VS Commodore possible electrical fault?

donz01, Nov 7, 9:53am
I was stopped at the lights and noticed my tacho was bouncing around 7000 rpm however the car was really just idleing in drive waiting for the lights to change. It did the same at the next intersection as well. It still run smoothly with everything working correctly until the next intersection and again the needle was up around 7000 rpm. Turned the car off when i got to the shop and then it would not start when I went to leave. Dash lights come on,Fuel pump runs, stereo starts but starter motor is not getting a signal to start. I have checked all the fuses and also relays under the bonnet. Are there any more fuses or relays hidden somewhere else or any ideas what may be the problem. My original thought was a faulty ignition module as I believe that is where the tacho gets its signal from but I tried another module with no luck. The battery is only 3 months old and is reading 12.7 volts and I also believe it is ok.Thanks in advance for any ideas and help.Cheers.

unbeatabull, Nov 7, 6:18pm
Tacho will most likely get its signal from the CKP (Crank Position Sensor). If there's a fault with it, the engine won't crank.

donz01, Nov 8, 5:36am
Hi unbeatable, I just used a short bit of wire to cross pins 5 and 6 at the diagnostic port under the dash and get a code 12. Looking on the forum where I found the info it lists the following,12* No RPM Signal (OK Stationary Engine). This is true as the motor will not turn over at all. I am going to double check all starter motor wiring etc and see if the starter is actually getting the signal to start from the ignition very shortly.

donz01, Nov 8, 6:08am
Just double checked terminals were tight on the starter motor and used a multimeter to check the voltage at the starter main battery feed 12.7 volts and then checked the trigger wire for the starter solenoid 10.8 volts. I will make a wire to try and give the solenoid 12.7 volts straight from the battery and see what the starter motor does.

unbeatabull, Nov 8, 6:39am
10.8 should still be enough to make the starter try to crank, or click the relay at least.

No RPM Signal would point towards the CKP Sensor or something in the wiring between.

franc123, Nov 8, 1:12pm
There is a ground cable on those old 3.8's attached to a stud below the alternator somewhere that if wasn't making good contact caused crazy problems, try cleaning it up, ditto the main engine ground cable. An easy way to eliminate CAS problems is to dump cold water on the sensor the next time it cranks but doesn't fire up, if it fires instantly then you need a new one. Pretty common on older V6 Commodores.

donz01, Nov 9, 11:27pm
I just put 12.7 volts to straight the solenoid from the battery and nothing happened at the starter motor.I will get it out and have it checked.

donz01, Nov 10, 7:19am
Checked the starter motor and it was all good out of the car. Found an earth cable that goes from the battery to near the alternator and it has not had the terminal correctly crimped to the cable. I wriggled it and the cable came out of the terminal. With the cable disconnected the tacho goes right around to touch the speedo needle at 0 km and the car will not start. Touch the cable to any earth on the engine and the tacho reads true and the engine starts no problem. Franc123 was spot on. Thank you very much for your advice. The faulty earth cable was only fitted about a year ago. The auto sparky that made it for me may have used a terminal too big for the cable so I will take it back to him on Monday to get another terminal fitted correctly.

franc123, Nov 10, 8:48am
Bloody good result, its the sort of thing that can potentially take hours to find if you don't know about it, and can even stump the experienced. When you described what was happening it sounded exactly like a problem I had with a police car back in the days when they were using those models. Post back anytime if you need other help, from your posts you clearly know what you are doing and know what tests should be done.