My 1998 12V has been intermittently cranking slow for about 6 months. Recently it started happening more often and more severely. It would crank slow and then sometimes stop turning at all but with the starter still trying. I assumed it was the batteries so the first time I couldn't get it to start at all (about a week ago) I bought 2 brand new Exides. No change.

I can still get it to start after a few tries and once it starts it runs great. No smoke, no noise, good power. I checked the resistance of the battery cables and they were fine so I pulled the starter. It had never been out before and with 220k and a lot of short trips I figured it was toast and took it to be rebuilt by a very good local shop. They reported that the contacts were rough and the armature was shorted and did a full rebuild. I put it back in tonight and... no change. By watching the fan when cranking it seems to go normal speed until it gets to a certain point and then the engine just stops and the starter strains against it. Again once it does start it runs great.

With a socket on the alternator I can turn the engine but it is pretty stiff in some spots. I checked the coolant and oil and they aren't mixed so I don't think a leaking head gasket is allowing water into a cylinder and hydro-locking it. I also took the belt off in case the AC compressor, alternator, or something is causing an issue. No change without the belt.

That's as far as my diagnostic brain can go tonight. Any ideas here? Could the PS pump, vacuum pump. or injection pump cause this?

 

Make sure theirs full battery voltage from the key switch to the solenoid,low voltage to the solenoid will make a starter seem weak. If it seems week run a jumper straight to the starter solenoid see if that helps .

 

I would also measure voltage drop on the positive cable. Do this by hooking a voltmeter in parallel with the positive cable from the pos battery terminal to the starter end of the cable. A good cable should read no more than .5 volt during cranking. Then do the same on the negative side to measure voltage drop on the negative side. run the voltmeter from the negative battery cable to the starter housing. Same spec during cranking.

 

I would use a barring tool and bar engine over several times to confirm binding is not internal engine problem first. Pull the lower cover on bell housing and check flex plate for any damage to gear. If you don't have a barring tool try using large screw driver to pry gear on the flexplate, the engine should turn over smoothly without any excessive force needed to turn crankshaft. Mark the gear so you know when you made complete turn of crankshaft.

 

Remove the engine grounds, clean very well and try it.

 

A voltage drop test is best for troubleshooting this problem. Only do not use the cable terminals. Go from battery positive POST to starter cable positive POST. Then battery negative POST to starter housing. If you get a large voltage drop then you can test voltage drop at the terminals.

A resistance test of the cables is not a reliable test in a high amperage circuit. If a cable is corroded 7/8th thru, that last 1/8 of a cable will pass a resistance test. The cable will not pass high amperage. So starter does not work right.

 

I'll check the voltage drops tomorrow morning. I have a very strong suspicion that is the problem. Thanks for all the help, everyone. I'll keep you updated.

 

I checked voltage drop both + and - from the battery to the starter yesterday while cranking. There was less than a half volt on each which surprised me. Then miraculously while checking the ground side "one last time" it started cranking really fast. I shut it down and restarted it a half dozen times and it cranked perfectly every time. So now I "fixed" it but without doing anything... great. I'll see if I can get it to show the bad symptoms again tomorrow. Any ideas?

It looks like my + cables could use replacing so I'm looking for a source of some 2/0 cable and terminals where they wont cost me a mint. I'd like to make new cables for another car and a couple sets of HD jumper cables too while I'm at it. Anyone have a favorite source for welding cable?

 

Lucky for me my ctd cranks like it was hit by lightning with my 2 AGM batt's but I had the exact same problem with my VW Diesel as your are with your RAM- Doubble Check your grounds really well as I eneded up adding a second ground on my VW and it fixed it right up. Unplug your grid heater and see if it cranks faster

 

Was the less than half a volt reading gotten during the slow cranking or when it started cranking good. If during the slow cranking, it is a strange reading.

If the problem reappears jump the starter and see if it will work normal. Or run a jumper cable from battery positive to the solenoid terminal. If you get a normal start, the battery, cables, solenoid and starter are all good. You then would need to look elsewhere for the problem, maybe the starting circuit, starter relay.

 

Today it was back to cranking slow. No more than a half volt on either the + or - side whether cranking fast or slow. I tried jumper cables to the starter on Saturday and then again today. No noticeable difference.

My next door neighbor saw me playing with it an came over. He's a long time gearhead and retired electrician. His opinion was that the voltage drop we saw wasn't enough to be an issue. We redid all the battery terminals because he had the stuff on hand just to make sure we eliminated that possibility. Still cranking slow. He and I are both confident that the batteries and cables are not causing problems.

I'm taking the whole truck to the shop that rebuilt the starter this week. I think they missed something. I'll update with what their diagnosis is.

 

very possable, there are 2 starter contacts so if only one is makeing contact that would nearly stop the starter

 

My bet is a battery cable is corroded almost all the way though way under the insulation where you can't see it.
What happens is a heavy load or moving the cable around can temporary "weld" the cable back together.

 

Wouldn't this show up as a voltage drop across that cable when its under load? I get 12.8V at the battery which drops to about 10V when cranking. At the starter I get the exact same numbers.

 

It will show an amperage drop under load.
Voltage may be the same.