Mechanics

ALTERNATOR ISSUE

2004 Chrysler PT Cruiser • 4 cylinder FWD Manual • 80,000 miles

I just had a new alternator installed in my 2004 PT Cruiser (base model) in addition to a new battery. I had an issue initially with the negative wire on my battery terminal not making good contact. I fixed that and the car started fine.

Drove the car around the block and returned to find the dash cut out, followed by the car shutting off. The battery was drained to approximately 8 volts.

It seems something is preventing the alternator from sending the charge to the battery and I have no idea what to look for or where to look. The fuses all appear fine and the starter relay was recently replaced new. The alternator that was just installed was new from autozone.

Any advice on what could cause this and how to resolve it would be greatly appreciated.
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Tundah11
January 10, 2011.



Measure the voltages on the three wires on the back of the alternator and tell me what you find. The larger one must have full battery voltage all the time. If it's missing, look for a very large blown fuse in the under-hood fuse box. The two smaller wires must only be measured while the engine is running. They will have 0 volts when the engine is stopped.

Caradiodoc

Caradiodoc
Jan 11, 2011.
Ok, was just able to do that test and immediately on the large wire connected to the alternator, there was no voltage, indicating the fuse as you mentioned. Can you guide to which fuse it would be and where it is located exactly?

Thanks!

Tiny
Tundah11
Jan 15, 2011.
I also got computer diagnostic codes on the dash of: •U0155-LOST COMMUNICATION WITH CLUSTER/CCN
P2503-CHARGING SYSTEM OUTPUT LOW

Tiny
Tundah11
Jan 15, 2011.
That same large wire off the alternator puts out 1.25V while engine running.

Tiny
Tundah11
Jan 15, 2011.
The battery is maintaining 16 volts engine running for 20 minutes at idle and was fine, put under load with ac and lights, still fine. However, battery light comes on in dash almost immediately after starting car. Last time this was all the same, but it died after a ride around the block.

Tiny
Tundah11
Jan 15, 2011.
Disregard the last, voltage just started dropping until the car died while at idle.

Tiny
Tundah11
Jan 15, 2011.
The alternator itself was extremely hot after that idle session. I know this because of my scalded ring finger. Lesson learned there.

Tiny
Tundah11
Jan 15, 2011.
Everything you found suggests there are two or more shorted diodes in the alternator and the unit will have to be replaced. Diodes are one-way valves for electricity. There are two sets of three of them. When one diode in each set shorts, there is a direct path to ground through them which would melt the output wire. Older cars used a fuse link wire that was soldered in part of the wiring harness. Yours should have a large fuse bolted in the under-hood fuse box.

When the fuse is blown, you will not have battery voltage on that output terminal. With the engine running, the shorted diodes will let the very high current run back and forth wide open inside the alternator rather than being directed in one direction out to the battery. It takes 0.5 to 0.6 volts before a diode will turn on and conduct in the forward direction, and since they work in pairs, that's why you are still getting 1.2 volts at the output terminal. Once the turn-on voltage is reached and the good diodes start to conduct the current, it immediately goes backwards through the shorted diodes instead of to the battery. That high current is why it gets hot.

Intermittent output is caused by worn brushes which can be replaced easily, often without removing the alternator from the engine, but diodes are a different story. Because of the high current and the very low failure rate, they are soldered in and are relatively difficult to replace. That's why you're better off replacing the whole alternator and fuse.

Caradiodoc

Caradiodoc
Jan 15, 2011.
Thanks, I was leaning towards a computer issue, but this makes more sense. I didn't want to accept it could be the alternator as this is the 3rd one that's been installed. The first was original with car, the second was used from a junk yard, and the current one is new from advanced auto parts, but at least it has a lifetime warranty.

I'll take it back and give it a shot with another one.

Thanks for the info.

Tiny
Tundah11
Jan 15, 2011.
If you want to double-check, verify there is no voltage on the output terminal, then disconnect that wire and use the ohm meter to measure resistance to ground on that terminal. I think you'll find close to 0 ohms. On the unlikely chance you find an open circuit, measure on the wire. If that reads low resistance, look for a spot where it is pinched or rubbed through and touching metal.

I don't know why you would be having so many failures. Other than the brushes wearing out at high mileage, Chrysler has very little alternator trouble. It's the GM piles that fail very often. It's real common to go through four to six of them in the life of the car.

Caradiodoc

Caradiodoc
Jan 15, 2011.