Temperature sensitive Tach

[4Cornered]

Hi,

I have a 1984 GL 4WD Wagon (EA-81). The tachometer is acting strangely. I started the car this morning (22 degrees F, low of 16 overnight). The tach showed no response. I drove the car about 8 miles, closely watching the temperature gauge and the tach and being a bit of a hazard to others. The temperature gauge showed a typical increase to normal operating temperature. The heating system was putting out heat and the fan was running. The tach slowly moved up to about 1700 rpm at 55 mph, which is incorrect. After unloading with the car idling for about 3 minutes (the electric fan came on briefly during this time, 26 F outside air) the tach was responding faster, but still reading low. Drove 2 miles, then 10 more minutes idling while loading. Drove back with tach responding in normal fashion, showing roughly 2700 rpm at 60 mph in 4th gear. Tach has been working fine all summer.

 

My questions are:

1) Any ideas what is causing this?

2) Any suggestion on trouble shooting?

 

My best guess at this point is lubrication failure in tach or binding due to material contraction.

 

Thanks for any thoughts.

 

[carfreak85]

I've had this happen to my car once before in similar conditions.  I'd have to think something is freezing and keeping the tach from operating properly, but it's not a common problem for me, so I've never pursued it beyond doing some research.

[robm]

I have had my speedometer do something like this, but only at extremely cold temperatures, like -35 deg. C.  And then, it was reading really high, and very noisy.  A few mile of driving fixed it.  I can't remember if I had heat in the car on that trip or not.  

[Knucklehead Saloon]

Check your wiring connections... my tach played up until I moved the ignition on hotwire. I originally ran it from the fan circuit but it went crazy after the motor warmed up. Changed it to the stereo circuit and its good as gold. Cheers.