6-71 Starting Problems

[keithshotrodshop]

I finally was able to make the final leg of the journey and get my latest silversides home. It is a 1948 PD3751 with a very healthy 6-71. 5 months ago, when I first started my drive home with it, it started up every time without a problem. Well, I had some trouble on the road, and ended up pulling it into a repair facility in Cleveland. I had several different things done to the bus, including a conversion from a positive ground system to a negative ground system. Now the bus will not start when it is warmed up, only when it is cold. When it is warm it trys, but it can't even turn the motor over half a turn. The batteries are fully charged and healthy. My first thought was that the starter was dying. So I swapped the starter out from my other bus, which I have driven thousands of miles, and it is doing the same thing.

I did notice that the wire from the starter to the junction block was badly cracked and corroded. I replaced it with a new one, and it is doing the same thing. The wire from the junction to the batteries is also in poor shape, but not horrible, so I plan to replace it within the next few days. Do you think that will be the problem? If not, is there something else I should be looking for? Do you think it has something to do with the conversion over to negative ground, now that the flow of electricity is going the opposite way through all these old wires?

[Dreamscape]

Are any relays getting hot?

[gus]

This is a voltage/current problem to the starter. The bad cable will be very hot after some starting attempts with the engine hot.

You have a poor connection somewhere or maybe a missing ground, possibly from the engine to the frame/body. It could have been left off during the neg/pos conversion. I'm sure your 4103 body is grounded to the battery at the battery box like my 4104 and this connection must be kept clean since it is two different metals in contact.

Also completely remove the ground bolts in the body and clean them so they shine. They are probably rusty. If so, change them to new bolts. Change all the corroded washers at the ground bolts and make sure the Al bulkhead surface that contacts the washers or cable end is shiny clean. New cables will not help unless the connections are clean.

Fully charged batteries (are you absolutely positive about this, did you load test the batteries?) and the starter changeout prove that the wiring is the problem. Relays are not the problem. It takes little power to operate the relay and solenoid, but lots of power to spin the engine.

The polarity change has nothing to do with power, the starter could care less.

Clean every large power cable connection and I mean every one. You will be surprised at the amoung of corrosion you will find. Also make sure your battery posts and clamps are clean so they shine. Dark colored lead connections are corroded.

Large twisted cables very often severely corrode inside the plastic/rubber cable cover and some may need to be replaced. The cable ends can also corrode badly where the connect to the cable.

It is harder to start when hot because there is higher compression opposing the starter.

[Stan]

My guess is that the polarity change reversed the electrolysis in the copper lug/aluminum chassis joint and there is no longer a good connection at that point. Gus' suggestions should solve the problem.

When trouble shooting a starter problem, always use the ground terminal on the end of the starter as a reference point. Don't connect your meter lead to the frame or chassis until you are sure that there is not a voltage drop between the ground bolt and the frame.

[Lee Bradley]

Years ago, we had a similiar problem with a 6-71 on a sawmill. Crank great when cold but would not crank hot. After starters, batteries and cables, we found out our fuel supplier had gotten some gasoline in a diesel compartment of their tanker. Diesels don't like gasoline; any chance you could have gotten some gasoline?

[keithshotrodshop]

No gasoline, and I actually have a PD3751, which has no relay. It is set up just like a Chevy V8 with a solinoid on the starter that has a cable going directly to the battery. As for the ground cables, they are in great shape, clean, and there is about 4 of them going from the block to the body, so I'm thinking the problem lies in the wire going from the junction block to the battery. I'm planning on digging into it in the next couple of days, so I'll let you all know. Thanks for the help.

[gus]

Keith,

I'm pretty sure you will find that your solenoid is energized by a relay in the RH rear electrical compartment. This relay allows a very small wire to be connected from the starter switch all the way to the rear of the bus. I don't think this relay is your problem because it is low current and your problem is in the high current circuit. This relay is just a switch to energize the solenoid.

All old GMC starters have a cable directly from the battery to the starter solenoid but the energizing coil inside the solenoid is hooked to the starter relay. I think most heavy duty vehicles use this system, all 10 of mine do.

This direct starter to battery cable is the reason it is a very good idea to have a battery cutoff switch- in case the solenoid contacts ever stick. Trying to start the engine with a weak starting system is one of the best ways to make these contacts stick. Then the starter stays engaged until something burns up-like the starter or a cable, even with the engine running. Don't ask me how I know this!!

There is very little mechanical difference between the 3751 and 4104, mostly in the suspension. The drivetrain is probably exactly the same and the elect system is also probably essentially identical.

[Stan]

Quote "As for the ground cables, they are in great shape, clean, and there is about 4 of them going from the block to the body," Unquote

None of those cables ground your starter. A DD starter is isolated electrically from its own case. The ground terminal is a large bolt sticking out of the end of the starter. That bolt should directly to the battery (preferred) or else to the engine cradle and then to the battery.

[roadrunnertex]

Well,
I had the same problem with  PD4104 many years ago.
The starter would work fine when it was cold.
But once it got hot from running the engine after it was started.
It would not turn the old 6/71 over once it got warm.
Even though you made mention that you had swapped with a used starter and the problem did not go away.
I sent the old starter out for a rework and it had a bad (shorted) field core worked fine when cold but the old field core would open up when warm.
Got the starter back from overhaul no more problems.
jlv

[Dallas]

Many times a starter that will turn fine when cold won't turn well when warm because the end bushings are worn and need replacing.
I don't think that's the case here, but thought I would bring this up because many starters get rebuilt when all they actually need is a pair of bushings.

I've seen the bushings on a Delco starter that were so bad that the armature shaft was running on the cast metal of the housing.

As a quick fix when you find this condition and absolutely, positively have to get the engine running, try removing the end plate from the starter and using a punch, blunt screw driver or even a nail and dimple around the end of the bushing at equidistant points.
This swells out the bushing and allows the shaft to run true.
Do the same thing to the bendix end of the starter.

Note: DO NOT pound hard on the bushings as you will deform them too much, just a light tap in four or six places will do it.

This is just a temporary fix and will not solve your problems, but it will possibly get you out of a bad situation. You may even get 50 or 100 more starts out of the starter before rebuild.

Just thought I'd mention this little quick fix.

Dallas

[kingfa39]

I think if you follow the thread gus wrote you will probly fix the problem, i go over all my grounds once a yr, the one that goes to the bulkhead is often a problem, also do yourself a favor and check the relay in the right hand rear electrical panel, mine went bad and left the contacts flopping around, we were sitting in it having coffee and the engine started turning over, burned up the starter before i could get around there to stop it, it can engage going down the raod as well, however that wont be your current problem. Gus has the answer in my opinion, grounds are often over looked
Frank Allen