Jumping Tach?

[explicit408]

I have an intermittent problem with my tach gauge. It went away for a while after I did a tune up but came back yesterday when it started raining. At idle it will drop two hundred rpm and I notice a small change with the engine shaking a bit. When driving on the highway the needle will jump around and read about 2 hundred rpm higher that what its supposed to but I don't notice anything different with the way the engine operates. I have ruled out the basics like spark plugs, wires, pcv valve. It has to be something electrical or a sensor. Any ideas?

btw it's a 1993 rodeo v6.

[Ramblin Fever]

Auto or 5-spd??

When it's idling, you said you felt it with the engine shaking a bit, that to me isn't a sign of a bad tach, but rather a miss in the engine. If the tach itself was the issue, you wouldn't *feel* it in the engine.

At highway speed, your tach *could* be off if your clutch or tranny's slipping, or again, if you have a miss.

Which sparkplugs did you use?

[explicit408]

Its a 5 speed. I used ngk v-powers. When it happens at idle the engine changes very slightly, but runs fine does not feel like its going to die. I'll post a video of it when i get home.

[Ramblin Fever]

My Mom's 22yr old 5-spd BMW is doing this exact same thing, in which the tach is only working half the time. At first - cause she just bought this car over the summer with 200k miles on it, but it's in immaculant condition - she was a bit leary of not having it. But then it dawned on her, that she has *never* driven by the looks of the tach, but rather the sound of the engine.

Personally, if it for sure is just the tach and not an actual miss, I got to tell you, if it was mine, being a 5-spd & 14yrs old, I'd be tempted to just leave it alone.

Obviously it's not an issue with something that's only occuring while you're driving, i.e. clutch slippage. And if the truck is running great otherwise.

One thing I've learned about older vehicles, if you start messing with sensors/parts that don't really *need*, but rather *want* to replace, other parts/sensors that you end up moving to get to the one you're trying to replace end up going out too, and it ends up becoming a viscous cycle.

We went through this mess with our late 22yr old Toyota p/up, we'd replace this part, just to have that part fail, etc. etc.

I don't know what the fix is, to be honest with you; but is it something you look at when you're driving or do you go by sound?

My Rodeo is an automatic, and I DO rely, sometimes, on my tach, but only to make sure my tranny isn't slipping - and sometimes, with all our mountain driving, I use my tach as a gauge to tell me when to down-shift the tranny.

I remember my first Rodeo, '91 automatic, first truck I'd ever owned that had a tachometer in it.

[explicit408]

Thanks for the advise Ramblin. The damn thing has been working fine for 2 days now.