Intermittent Tach issue (stumped)

Kurt K

SCCoA Member
I've had an intermittent tach issue for a few years now. I will be cruising around and then the tach will go to zero rpms--sometimes just briefly, sometimes long enough to throw a check engine light. The issue seems to occur more recently the hotter, or longer I've been driving the car. I finally decided to change my cam sensor with a brand new Motorcraft sensor. Unfortunately, this did not fix the issue and about 20 minutes into my test drive, the tach went to zero and through a check engine light. I have not tried pulling codes. The light goes away as soon as the tach starts working again.

Ideas?
 
I can tell you this. I installed a WOT box on my 5 speed car. I wired the rpm signal wire into the crank sensor. And the coil power wire goes thru the WOT box. When I set it on the 2 step for a few seconds the tach drops in and out as the WOT box controls the rpm and the check engine light will illuminate then go out a few seonds later after I stop the 2 step.

So maybe something to do with the coil or coil power wire?
 
Good evening


i would inspect the connectors at the DIS harness. Might have two separate problems. The shield connector for the intermittent EEC light and exposed/frayed wiring connectors on the CAM sensor and DIS for the intermittent RPM drop.
 
I had the same problem in my car for a long time. I swapped dis modules and thought that fixed it for a few months. Swapped the crank sensor and thought that fixed it. Later swapped the cam sensor and thought that fixed it. My next step was to swap the section of harness by those three items. I suspect that it worked for a while because I was moving the harness everytime I replaced one of those items. I ended up doing the 351 swap before I ever solved the problem. I also have a gauge that reads rpm for the shift light that picks up it's signal from the dash harness. I doubt that's it, but that is an idea that I had not thought about.
 
For what it's worth, I had a similar situation where it was a bad tach and I went through all the same steps and head scratching as you are doing.
-Tim
 
Going through exact same problem. I have replaced cam sensor, dis module, coil pack in that order. Wierd part is I never got the upshift light while cranking which means bad crank sensor. After replacing all those parts,now I get the upshift light and will not start what so ever,not even intermittently. I have a new crank sensor that I will be puting in this weekend. Fingers crossed, if that doesn't work I'm stumped after that.
 
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