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tachometer problem


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I have a 1986 comanche custom with the 2.5. The problem is when the truck is idling the tach is reading 3000 rpms and when I rev it the tach goes way past 7000 rpms. I pulled the tach out and I don't see any adjustments on it. What could the problem be?

 

Also, the oil pressure gauge is reading 70 to 75 psi, is that right? What should the reading be when checked with a multimeter?

 

Thanks in advance for any help,

 

Jon

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When swapping tachs, in case of a mismatch, the difference will be 20%, Not 300%, A 4 cylinder tach in a 6 cyl will read high. A 6cyl tach in a 4 cyl will read low.

On a cold morning oil pressure can read 70PSI but should drop to normal as engine warms up.

OP sending unit resistance

0 PSI = 1ohm

40 PSI= 46 ohms

80 PSI= 87 ohms

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if it was a ground problem wouldnt all the gauges be messed up?

 

I checked the oil pressure cold and warmed up and it was 83 ohms cold and 80 ohms warmed up. My buddy has a wrangler with the 2.5 and his oil gauge reads 70 to 75 all the time....does that sound right???

 

Could there be a relay or something messing up the tach or do I just need to find a new one??

 

Thanks,

 

Jon

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if it was a ground problem wouldnt all the gauges be messed up?

 

I checked the oil pressure cold and warmed up and it was 83 ohms cold and 80 ohms warmed up. My buddy has a wrangler with the 2.5 and his oil gauge reads 70 to 75 all the time....does that sound right???

 

Could there be a relay or something messing up the tach or do I just need to find a new one??

 

Thanks,

 

Jon

You would think so but not necessarily. One of the reasons bad grounds are so hard to find and diagnose. On the '86 2.8 the tach signal goes direct from the ign coil to the tach. You can unplug the connector at the coil and check the output with another tach or voltmeter. The tach plugs into a PC board behind the instrument panel. Contacts here may be dirty.

70psi oil pressure is not right. According to the resistance reading you have a bad sending unit or the wrong one, one for the idiot light instead of gauge. Also threads may be dirty making for poor contact.

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  • 3 months later...

I too have the same 3000 rpm at idle issue. I'm prety sure the guage cluster is the origional in the truck as the milage coinsides with all documentation.

if it's a ground issue, how would your ground it?

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