug 28, 2012 5:35 pm 
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Joined: Fri Jul 13, 2012 10:14 pm
Posts: 100
Location: Kentucky
The wiring behind the gauge cluster looks good, but I did notice something tracing the wire from the tachometer to under the hood. It runs from the tach to the distributer, where it is also tied in to a condenser. In my mind, this has to be the problem. Does the tach not read the rapid charging/discharging of the condenser in order to tell RPM? The condenser looks ancient, and is clearly in disrepair. The next task is actually finding another to replace it with. I don't have a part number for it, and the markings on the bottom are almost totally illegible. I assume any 12V condenser will work in its stead?


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 Post subject: Re: Tachometer Woes
PostPosted: Tue Aug 28, 2012 5:38 pm 
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Joined: Fri Feb 15, 2008 11:16 pm
Posts: 791
Location: Saratoga N.Y.
Try it without the condenser :idea:
Its probably something simple but overlooked??

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 Post subject: Re: Tachometer Woes
PostPosted: Wed Aug 29, 2012 11:26 pm 
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Joined: Mon May 25, 2009 4:11 am
Posts: 272
Location: Melbourne Australia
Verbatim from the factory body manual.

"The engine tachometer utilised on the model S indicates the engine revolution by actuating the direct current ammeter with the pulsation created during the opening and closing of the distributor points which is applied as transistor signal to charge the condensor provided on this output side, and the discharge current functions the movement."

Well yes, whatever. Other than checking the connections at coil, I would have to think the tacho itself may be suspect.

The condensor is not actually doing anything when you turn the ignition to On, without the engine started.

The Bosch part number for a 2T condensor is GD209

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