After driving 100+ miles in temperatures between 30* and 40* Fahrenheit, my dash lights will dim and the in-dash voltmeter darts left to the peg. I assume this is the grid heaters cycling as all will return to normal in 10 to 15 seconds. Sometimes this cycling continues and sometimes it stops. At the advice of ThrashingCows, I replaced the Air Intake Temperature Sensor ("AITS") with an OEM Mopar unit (yes, they are still available) -- but to no avail .

Other than a gremlin in the Powertrain Control Module ("PCM"), what could cause this? I assume the grid heaters should not cycle on a warm engine, even at 30*F air temperature? Other than diming the headlights and discharging the battery, will it hurt anything?

TIA.

 

make sure it's the grid heater, it could also be the voltage regulator in the ECM kicking in and out

 

At least on the second gens, the grid heaters will stop cycling when the truck reaches approx 20 MPH ( some say 17, I don't see it until 20 ).

If the first gen is the same way, and I see no reason it wouldn't be, then you definitely have an issue, and I would say it isn't related to the Air Temp sensor. Perhaps somehow the PCM is losing power and restarting?

I'm going to let someone with more first gen experience chime in here.

 

I think the first Gen are based on intake manifold/ intake air temperature.

 

If the CPS craps out, the generator will quit. Whatever load is on the electrical system will pull down the voltage. At night with the headlights on it is pretty dramatic.

There's the CPS, it's adjustment, and 3 connectors between it and the PCM that would be suspect.

If you truly suspect grid heater, just disconnect one wire from each solenoid coil, and see what happens.

 

There is a temp sensor on the intake manifold that sometimes goes bad.
I have also seen bad connectors for that sensor go bad also.
I have a bypass switch in my dash so that I can shut the system off
when I don't need it, like when you shut the truck off and then start again
and the heaters start cycling all over again.

 

Sorry my advice did not work for you. Are you sure you replaced the proper air intake temp sensor? There is one for the grid heaters, and one for the KSB.

But as Mr. Martin suggests, disconnect the wires at the solenoids will confirm if it the grid heaters, or the charging system crapping out.

 

Thanks to all for the various suggestions. The problem is HIGHLY intermittent -- sometimes the truck will run for a solid hour at 70 mph with no issue and then the headlights begin diming in 15 to 20 second intervals.

Do you think it possible the Crank Position Sensor ("CPS") could go bad in such a highly intermittent way? If so it is relatively easy to find a new one and replace it? (Because the problem is so incredibly intermittent, I fear that diagnosing the problem is not an option -- so I am stuck with replacing parts until I get it fixed.)

Are the two Air Intake Temperature Sensors identical? The one I replaced had the same number stamped into the side of it as that referenced in your previous thread. (I used a Mopar replacement unit.)

A good suggestion; thanks to both of you.

 

Actually the sensor is a highly reliable hall effect unit, connect by 3 layers of iffy connectors to the PCM which is where the problems usually are found.

Judging from the rythm, it looks like it could be a grid heater problem. However I find something "different" in your observation.

Usually, once the battery is well charged, grid heater operation only minimally deflects the volt meter. You say it pegs left.

I suggest you either disconnect the PCM leads to the grid heater relays, or better yet, rig test lamps to the relays and position them in the cab so you can see them light. Use small bulbs or LED's so you don't overload the PCM, which is expecting about an amp draw from the relays.

The PCM might recycle and run the grid heater as if it were a new start up if the intake manifold is cool and the ignition switch cycles, even for an instant, ie bad connections. This whole problem might be a simple as a bad ignition feed to the PCM.

Don't do a lot between tests. Test/clean one thing at a time and retest. If you do too much you risk the chance of initiating a dual fault, which really gets to be fun to find.

Just because a fault is intermittent doesn't mean it can't be diagnosed. Throwing parts at it is no more valid than if the fault were solid. The trick is to set traps so you can verify what's happening, thus the lights on the relays setup.

The best way to go is to trap a measurement or indication that explains the symptoms, attempt a repair, then use the same trap to verify the repair.

Among other things that I am using for theory and diagnostics is a copy of the 1993 5.9L Cummins Diesel Truck Powertrain Diagnostics Procedures manual from Dodge. It expects you to have a DRB II scan tool, so it takes some modifications to procedures.

hope it helps
J

 

No they are not the same. One has 2 pins and the other has 3 pins. As well as they are different size threads.

 

Okay, the one I replaced was the one referenced in your recent thread (that controls the grid heaters).

In looking at my symptoms, is there any possibility my issues could be caused by the other temperature sensor being faulty?

TIA.

 

Yesterday I drove about 150 miles. The voltmeter jumped to the far left (not quite pegged) but it stayed there for about 45 minutes as opposed to cycling. Then it returned to normal for a few minutes, jumped left for another 45 minutes, and then returned to normal for the rest of the day. It seems John Martin is correct in that this isn't a grid heater problem.

This morning I started the truck for a few seconds and then disconnected one grid heater cable. The voltmeter showed the grid heaters still cycling, so I disconnected the other cable (simply to convince myself that this isn't a grid heater problem). I also pulled the electrical plug off the rear thermistor (the one for the KSB) because it looked dirty. I cleaned and replaced it.

Now the starter won't even manage a click . My battery charger is showing the battery is fully charged and the dash lights work well.

Help! I'm 120 miles from home. TIA.

 

Did you try jumping the starter to see if it will crank?

Sounds like what mine did, but it let the smoke out of the front body grounds after the lights dimmed a couple times.

If it'll crank you can at least jump power to the FSS and get home.

 

jumper the brown wire going to the starter to 12v pos. batt. and see if it cranks

 

There is VERY large red cable along with a much smaller black wire, both going to the solenoid on the side of the starter. In reading the sticky on rebuilding the starter, the small black wire is called the "Engine Start" wire.


Guys, I'm woefully ignorant -- I'll need some help identifying the "FSS."

TIA