Hi,

I've suddenly come down with an intermittent (only happened twice in the span of 2 weeks) P1637 fault. The first time it happened, I wasn't even aware of it as it only set the code (I was getting ready to go through emissions which is why I checked my codes). Tonight however, it happened and I kept getting the "Charging System Fault" error on the cluster. I was on the highway (30 min cruise home from work) and the entire time both the analog and digital gauges (on the cluster) read 14.1 Volts.

When I got home, I ready through this thread:
Need electrical help! Code P1637

Here is what I could check:
- My alternator is connected directly to my battery terminal using 4 gauge wire (I replaced and rerouted the factory wire when I did my headers over a year ago). With a volt meter on the alternator and battery terminal, the meter reads 0.03 Volts (0.2% voltage drop so negligible).
- I noticed that after idling for a while, the gauge cluster said the voltage was 13.4 volts while the volt meter at the battery read 14.00 volts. I brought the RPM up to/held at 3000 for about a minute and the volt meter held at 14.0x while the gauge cluster floated at 13.4-13.5 volts. Not sure why the gauge cluster would be seeing a lower voltage.

Everything was too hot (plus it's late and dark outside) to go any further, but what should I look at next?

 

If you read 14.00 volt at the battery and the gauge around 13.5v this can be caused by a voltage drop across the ignitionswitch .
Try to measure the voltage with your volt meter somewhere after ignitionswitch .
If you measure 14.00 volt on the battery and 13.5 after the ignitionswitch , you have have voltagedrop across the ignitionswitch contacts on 0.5 volt .
which is too much and will probably increase in the future and then I think your ignitionswitch needs to be replaced.

 

The DIC is downstream the ignition switch, so its reading is affected if the switch develops high contact resistance.

 

Thanks (both of you). I put the car up on stands this morning and I'll investigate this afternoon after work.

 

Before you get too involved with the alternator or switch do a good battery test. A low, weak, or dying battery can cause the code esp if it is intermittent.

 

But when running? I never have the issue on cranking (which from what I read is when the dying battery seems to be the issue), only while driving.

 

If it turns out to be a bad alternator (which I doubt) DO NOT trade it in at a local parts store. Get it rebuilt.

 

Now that I think back it was only when cranking buy is was intermittent which made me think at first it may be an alternator problem. Your battery may be fine but any electrical troubleshooting (esp on a C5) always starts with a good check of the battery condition and connections. What does the battery read across the terminals with the motor and key off after sitting overnight?

 

I'll check the battery voltage when I get home this evening (it's been sitting since last night). I expect it's good because it's always fired right up (no delay/no slow start/etc.) but yeah, C5s are finicky when it comes to voltage.

 

When I got home this evening, I checked the battery voltage and it shows 12.4 volts. It's been sitting since last night (~24 hours).

 

Okay, so I finally got around to moving further on testing today.

- I had the battery load tested and it passed, voltage 12.6 volts.
- All battery connections are good/clean/tight.
- I haven't checked the voltage across the ignition switch yet but that's next.

I checked the 4 pin plug on the alternator and got the following:
pin A - not used
pin B - 10.4 volts (with key on. 0.00 volts with key off)
pin C - 12.6 volts (with key on. 0.00 volts with key off
pin D - 12.6 volts (at all times)

I have no idea what pin C is supposed to be, but everything I've read says pin B should see 5 volts with the key on. I've checked these voltages both through the alternator (using a spare plug to check the voltage at the pins) and grounded. No change in voltage between alternator pins and ground.

So with pin B showing 10.4 volts, that doesn't seem right. The question is, where's the issue. Reading the schematic, pin B voltage is shown as coming from the ECU so that (wearisomely) suggest the issue might be the ECU. BUT, I came across another thread (P1637 L code 2003) where someone was getting a P1637 code and 10.x volts on pin B, but his issue was solved by a new alternator (which seems odd because again, voltage is coming from the ECU right?).

 

I just tried something different. I put the 4 pin plug back into the alternator and stuck a needle in pin B.

When I turn the key on, it reads ~1.5 volts.
When I start the engine, it comes up to around 8.5 volts but has some variation (sometimes jumps down to around 4 volts and then pops back up to the 8.5 volts).

What's that mean?

 

Pin B measures 10V. The service manuals saying 5V are wrong.

Pin C is a duty cycle so it's a square wave. The 12.6V is likely because the PCM has a pull up resistor to battery voltage on that terminal so the voltage goes there without the connector plugged in. When connected, the alternator regulator produces a PWM signal that drives the alternator field and is also output on the C terminal/

 

Thanks!

So in other words, everything looks correct

 

This is how my generator signals looks like on idle with minimum power consumption ( low duty cycle)
B(green) generator output , A(red) battery voltage and C(blue) duty cycle signal

 

What are you using to log that?

 

I use my Autel MK908P scanner together with my 4 channel Autel MP408 scope module .

Shown here when I check the voltage on one of my injectors

 

Okay, so odd.... the problem was getting to be regular but after I pulled/cleaned/reinstalled the 4 pin plug on the alternator, the problem hasn't resurfaced. Dirty pins? Perhaps..... I hate these kinds of things because now I don't know if the problem has actually been resolved or not.

Still no solution to the gauge cluster showing low. It seems the ECU is actually getting a low voltage because according to HPTuners Scan, the ECU voltage reads 13.4 Volts while the battery voltage is 14.4 Volts. So I need to trace down how the ECU gets it's voltage and track the voltage drop.

 

I dug further into the voltage drop issue this morning.

At cold idle, the voltage at the battery is 14.4 and the voltage at the ignition switch is 14.2. There is not voltage drop through the ignition switch.

At this point, I moved to the fuse box under the dash (passenger's footwell). On testing the "battery cable" (the 8 gauge wire that runs from the underhood fuse box to the under dash fuse box) voltage, I found it's voltage to be 14.23 volts. So I'm losing almost 0.2 volts from this cable. It seems to get worse as the vehicle heats up (perhaps the wire is getting warm and resistance is going up?).

So it looks like I need to replace the 8 gauge cable feeding the under dash fuse box. If I'm rewiring, I might as well run 6 or 4 gauge.

 

Do not buy cable from the auto parts store, they charge too much $. Get it on-line or from a car stereo shop. They will not have 6-gauge but will have 4-gauge. Might as well do it right. I would also recommend doing the same 4-gauge from the battery to the starter. Put some type of shielding on it cause it gets hot down by the headers.
(I say not a auto parts store as when my black 99 had a electrical problem my tech purchased a new cable for the battery to alternator. He got it from a auto parts store, I about chit when I saw the price.
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