bouncy tach.

T

tmz93sc

Registered User
as the title says i have been having problems with a poor idle on my sc. im getting the bouncy tach between 700 and 900 rpms upon start up. i unplugged the cam sensor and started her up after a few tries and the tach dropped to zero and the engine light came on and i could still hear the car stumbling, so i can rule out the cam sensor right? i then swapped out the ignition module with one that i thought was good from a previous sc that i had, it ran fine at first until i went to the store, start it up and it could barely hold idle... needless to say i get back home put in the old module. im leaning towards a bad cam sensor but not sure any suggestions :confused:
 
i thought it might be a bad crank sensor also but im not getting the upshift light when i start it up. today i decided to unplug the cam sensor again and drive it around and it seem liked the problem went away, there was a slight miss that was gone. i drove the car around town with no problem until i went on the highway and gave it some boost and the car fell flat on its face and stalls on me, i pull over and plug the cam sensor back in, drive back home with no problems. i should have mentioned i have no way of pulling codes, any direction would be helpful
 
When you start and run without that sensor plugged in, the computer substitutes default values so you can limp to a shop for repairs.

Some parts stores will read the codes for you - check near where you live.

When you disconnected that sensor, what did the wiring look like? Oily...damaged...ragged?

If you search the forums here you should be able to find a considerable number of similar bouncing tach episodes with suggestions on how to troubleshoot and fix, etc.
 
KMT said:
When you start and run without that sensor plugged in, the computer substitutes default values so you can limp to a shop for repairs.

Some parts stores will read the codes for you - check near where you live.

When you disconnected that sensor, what did the wiring look like? Oily...damaged...ragged?

If you search the forums here you should be able to find a considerable number of similar bouncing tach episodes with suggestions on how to troubleshoot and fix, etc.
Click to expand...

kmt thanks for the explanation about unplugging the cam sensor that makes sense now. i did a search about bouncing tach and ordered a new motorcraft cam sensor from rock auto, as far as the wiring on the sensor it looked ok to me
 
Actually, when you start run with the cam sensor disconnected the EEC-IV takes a guess. It has crank position, but it doesn't know intake or exhaust stroke. It uses a guess strategy based on cycles of key on - engine start - key off. On each cycle it makes a new guess until it starts. Once it starts the EEC-IV then makes assumptions to generate any other values needed. This is a function of the EEC-IV not the DIS/EDIS.

Bouncy tach is indicative of a DIS or CAM sensor problem. The laying down under boost can be many things, mostly related to ignition or fuel. Plug wires and spark plugs are the first place to look for that particular problem. Then sufficient fuel pressure, and coil pack or DIS module
 
the ignition control module i have now is about 3 years old, it is a standard motors module, bill at scp did not have the motorcraft module in stock at the time. has anyone run a aftermarket module with success on their sc i wonder if this is part of my problem to ?
 
I've run both - and while any part can have trouble, any time I've had issues I could lay on the DIS, it was an early Motorcraft. See this thread for discussion on DIS differences: http://www.sccoa.com/forums/showthread.php?t=98979&highlight=reduce+exit+temp

Be sure to lightly coat the bottom edge with the appropriate compound before installing and check all grounds, replacing if suspect as corrosion can hide inside the sheathing. Check the condition of the ground and mating surface from the pass. side motor mount over to the body...
 
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well i finally get around to installing a new cam sensor yesterday, and what should have been a easy 5 minute installation turned into a hour and a half headache. the bolt on the left of the sensor is completely rounded out, i even took off the sc belt to get my hands in there with needle nose pliers just to get it to turn with no success. does anyone know a place where i can get a tool to extract a small bolt like this ? :
 
If you got one bolt out, you might as well just pry the old sensor up/away until it snaps/breaks apart, then WD-40 the bolt, wait a bit and unscrew it with vice grips.

Get two news ones for the install.
 
KMT said:
If you got one bolt out, you might as well just pry the old sensor up/away until it snaps/breaks apart, then WD-40 the bolt, wait a bit and unscrew it with vice grips.

Get two news ones for the install.
Click to expand...

you know something i had thought about doing this but was afraid of doing damage to the stalk (housing ) ?? i,ll have to try this thanks kmt
 
Be careful, of course, but I don't see as if you have much choice, given the small diameter of those bolts.

Just work the free side up gently until you can get under it - maybe score the top of the sensor to help it fracture. Study the new one for weak points and good luck :)
 
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