1996 Silverado 6.5 turbo diesel 5-speed. Will not turn over, will not accept a jump

Need help please! Problems started 2 days ago, after weather got colder. Related? Can hear starter engage (loud click) but not enough juice to spin engine. Only way to start is to get rolling and pop clutch. Will not start when I attempt to jump start. When running, battery emblem in instrument cluster is illuminated and voltage gauge shows very low. Batteries (two 12-volt 800cca batteries) old and worn out? Alternator gone bad?

Battery light on and low voltage is usually an alternator issue. If the batteries are not fully charged by the alternator, then they can not start the truck. Make sure you have the batteries fully charged before replacing the alternator. Alternators are not designed to fully charge dead batteries. They are only designed to maintain voltage used well the truck is running

Batteries are shot. Same thing happened to me with no warning. Wouldn’t take a jump. Borrowed a car from work and got two new batteries for my diesel and all was well. The starter is 24 volts so you need both in good shape.

Have a load test done on the charging system. That will take the guess work out here and point to the real problem. There could be a problem with the starter solenoid contacts causing the starting issue. Dirty solenoid contacts can disable the starter motor from turning. Corroded battery connections or internal wiring can also do that.

Problem solved. The thing randomly started so I took and had the charging system and batteries tested. Alternator was pushing just over 14 and both batteries tested bad. Replaced both batteries. Upon inspection, one of the batteries was bulging severely. What does this possibly indicate?

A bulging battery equals a dead/shorted out/ or overcharging battery. It was time for new ones no doubt. Be thankful that bulging battery didn’t damage the electronics.

The bulging battery could also be a sign that it had frozen at some point in time if it was severely discharged during the winter time. You were wise to replace both batteries at the same time since they work together when starting the engine and running things. That should keep things close to equal for keeping them charged up and easier on the alternator. Keep the battery connections clean as a lot of folks have found out the hard way what can happen when you don’t do that simple maintenance task. It sounds like the charging circuit is working okay and I assume the testing showed that to be the case. Thanks for the update.

@mrmartin6 What sort of damage could result? I also had the blower motor malfunction around the same time. The air will not come on except on “high”, and even then the fan barely spins. Related?

If the battery shorted internally it could have sent a voltage spike thru the system. But you would hope that the fusable link or other fuse would blow first. But it could have fried the fan motor relay. Check that fan relay.

When trying to jump it are you going to a ground other than the battery?

When the blower only works on the “high” setting it usually means that the resistor pack for the lower speeds needs to be replaced due to broken resistor coils. This is a very common problem. The resistors are mounted on a panel that is mounted on the air ducting near the blower motor for cooling. Wires from the motor should lead you to the panel. There could be another issue going on since the motor doesn’t seem to work well at high speed also. You could try running power directly to the motor to see if it works okay that way. If it does this would mean there is some trouble in the connection to power. The relay contacts or the socket connections for it is a logical spot for the problem. Look for signs of burned connections. If there is something causing a drag on the motor to slow it down that could cause excessive motor current and damage things.

Most blower circuits supply 12 volts to the blower motor via the relay and the resistors along with the switch connections are on the return side of the circuit. If your design is that way also you should be able to manually ground the return side of the motor wire and make it run at full speed if there is no problem with the wiring on the power side.

Batteries work using a temperature dependent chemical reaction. The lower the temperature, the less energetic is the battery. I expect that is why you are noticing this now, winter and all. Good that you had the batteries tested and replaced. A severely defective battery can damage the alternator. So you might well have saved yourself some money by addressing this promptly. I suggest you take this money you saved and buy some Christmas presents!

@Cougar -

Thanks for the clarification. Vehicle electronics were never my strong suite.


Replace the blower resistor pack, as others have stated

Make sure the blower pigtail and the resistor pigtail aren’t melted

GM is well known for melted pigtails

Another thing, inspect the connector and terminals on the blower itself. They sometimes also melt

@Bing: Are you sure the starter is 24V? I thought the batteries were hooked up in parallel, giving 12V with double the reserve capacity/cranking amps.

All the diesel engines I am aware of use 24 volt starting systems then revert to 12 volts for the running operations.

All of the class 8 diesel trucks in our fleet have multiple 12V batteries hooked up in parallel

The starters are clearly marked 12V

Thanks for the info @db4690. I must admit, the trucks I was referring to are older models. It sounds like a change in design has happened at some point in time. I assume that this was perhaps done to save on costs.

We’ll it’s been over 20 years but that’s what I remember and why a new one was $300 back then. I’ll have to look it up when I get back home. It needed more than just capacity to start though and had to spin much faster even with glow plugs. It’s not an era I look back on fondly.


Now that you bring it up, I’m trying to picture the US Army GM diesel trucks and HMMWVs from the mid- to late 80s, because we had them in the depot. They used the GM 6.2L V8 diesel, and they had multiple batteries. I’m not entirely sure, but I seem to remember that the 2 12V batteries were hooked up in series, which would imply a 24V starter

We have a 1996 GMC 6.5L V8 diesel in our fleet right now, and it’s definitely a 12V system.