Over the past few weeks, it seems my Cherokee is cranking more slowly than normal. It still fires right up quick, so it's not anything to do with fuel pressure or spark. It just acts like the battery is starting to go. It's a good-quality AGM battery barely a year and a half old, so I decided to break out the multimeter to do some diagnostics.

I'm sitting right about 12.6v with the ignition off (40°F ambient temp), and here's a video showing the voltage drop while cranking. Note that I disconnected the injectors to let it crank without starting, since I was trying to see how low it dropped while cranking.

Not bad, right? I'm running upgraded battery cables/terminals (from XJ Wonders maybe?), and it doesn't seem to be a ground issue. Time to replace my starter? I'm assuming it's original.

 

Try measuring at the starter and see if it's any different, but yeah I'd suspect the starter.

 

may I suggest a new BAT + to STARTER wire. old one runs through a harness. it is about 40 inches long. I cut off the ends and ran a new 51 inch wire from Oreilly's

 

Does seem like a hard, relatively quick descent down the hill.... One of the preliminary tests should include a load test on the "new" battery...Even AGMs aren't what they were just a few years ago...I stopped buying them because they didn't seem to perform any better {some actually worse} than a standard lead acid...Secondly I'm wondering what the amp draw is for a new fresh starter compared to what's happening with yours...

 

Funny you should say that, as rumor has it my wife is getting me a load tester for Christmas. In the meantime, I went ahead and replaced the starter, and it cranks strong now.

 

During/after cranking feel around the connections for something hot. I've seen loose connections cause similar symptoms. Also curious what voltage you get at the starter when cranking.

 

How old is the starter? When vehicles start to crank slow, often it's due to worn brushes. The stubby brushed (if that's your issue) don't have enough spring pressure to press against the commutator bars. A guy can buy new backing plates with new brushes (for Mitsubishi gear drive starters) and rebuild the starter, pretty cheap. Or get a replacement starter. This assumes your cables/clamps/battery are good. It's easy to check....remove the starter and remove the backing plate.

Also the starter's solenoid can go bad eventually. Every time the solenoid plunger is drawn back, a small sparking occurs the micro-moment before it closes the starter circuit. Eventually all that sparking for all the times you attempted to start the vehicle, wears out the copper starter contacts. The contacts start out 'thick' but wear down eventually. When they wear to a certain point, they become extremely pitted and may not make good contact, and thus don't transfer the amount of current they should. And when that occurs, a starter can operate slowly (not enough juice at the starter solenoid).

So..... there are a couple reasons that can cause weak starter operation. Could be a combination of things going on. Best to take it apart or consider getting a new/rebuilt one. Again, assuming all other wiring/battery are good.

After rebuilding my Jeep's starter, it cranks like a honey now!! Yeah, the brushes in mine were worn to a nub.

 

I second this. The problem has to be in the starter or solenoid if:
- the battery is holding voltage during cranking
- the cables are good
- the grounds and connections are good
- it cranks when you turn the key (circuit works)
A rebuilt starter will fix it.

Also worth noting that not all the starters are the same. I've seen some sold that are barely half the length of others, and I do believe the smaller ones don't crank as hard.

 

Had the old one previously been oil soaked? I've seen where the oil gets into the solenoid and contaminates the contacts, causing a poor connection. I had a YJ that the oem starter solenoid kept getting full of mud somehow. I eventually gave up cleaning it and replaced it with a NAPA house brand and she cranked twice as fast as the old one.

 

As far as I can tell, it's the original starter. It's made by Mitsubishi. I'm not sure about oil, but it was definitely covered in mud. My guess is the brushes are shot, but I'll have to take it apart one of these days and see what's going on.