Starter cranks at half speed

I was helping a friend with a Cat 931C that wouldn't start. Figured it was a fuel issue and found a bunch of gunk in the screen going into the lift pump. Cleaned it out but trying to prime the engine it is only turning about half speed. He said it was always slow like that. Battery was fairly new and we tried a boost pack but no difference. Cleaned all the connections but the positive cable connection on the solenoid got hot. Can a bad solenoid get hot and cause slow cranking? Never tried to engage hydraulics when cranking. Have read that if hydraulics are somehow engaged activating a function is easier than running against a relief valve. I think the starter has to come out but want to look at all other possibilities first. The guy he bought it from said he did something to it and it puts out 90 HP instead of 70 which I think might be BS. When it starts it ran good but could the engine be excessively tight?

Moisture in the fuel/white smoke engine could be tight from "cylinder wash" adding oil to the fuel helps to recoat & unsticks rings . Chassis ground is common. Pull the solenoid energize it work it could be rust or gummy from sitting emery cloth handy. Good luck bud

Yes bad contact inside solenoid could cause it to get hot I would also check wires to master switch and the ground wire I'm thinking there was a ground strap between engine and frame that being missing could cause a problem

A compromised engine-frame ground strap was the problem on our Deere 50d. New gear reduction starter/all other cables new (built from heavy welding cable). No improvement….then realized frame-engine ground bad! Now turns over at very high speed, starts immediately at all temperatures.

If an electrical connection is getting hot it is all most always a sign of a bad connection. One exception is when the brushings/bearings in the starter are bad and it is dragging. It can pull so many amps to spin it that it can heat up good connections and cables. It is always good to check the grounds. Only as many amps will leave the positive cable as can return through the ground cable. I'm leaning more towards a bad connection, cable end, or dragging starter than a problem on the ground. If a positive connection is heating up from high amp draw or a high resistance connection, the ground is likely able to carry the load. I have been wrong many times though.

CCA copper clad aluminum used for negative strap or cheap wire,the copper cracks moisture get into aluminum core & magic corrosion starts get resistance & heat

He did work on two ground cables and even drilled the hole in the lug a little bigger to use a bigger bolt. Can't take the solenoid off without removing the starter. The machine has an added master disconnect switch. Could it be contributing to the problem? It's like when you first hit the starter it wants to spin faster but only for about 2 seconds. Then it almost stops like a dead battery but picks up and spins at half speed or maybe less. Apparently it has always spun slow.

Mr. Wrench said: If an electrical connection is getting hot it is all most always a sign of a bad connection. One exception is when the brushings/bearings in the starter are bad and it is dragging. It can pull so many amps to spin it that it can heat up good connections and cables. It is always good to check the grounds. Only as many amps will leave the positive cable as can return through the ground cable. I'm leaning more towards a bad connection, cable end, or dragging starter than a problem on the ground. If a positive connection is heating up from high amp draw or a high resistance connection, the ground is likely able to carry the load. I have been wrong many times though. Click to expand... Ground cable is pure copper but large dia. wire that is really stiff. A little corrosion but not much. Cleaned it up with a wire wheel and cut a little off the end. It had a temporary battery post connector and cleaned that up too. Don't think it's the problem though.

Mr. Wrench said: If an electrical connection is getting hot it is all most always a sign of a bad connection. One exception is when the brushings/bearings in the starter are bad and it is dragging. It can pull so many amps to spin it that it can heat up good connections and cables. It is always good to check the grounds. Only as many amps will leave the positive cable as can return through the ground cable. I'm leaning more towards a bad connection, cable end, or dragging starter than a problem on the ground. If a positive connection is heating up from high amp draw or a high resistance connection, the ground is likely able to carry the load. I have been wrong many times though. Click to expand... Accidently grounded the positive wire and had a big spark at the starter so it was getting good power there. It's a ND starter with two solenoids. I don't know if that makes any difference? I've read if the starter itself isn't grounding properly could cause a slow turn but I'm leaning to the starter/solenoid being the problem. Want to rule everything else out first though because the starter is a royal PIA to remove and put back in.

Old school but one of these might help

Battery had 1035 cranking amps so definitely not the problem. Talked to a reputable auto electric shop and they said it's most likely a starter issue. Now the fun begins. I can help just don't want to be the one to crawl under it. One really good thing was he parked it with the lift cylinder support on holding the loader boom up. It's an even worse PIA to pull the starter with the boom down.

Simple test if it is a 12 volt system, Stab the negative post of the battery with one test lead and stab the frame of the starter with the other test lead. Try to crank for 5-8 seconds while watching the meter. If you read more than 0.5 volts during the cranking while a lot of current is flowing then you know you have poor cables, disconnect switches or connections somewhere. Never trust a battery without seeing how far down it drops under load, or you will get fooled one day. Also a cable can have a major corrosion happening part ways down and won't be seen but will record with the test. Simon C

Can also test the Positive side cable by hooking meter to Positive post and other lead on Starter Solenoid Hot post. Any loss over 0.5-0.7 volts on the meter while cranking for 5-8 seconds will let you know the positive side is insufficient. Simon C

Mr. Wrench said: If an electrical connection is getting hot it is all most always a sign of a bad connection. One exception is when the brushings/bearings in the starter are bad and it is dragging. It can pull so many amps to spin it that it can heat up good connections and cables. It is always good to check the grounds. Only as many amps will leave the positive cable as can return through the ground cable. I'm leaning more towards a bad connection, cable end, or dragging starter than a problem on the ground. If a positive connection is heating up from high amp draw or a high resistance connection, the ground is likely able to carry the load. I have been wrong many times though. Click to expand... These are all good things to be mindful of also. Too much draw will turn even fairly good connections into insufficient. Simon C

Thanks for this. The owner is a motorcycle mechanic and has also worked on cars and stuff so is pretty knowledgeable about electrics just not heavy equipment. Same principles though. He cleaned the grounds and positive cable connections. The battery isn't even a year old and he has a battery load tester. I'll mention these other tests to him though.

Welder Dave said: Thanks for this. The owner is a motorcycle mechanic and has also worked on cars and stuff so is pretty knowledgeable about electrics just not heavy equipment. Same principles though. He cleaned the grounds and positive cable connections. The battery isn't even a year old and he has a battery load tester. I'll mention these other tests to him though. Click to expand... A carbon Pile load tester that can load the battery to 1/2 of the CCA is reliable, but the electronic ones are ok, but I would not stake my life on the results. Have seen many that passed on electronic, but failed under real world battery load. Of course something drawing too many amps will pull a perfect battery down too far. That is what I hook up my Fluke 337 ( 1000 Amp DC Clamp On ) on the cable to see what is really happening. Simon C

Simon C said: A carbon Pile load tester that can load the battery to 1/2 of the CCA is reliable, but the electronic ones are ok, but I would not stake my life on the results. Have seen many that passed on electronic, but failed under real world battery load. Of course something drawing too many amps will pull a perfect battery down too far. That is what I hook up my Fluke 337 ( 1000 Amp DC Clamp On ) on the cable to see what is really happening. Simon C Click to expand... I agree on the load testers. The electronic ones run some kind of a test but I don't think it is as good. A carbon pile load tester puts a load on it a lot more like a starter. I was working on a machine one time that would not crank very good. The battery tested fine. The cables and connections looked ok. When you cranked it the battery voltage pulled way down and the cables and connections got warm. It presented like a bad battery or bad connections. Spent some time messing around cleaning connections and stuff. Nothing helped. I finally decided to take off the starter and take it apart. Inside I found that part of one of the magnets had broken off in a wedge shape and was jamming between the magnet and the armature. Put on a new starter and it cranked fine. Some things I learned from that experience: The numbers don't lie but don't get confused by them. I could have replaced cables and battery and not fixed the problem. I wish I had checked the amp draw, but I did not think to at the time. If you are not familiar with a machine it puts you at a disadvantage. If you don't know what is right for that machine it can be hard to tell what is wrong. Like if you don't know what a good starter draws it can be hard to know if it its drawing to much. Sometimes you have to go with your gut instinct. And than like all things electrical, you can get to confident and it will turn around and bite you.

The battery isn't even a year old but we tried a few different boosters with no improvement. Cleaned all the connections really good and the solenoid still got hot pretty quick. Pretty sure it's the problem and/or the battery. It sucks the starter is such a PIA to R&R.

Wonder if it's a motor problem and not a starter problem. How well does it run when working?

Apparently it runs great and always started right up at the low cranking speed. The seller told him something was done to it and it puts out 90 HP instead of 70. It could be BS but makes me wonder if it's related to the slow cranking?? I don't know if the injection pump can be turned up or what else could be done to gain 20 HP? If it was easy every 3204 would be turned up. Reading online about the only way to add 20 HP would be to add a turbo and it definitely doesn't have a turbo. I also read if the engine was excessively tight it wouldn't keep running for long. Regardless I think the only thing to do is pull the starter and hope it's the problem. Some people have said the Nippon Denso starters were never that good. Also D3/931's were known for electrical issues. We can do a couple more tests but I think the starter is getting lots of current.