What is the deal with a weak battery not starting a diesel truck? Is it that the glow plugs are not hot enough? Or that the engine isn't turning fast enough, even though it sounds like it is.
I see weak batteries talked about a lot as far as a reason for not starting, but it doesn't make too much sense to me. It seems if they are strong enough to crank the truck, it should start. Can someone please explain this further?
My reason for asking is that I suspect I have bad glow plugs (the relay is fine and it starts after running the block heater). But I have the original batteries (on my '97) and am wondering if this could be the problem. It would be a much easier fix, but I don't want to buy new batteries and then change the glow plugs anyway... The truck cranks just as fast and strong as it ever has. 146k miles by the way.
Any input would be appreciated.

 

Sounds like you do have bad glow plugs to me.
Bad batteries will allow the voltage to drop too far during cranking. Less that 10 volts, and the IDM won't fire the injectors. And, the cold just increases the rate of voltage drop, so, that's why bad battery compaints show up most often in the fall.

 

If you see white smoke when you crank and no start its has a glow pug problem you have fuel. No smoke during crank no fuel! If the batteries are bad when you crank the vlotage is not enough to let the injector Drive module to provide 115 volts to open the injectors for the fuel.

 

I just had this happen to me the eye in the battery on the drivers side was red so I replaced the battery since the passenger side battery still showed green in the window but little did I know that the window only shows one cell of the battery and the truck started fine for a day or two then stranded me 40 miles from home then after some checking around I found out that you almost always have to change both batteries when one goes bad unless you catch it immediately because the bad battery pulls off of the good one and melts the lead together in the bad one so I replaced the other one and no problems at all.

 

So, if I understand correctly, if my batteries are too weak, I get no fuel, so no white smoke. I have white smoke so must be my batteries are good and my glow plugs are not.
Thank you for the help

 

Think about holding two 9-volt batteries in your hand. If you hook them up in series you have 18 volts. Even though you have 18 volts you can't crank a truck with them. That's because the batteries don't have enough capacity to put out the amps necessary to feed the truck. The batteries have to supply the starter motor, PCM, IDM, glow plugs, and no telling what else to crank the truck.

If you have a poor battery cable connection somewhere you could be running on one battery basically. All batteries have a certain amount of capacity. If your battery/batteries are weak you will get an excessive drop in the voltage as the current rises. This means that a 12 volt battery under a load could only put out 10 volts, and that's if it's good. If the battery is bad you might only see 6 or less volts.

The way to know for sure is to have a battery load tested. The battery is required to put out a certain ampereage for a certain time, while still keeping a certain voltage. The specific amounts all depend on the size of the battery in question. Load testing is the way to make sure that you are replacing the correct part. You can also load test through the battery cables. If you get worse performance with the cables on than with the cables off, then at least part of the problem is in the cables. Poor cables can also lead to premature failure of the batteries. This is because the charging system must put out too much voltage to have enough left at the battery. Over charging is a sure way to kill a battery. A poor cable or connection could actually bring down the whole charging system, regardless of how many other parts are changed out.

IMO I think that many of the batteries that go bad go bad due to problems elsewhere in the system. If everyone had their batteries load tested before they changed them out, I'd bet that close to a third of them did not need to be changed out.

Here is a link to a very informative site that has some very good battery information:
http://www.uuhome.de/william.darden/
Maybe this will help explain the battery end of things.

As far as the glow plugs are concerned, I think that the only way to know for sure is to read the amperage going to the glow plugs. If it's excessively high it would be due to a grounded glow plug. If it's low it would be due to plugs that are reading open. The more bad plugs, the lower the amperage. (at the GPR)

My 97 was giving me some starting problems last week and I discovered that the GPR was letting voltage pass through it like it is supposed to, but there was no current (amperage) flowing through it. This was due to internal resistance in the GPR, probably due to worn and/or pitted contacts in the GPR. Just checking it with a test light indicated that it was good. That's because a test light can only check for voltage, not current.

The block heater letting it start makes me very suspicious of your glow plug system. I would suggest that you check the current on the big wires on the GPR and see what's going on. I don't know what the specific values are supposed to be, but I would guess that you have little or no current to the plugs even though the GPR clicks and there is voltage on the output terminal.

Sorry if I made something more complicated than it had to be. Hope this helps.