Vaughn MacKenzie

Well I found out recently that the Voltmeter is Fake along with the Fake Oil Pressure Gauge.

A couple weeks ago I disconnected my grid heaters 'cause I got annoyed at them coming on incessantly. Well now when I fire up in the morning the voltmeter faithfully dips to 11v or whatever it is, then pops up as soon as I hit 20mph
I watched my headlights carefully, and sure enough they do not dim, but the Virtual Voltmeter is programmed to represent the expected voltage dip whether or not it's there

Whatever happened to the simple concept of actually monitoring a parameter directly then reporting a value to the gauge/ECM??

As far as the grids, I disconnected 1 wire at each relay. I get a touch of blue smoke but it's been rather mild recently. I think I've started it once or twice when it was around freezing after sitting all night unplugged and it did make a bit of blue smoke but it cleared right up.

Vaughn

Commatoze

Wow, how much more bogus can they get?

bigblock2stroke

I don't think the voltmeter is digital. Yes, the ECM is sending the signal to the gauge like the oil pressure, but the ECM is actually reading the voltage of the batteries unlike the oil pressure.

The ECM controls the current from the alternator. It would not be able to control the current from the alternator if it didn't read the voltage on the batteries. The ECM does not control the pressure from the oil pump.

Windscreen

According to the factory service manual the voltage gauge should actually give correct readings, but AFTER the grid heaters are done cycling.

To quote from page 8J-43:
System Voltage Message - Each time the cluster
receives a system voltage message from the PCM
or ECM indicating the system voltage is between
about 9.5 volts and about 15 volts, the gauge needle
is moved to the relative voltage position on the gauge
scale.

Further down, it goes on to say:
On vehicles equipped with the optional diesel engine, the
ECM is programmed to restrict the voltage gauge
needle to a position above the graduation on the far
left end of the gauge scale and suppress the check
engine indicator operation until after the engine
intake manifold air heater has completed a pre-heat
or post-heat cycle.

The service manual does, however, confirm the oil pressure gauge is just an idiot light...even though on the Cummins the L & H marks are replaced with genuine numbers. I guess DC figures it takes a bit more effort to fool us diesel owners.

Steve

Commatoze

Do you guys understand what's happening here? In Vaughn's case, he disconnects an extremely high load component, the grid heaters; pulling huge amounts of current, nearly 200 amps. If you look at the actual battery voltage (which I have) during cycling with an external DVM, the supply voltage drops to nearly 11 volts at idle, when the heaters are on. But in his case the heaters are disconnected. The ECM is deflecting the volt meter downward as if they were present in the circuit. What it's doing is sending a signal to close the relays and at the same time telling the volt meter to react to the load that in Vaughns case, isn't there. Tell you what, this is CHEESE....

Vaughn MacKenzie

So if what Windscreen posted is correct, the voltmeter acts as a "Virtual Gauge" until the grid heater cycle ends, then goes into a "normal" voltmeter mode??

This is just killin' me! It's just tooo darned much to expect a gauge to behave like a normal voltmeter all of the time

Casey Balvert

Come on guys!! For a paltry 40 grand you don't expect a REAL volt meter do you.

Commatoze

.. You know, I wondered about this before, because when I read the voltage from the internal fuse box, the voltage bounced right back up to 13.6 volts following a heater cycle. Yet, if you look at the guage on the dash, it slowly returns to it's "normal" position on the guage face. If you observe the headlights, they too pop back to normal brightness after the heaters cycle.

Casey Balvert

Wasn't that smart of DC to make the headlight also function as voltmeters? What will they think of next?

Peterbilt

It's amazing what goes through their minds. It seems it's a lot more complicated and expensive to do what they are doing, although firmware is cheap. My .02.

Pete

stevenknapp

One thought.

The A/D converter in the ECM is in the ECM. So it reads the voltage that the ECM reads. The relays are driven from the ECM. From my understanding these relays alone draw some current. Not as much as the grid heaters, but their power is sourced from the ECM

It seems possible, and maybe even reasonable, that there would be a voltage drop across the ECU's supply when the relays engaged, which is would then report to the IP.

Vaughn MacKenzie

Yeah but when the relay control wiring is disconneced it's kinda hard for them to load the ECM when the signal (voltage) is applied.

Vaughn

stevenknapp

Ah, the relays are disconnected, not just the grid heaters....nevermind.

SBishop

Vaughn,
Would be interesting if you could put a voltage meter on the batteries and see if anything was cycling, even though you have disabled the heaters.

Windscreen

As an engineer that does engine development, I can appreciate that DC did this to avoid nuisance warranty claims. I've read the FAQ for older trucks (2nd gen?) and there is a question in there something like, "Why does my volt meter bounce all over the place when I first start my truck." Most likely, there were more than a few owners that took their truck into the dealer because they thought something was wrong. In the end, it costs everyone time and money.

The engineering fix when the service guys are complaining to you: don't let the customer see the normal activity they may perceive as abnormal.

That doesn't mean I like this characteristic, but at least the gauge works normally the vast majority of the time. That oil pressure gauge is what really gets to me, though. Don't patronize me with a fancy idiot light (with numerical values for pressure, no less)! Either make it a working gauge, or just a light.

Steve
a.k.a. geeky engineer