Diesel Preheat/Heat Grid Question?

[tja1977]

I have two Ram tucks, the first is a (2020 Ram 3500 6.7L HO Limited) and the second is a (2020 Ram 5500 6.7L Limited). The question I have is regarding the Diesel Preheat Message for the 6.7L diesel engine.

On my 2020 Ram 3500 6.7L HO, if the temperature out side is in the low 60's (or below) the truck will have the Diesel Preheat Message come on the dash and it will count down for approximately 10 seconds before the truck starts. Once it starts the diesel engine runs very smooth.

On my 2020 Ram 5500 6.7L, I have never had the Diesel Preheat Message come on the dash. The truck will start immediately as soon as I press the start button. In the last week I have had a couple nights that the temperature has been down to the upper 20's already. When I start the truck, it cold starts with out any issues, but it runs very rough for the first few minutes during start up. I have had all the current recalls completed (W57, Y21, Y17). I have had the truck to the dealer and they actuated the Heat Grid with a Scan Tool. They came back and said everything is checking out correctly regarding the Heat Grid on the Scan Tool.

I have not plugged in the Block Heater on either truck.

Has anyone else had this issue? Are the 6.7L HO and 6.7L diesel engines calibrated different regarding cold temperatures? I am concerned that as winter gets colder I am going to have an issue with the Ram 5500 not starting, or should I even be worried?

Thank You in advance!

(I have attached pictures of the Heat Grid "Wait To Start Times" provided by my dealer below.)

 

[Southernspeed]

Switch to battery voltage display and try to start it. If the batt voltage doesn't drop you're not getting the grid heater switched. If that's the case I'd suspect the related temp sensor. I'd expect it to act just like a 3500 in that regard. The rough idle at start up would suggest it isn't working when it should be.

 

[H3LZSN1P3R]

There was a TSB for this i dont know if it included the chassis cabs though

 

[tja1977]

OK, So this morning the Temperature on my Dash showed 28 degrees. I already had the dash information set to battery voltage. I pushed the start button and the truck immediately started without any issues. I still did not get a Diesel Preheat Message on the dash. (I did get for s split second, a "Cabin Heat Up" Message, I think is what it said.) The battery voltage was initially showed 11.3, at 5 seconds 11.8, then at 10 seconds 12.3, then at 20 seconds 12.8, then at 30 seconds 13.8. Then after letting the truck warm up a few minutes I noticed as I drove the truck showed 14.2/14.3. I do have duel alternators, I am not sure if this makes any difference or not.

Are these normal battery voltage numbers? Since the battery voltage was lower during start up, does this mean the Heat Grid is working?

Do I have a bad Temperature Sensor that triggers the Heat Grid?

Thank You, In advance!

 

[H3LZSN1P3R]

OK, So this morning the Temperature on my Dash showed 28 degrees. I already had the dash information set to battery voltage. I pushed the start button and the truck immediately started without any issues. I still did not get a Diesel Preheat Message on the dash. (I did get for s split second, a "Cabin Heat Up" Message, I think is what it said.) The battery voltage was initially showed 11.3, then 5 second later 11.8, then at 10 seconds 12.3, then at 20 seconds 12.8, then at 30 seconds 13.8. Then after letting the truck warm up a few minutes I noticed as I drove the truck showed 14.2/14.3. I do have duel alternators, I am not sure if this makes any difference or not.

Are these normal battery voltage numbers? Since the battery voltage was lower during start up, does this mean the Heat Grid is working?

Do I have a back Temperature Sensor that triggers the Heat Grid?

Thank You, In advance!

Numbers indicate the grid heater is working, from what i was told its the intake air temp sensor that triggers the grid heater…. I wonder if it has something to do with the HO 3500 vs SO 5500 as my SO wont run the grid heater till 32* Even then its not always

 

[tja1977]

That is also what I am beginning to think, is the HO is different from the SO. The only question I have with that is, the Heat Grid "Wait To Start Times" provided by my dealer does not specify a difference between the HO and SO. I just don't want to be stuck in the middle of winter or tear the engine up due to something simple.

 

[Brutal_HO]

Diesel trucks have supplemental electric cabin heaters. You could be seeing the draw down voltage from that.

If you don't have AlfaOBD to do some testing, perhaps pull the fuse for the cabin heater and do the same test to see if it's the cabin heater or the grid heater drawing the battery voltage.


If there's any difference, it's not HO vs SO, it's pickup vs CC truck.


ETA: Scratch that first part, the heaters (PTC) don't come on unless the engine is already running and battery voltage is >12.6V.

Diesel Cab heater

I hear that the ram trucks with diesels have a extra cab heater, electric maybe? Has anyone used it? Have any info? Cab warm up super quick?

hdrams.com

For reference, the fuses and relays for the PTC in the Aux PDC as shown.

 

[Southernspeed]

We are getting down into the mid 30s some mornings now and the last few days is the first time I've had to wait for the pre=heat. I didn't time it but I'd guess at 4-5 seconds. That's with the truck kept in a garage so it's a bit warmer than outside temps. 3500 HO 2021

 

[Jacob]

I have an issue with mine. I would say the grid heater typically comes on below 60 degrees ambient temperature with engine cold.
But several times it has been 10 to 30 degrees with engine completely cold and the grid heater didn't come on. Probably 1 out of 10 times it doesn't come on when I think it should. I would like to get it fixed but doubt the dealer would be able to duplicate the fault.

 

[Jacob]

Also on my truck, when the grid heater, does come on, there are two different messages that it displays but cant remember the exact wording. After the grid heater process I usually have to hit the button again but a few times it has automatically cranked and started.

 

[tja1977]

Brutal_HO​

Thank You for the Link to Diesel Cab Heaters. I read over the entire thread and another link on the Cummins Forums regarding the Diesel Cab Heaters. If I am understand correctly, it does seem that the HO and SO engines might engage the Grid Heater at different temperatures due to the compression ratio being different in both engines. So, maybe my dealer gave me a copy of the Heat Grid "Wait To Start Times" for an HO engine, because my Ram 3500 HO follows the Heat Grid "Wait To Start Times" table exactly.

 

[Will_T]

I have an issue with mine. I would say the grid heater typically comes on below 60 degrees ambient temperature with engine cold.
But several times it has been 10 to 30 degrees with engine completely cold and the grid heater didn't come on. Probably 1 out of 10 times it doesn't come on when I think it should. I would like to get it fixed but doubt the dealer would be able to duplicate the fault.

Not sure if I really have an issue or if there is a reason for it but mine does the same. I have had it preheat many times in the 40's and 50's. But I have also had it start immediately without a preheat at least a few times when temps were in the 40's. Those are the coldest temps we have had so far so I don't know if it happens at lower temps also. All times the truck was cold having set for at least a day or two since the last start. I recall being surprised at the immediate start after putting foot on the brake and pressing the button once and thinking along the lines of: "wonder why there was no preheat, it is 41 degrees and yesterday it preheated at 55 degrees". Seems a bit inconsistent.

 

[tja1977]

I have been testing my Ram 3500 HO/Pickup and my Ram 5500 SO/CC over the last couple weeks and the Preheat/Heat Grid times do seem to be different. My Ram 3500 HO/Pickup follows the Heat Grid "Wait To Start Times" chart (provided by my dealer above) exactly/correctly. My Ram 5500 SO/CC does not seem to preheat until the outside temp is 26-27 degrees Fahrenheit. Anything below the 26-27 degree temp and it will preheat 5-10 seconds depending on the temp. My coldest night so far has been 16 degrees and it preheated almost 10 seconds. I hope this helps someone else out who might feel like their truck is not preheating either.

 

[Brewbud]

Low 50s and the grid heater will come on in my SO. I usually bypass it. It will also do a preheat if it was cold overnight but the air temp has quickly warmed up. Last Saturday it was in the high 30s overnight. It was almost 70 by midmorning when I started the truck and it wanted to do a preheat.

 

[#FJB2024]

Diesel trucks have supplemental electric cabin heaters. You could be seeing the draw down voltage from that.

If you don't have AlfaOBD to do some testing, perhaps pull the fuse for the cabin heater and do the same test to see if it's the cabin heater or the grid heater drawing the battery voltage.


If there's any difference, it's not HO vs SO, it's pickup vs CC truck.


ETA: Scratch that first part, the heaters (PTC) don't come on unless the engine is already running and battery voltage is >12.6V.

Diesel Cab heater

I hear that the ram trucks with diesels have a extra cab heater, electric maybe? Has anyone used it? Have any info? Cab warm up super quick?

hdrams.com

For reference, the fuses and relays for the PTC in the Aux PDC as shown.

View attachment 25227

The grid Heater might not be coming on because of a fail safe as if his battery voltage is too low to allow it to run

 

[Slackware1995]

Low 50s and the grid heater will come on in my SO. I usually bypass it. It will also do a preheat if it was cold overnight but the air temp has quickly warmed up. Last Saturday it was in the high 30s overnight. It was almost 70 by midmorning when I started the truck and it wanted to do a preheat.

I know that I am necro'ing this thread.

Yesterday my 2019 3500 6.7L HO engine engaged the pre-heater. The temperature was about 80F at the time. I just started it up, with the temperature at 67F and it engaged the preheated again.

Yesterday I had to use a battery charger set to 50 amps to start the engine. I set the main dash to battery voltage. After disconnecting the charger, the alternator was putting out 14.2v after a few seconds, so its good. After letting it idle for about 1/2 hour, I turned the engine off. I immediately restarted the engine and it showed the battery at 12.1v, which is way too low for AGM. The preheated was not turned on for that start.

That proves that at least 1 battery has a bad cell, I will be replacing both.

Today when I started the engine it showed 11.8v and by the time it finished the preheat it was at 10v. I'm shocked that it even started.

I don't know if this means that the preheated doesn't have low voltage protection or if my ecm isn't programmed correctly.

I was shocked to see the preheater being used yesterday as there is no need for it with the truck sitting in direct sunlight and air temps about 80f.

I even question the need for the preheater at 66F. I don't remember it turning on with it this warm before. I don't know the cut-on temperature, but it I always thought it was in the upper 50's.

All recalls have been performed.

I was shocked to find out that the engine block heater runs if it is plugged in. I had assumed that it would only run if the oil temp was below a certain level. After all, a cheap aquarium heater has that functionality.

I purchased the truck used last year (Nov 2024). It now (Jun 2025) has 36766 miles on it. I purchased the engine block heater cable on Amazon last November for $15.50.

The Cummings 6.7L engines all have the engine block heater pre-installed. If you pay for the optional "engine block heater" they simply plug in the cable.

I also purchased an outdoor 120v inlet receptacle. The block heater cable plugs into the receptacle which I mounted next to the passenger side tow hook. This is a black colored weather proof receptacle. It blends in well with the hook. When I want to use the block heater, I open the receptacle cover and plug the extension cord in. It was a $24 upgrade. If I had the dealer install the block heater it would have been several times as much as I paid and wouldn't have included the weather proof receptacle. I don't have to worry as much about corrosion or dirt.

 

[flan]

I know that I am necro'ing this thread.

Yesterday my 2019 3500 6.7L HO engine engaged the pre-heater. The temperature was about 80F at the time. I just started it up, with the temperature at 67F and it engaged the preheated again.

Yesterday I had to use a battery charger set to 50 amps to start the engine. I set the main dash to battery voltage. After disconnecting the charger, the alternator was putting out 14.2v after a few seconds, so its good. After letting it idle for about 1/2 hour, I turned the engine off. I immediately restarted the engine and it showed the battery at 12.1v, which is way too low for AGM. The preheated was not turned on for that start.

That proves that at least 1 battery has a bad cell, I will be replacing both.

Today when I started the engine it showed 11.8v and by the time it finished the preheat it was at 10v. I'm shocked that it even started.

I don't know if this means that the preheated doesn't have low voltage protection or if my ecm isn't programmed correctly.

I was shocked to see the preheater being used yesterday as there is no need for it with the truck sitting in direct sunlight and air temps about 80f.

I even question the need for the preheater at 66F. I don't remember it turning on with it this warm before. I don't know the cut-on temperature, but it I always thought it was in the upper 50's.

All recalls have been performed.

I was shocked to find out that the engine block heater runs if it is plugged in. I had assumed that it would only run if the oil temp was below a certain level. After all, a cheap aquarium heater has that functionality.

I purchased the truck used last year (Nov 2024). It now (Jun 2025) has 36766 miles on it. I purchased the engine block heater cable on Amazon last November for $15.50.

The Cummings 6.7L engines all have the engine block heater pre-installed. If you pay for the optional "engine block heater" they simply plug in the cable.

I also purchased an outdoor 120v inlet receptacle. The block heater cable plugs into the receptacle which I mounted next to the passenger side tow hook. This is a black colored weather proof receptacle. It blends in well with the hook. When I want to use the block heater, I open the receptacle cover and plug the extension cord in. It was a $24 upgrade. If I had the dealer install the block heater it would have been several times as much as I paid and wouldn't have included the weather proof receptacle. I don't have to worry as much about corrosion or dirt.

Block temperature vs outside air temp are 2 different things, depending on overnight temps cold soaking the block and how long your truck was in 80 degree temps it makes sense the grid heat was called for. I never let mine cycle, just double tap the start button and it will bypass the initial cycle. It will still cycle while the engine is running and intake air temps are met or vehicle exceeds a set speed, 25mph iirc.