Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
Jeep Grand Cherokee with 5.7l. When car has sat overnight and you try to start it it will start to crank and stop. It actually turns over it’s not just a click but it only does it for a second not long enough to start. Battery is new and tests good. I can bypass starter relay and starter turns over good but car will not start even when it is turned over with the bypass. I have no coil driver signal and injection pulse is weak. Asd relay is working because I have power to coils and injectors. Win module shows valid keys and theft is not kicking in. There is no codes in any module and I can talk to all. Can bus lines all look good. I am getting ground to pcm from trans safety switch. Have all power and grounds at pcm. Nows where it gets fun. If you jump start it it will fire up and run. It will start the rest of the day just fine until it sits overnight. I guess it usually only happens once it gets cold but it has sat in our shop and still won’t start. Iv done voltage drops on all battery cables. Cleaned grounds. Iv ran jumper cables from battery to jump post under hood. Only thing that will make it start is jump start. Will not start if you just have the battery charger on it. It has to have the high amp setting before it starts. I know I lose both control sides of the starter relay but I can figure out the no start even when it’s cranking by bypassing starter relay. Hook jump pack up fires right up. Iv used jumper cables to run extra grounds and from different powers. Nothing helps. I was leaning towards the tipm having a problem but I’m also losing pcm start signal.
AFTER MORE CHECKING TODAY THE GROUND SIDE DOESNT SHOW A VOLTAGE DROP. THE POWER SIDE I HOOKED TO THE SCOPE SINCE THE STARTER ONLY ENGAGES FOR A SECOND. IT SHOWS A VOLTAGE DROP OF ALMOST 3 VOLTS. WHEN THE STARTER FIRST CRANKS IT DROPS A VOLT BELOW ZERO AND THEN SPIKES TO 3 VOLTS RIGHT BEFORE IT STOPS CRANKING. I THOUGHT WE MIGHT HAVE A BATTERY CABLE PROBLEM SO I MOVED THE BATTERY FROM UNDER SEAT TO RIGHT BY THE JUMP POST UNDER HOOD. HOOKED NEW BATTER YCABLES DIRECT TO EVERYTHING. SAME PROBLEM. I RAN JUMPER CABLES AGAIN TO WHERE ALL THE OTHER POSTIVE CABLES HOOK ON TO. SAME PROBLEM. HOOK BATTERY CHARGER ON FULL START AND FIRES RIGHT OFF. I CANT FIGURE OUT WHERE IM LOSSING THE VOLTAGE AT.
Can you check voltage drop from battery negative terminal to block ground or starter case while cranking? If good go from block ground to body ground (fender strut tower etc). Then after go from battery positive terminal to positive post of jump start post under the hood?
I have checked ground side from negative on battery to body, block, and starter. Most it ever shows is about 30mv of voltage drop. When the starter engages I drop to 7 volts. I checked them from the positive battery post to the jump post under hood which is where everything splits from and have a voltage drop of 3 volts when the starter is tried.
