voltage gage goes to 8 volt mark

So, I'm driving along and I hear a chime, look at the dash and there's a check gages light on. Look over and voltage gage in pegged at 8 volts, pulled over and turned off engine, popped the hood and belt is still on.
restarted and after 10 seconds, gage back to 14 volts, drive 5 more miles and ding, gage at 8 again. I'm 2 miles from home, so I hooked'em to the garage, put a volt meter on battery with engine running=12.55 volts. Gage still at 8. Pulled alt. off and parts store tested it=Passed the test.
Anybody heard of this before and what do I look for.

 

Did you verify that the intake grid heaters were not improperly activating or stuck on when you were seeing the low voltage?

 

Check the crossover cable, I’m not certain about 2nd gens but on 3rd gens the alternator charges the passenger side battery and the ECM monitors voltage from the drivers side battery.

 

Intake heater is not stuck on. Both batteries show 12.55v+/- before restart and 12.16+/- after start. FYI alternator big post reads 79V AC while running. I'm assuming the 2 wire plug on the rear of the alternator is the control circuit from the ECM, I'm thinking of trying to check the integrity/continuity of the wiring, provided I can track the wire color in the loom.

 

Check the condition of the 140 amp fuse for the alternator. If this fuse is blown, it is impossible for the alternator to charge. The fuse is located in the PDC (power distribution center). Even if the alternator is not charging, battery voltage (DC volts) should be present at the alternator output terminal whether the engine is running or not running.

Check that the alternator output terminal is clean and tight as well as the connections at the 140 amp fuse. Check battery connections, too.

- John

 

I am a little rusty in my alternator functions, but the 79 VAC doesn't look right - too high by a lot. Usually should be less than 0.1 VAC ripple at the output.

 

OK, now I'm confused, in my research on my 99 I learned it was the PCM that contro0lled the output and not the ECM so could somebody verify was learnt wrong, please? BTW the alt charging the passenger side is correct on the gen 2 trucks also. I'm trying to trouble shoot a voltage drain on mine and that's how I found out about the charging system.

 

Can't help with the diagnostics on this Gen truck, but make sure the you clean all the connection points, including the fuse! Corrosion will steal voltage and cause all sort of problems! I think ECM and PCM terms are used interchangeably on your Gen truck. Good luck on the repairs.

 

ECM-Engine Control Module
PCM-Powertrain Control Module.

Two separate modules with different duties. The 12 valves only have a PCM.

Also, on 12 valve trucks the alternator charges the driver's side battery. The only leads from the passenger side are ground and the hot wire across the top of the radiator.

OP, one other thing to check is the battery temp sensor. Not likely in this case but it doesn't cost anything to check it with a multimeter.

 

@GAmes, Yes, I know the difference and was pointing out that the terms are thrown around without using them correctly! It's hard to keep up with all the different modules that are put into vehicles today let alone just a few years back!
The OP might want to do the upgrade of the cross top of radiator wire to passenger battery to help with charging and any noise issues that may be there.

 

People may use the terms "PCM" and "ECM" interchangeably on this era of trucks, but they are not the same and each have their own specific duties. In general the PCM controls the charging system, some automatic transmission functions, HVAC, etc. The ECM controls engine functions only.

The battery temperature sensor is located under the driver side battery and is wired directly to the PCM. It provides information to the voltage regulator inside the PCM. In turn, the PCM considers this information and operates the field circuit accordingly inside the alternator. The determined alternator output voltage (controlled by the PCM's internal voltage regulator) is wired directly to the driver side battery.

If there is an open circuit between the alternator output and the driver side battery, then voltage measurement at the alternator output could be very erratic and high (AC or DC).

- John

Sorry @Topzide , we were writing our posts at the same time.

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They wouldn't be used incorrectly if readers don't point out the inaccuracy as a matter of course. My favorite is the Engine Speed Sensor (ESS) on a 12 valve that is often called a CPS (crankshaft position sensor). It doesn't care what the position of the crank is, it simply measures damper revolutions. But even Genos lists it as a speed/crank position sensor.

 

Ah yes, I forgot that there were separate ECM’s and PCM’s on 2nd gen’s.

To make everything more confusing on 4th gen’s they have gone to calling them PCM’s not ECM’s.

Module, not unit.

 

With 79V AC that Alternator is shot, no matter what. End of trouble shooting.

 

Thanks. Corrected.

 

That's what I was inferring...
Then this passed test is what didn't make sense...

 

I'll check everything that ya'll have suggested, if no luck, I'll get a new alt. and see what happens.

 

140 amp fuse is blown, but why. (although it is 23.5 yrs. old) All connections were checked, crossover wire checked, Do I put another one in and see if it blows(they're only $5). I haven't jump started anything or done anything that would draw that much amperage(except grid heaters). I keep block heater plugged in when it's < 50 to help the old girl crank and lube flowing.

 

Yes. Get a couple just in case the first one blows again. Then the process of elimination begins.

 

I assume that you have reinstalled your alternator and associated wire connections. If you have, do not install the fuse. Instead, using a test lamp, clamp one end to the positive post of a battery and touch other end to the alternator output terminal. The test lamp should NOT glow. If it does, then there is likely a diode shorted to ground in the alternator, or the alternator output wire is shorted to ground (not as likely).

If the test lamp doesn't glow, all is well. Install the new 140 amp fuse. That fuse is not there to protect the alternator. It is there in case something shorts to ground between the battery and the alternator. If the fuse wasn't there and a short to ground occurred, an engine fire could easily be started as you have two large batteries there to provide the necessary high current flow for quite awhile.

If there doesn't appear to be a direct cause for the fuse to blow, I don't think I would be too concerned. It could be that the fuse is tired because it is over 20 years old. It could that the alternator output was accidentally shorted to ground during an oil filter change..., it would take much a direct short to ground to blow the fuse.

- John