It is not the grid heaters

[RTRAM]

I have a 93 CTD that I have owned since 13 miles. I have a new problem with the cold weather. It will start just fine and the grid heaters will function so the "Wait" light goes out. It cranks over and fires just fine. During the summer, the volt meter went up to 13+ after starting and if it was a little cool, the heaters would cycle. The problem is now that it is colder, the volt meter is buried under 8 and doesn't really cycle up and down. It is really apparent when I was driving at dark and the headlights were really dim. The interesting thing is that when the temperature enough that the I can feel the heater blowing out hot air, the voltmeter jumps up and reads normally around 13. This was a desert truck, so maybe it is just cold blooded??

 

[Topzide]

How are the batteries and charging system? Sounds like with the cold weather demand may have brought up a bad cell or failing charging system. I'd have the batteries checked first for load the voltage drop. Cables and ends would be the next thing that come to mind. Cold weather brings many frustrations! Mine at the moment are VERMIN! Carpet bombed under the hood and set up bait stations! Nothing in traps, but nibbling going on in bait stations!

 

[RTRAM]

I replaced the alternator already. The battery is a couple of years old. I may have to yank it and take it down to be tested.

 

[Topzide]

You can do a quick test on it. Disconnect it and charge it to full charge. Measure voltage, let sit at least 24 hrs. Measure the voltage again, if within a couple tenths, I'd say it's holding voltage. If it holds voltage, reconnect it and then have someone crank it while you watch voltage. If it drops below 10 volts when cranking, bad battery for sure. Good luck tracking it down.

 

[NIsaacs]

It might be the IAT sensor. I am not sure if it controls the grid or not? I know it does control the KSB.

https://www.genosgarage.com/product/sensor-3918461/engine-sensors-1

 

[petersonj]

It is possible the grid heater / heaters are continuing to cycle, or are stuck on for awhile after the engine is started and while driving the truck. The grid heaters consume more electricity than the alternator can put out, so if this was the case then battery voltage would fall.

I would disconnect the grid heaters and measure voltage at each battery with headlights on and heater fan on high while the engine is running. Battery voltage should remain near 14 volts. Drive the truck. Again, battery voltage should remain near 14 volts.

Post those results, then we can go from there.

- John

 

[RTRAM]

I did some more testing. It was cool this morning and the battery was at 12.2 volts. Before the alternator kicked in, the battery was at 9 volts. After I was driving it a while and the alternator was working, the battery and I turned the engine off, the voltage was at 12.9 volts. On the alternator, when cold the two field studs were at 1.0 of the top and 9.0 for the bottom. When warm, the top field stud went to 9.0 and the bottom was at 14.0 volts. Just as a reminder, I have a first gen and only one battery. Thanks for the replies.

 

[petersonj]

I did some more testing. It was cool this morning and the battery was at 12.2 volts. Before the alternator kicked in, the battery was at 9 volts.

It is not clear to me what you mean here. Was the battery at 12.2 volts before you started the truck?

What does "before the alternator kicked in, the battery was at 9 volts" mean? The alternator begins charging immediately when the engine is started. If the grid heater is operating (normal post cycling), the battery voltage will drop well below 12 volts while the alternator is charging and the grid heater is activated. Is this what you are saying?

The interesting thing is that when the temperature enough that the I can feel the heater blowing out hot air, the voltmeter jumps up and reads normally around 13.

This statement from your original post indicates the grid heater just stopped cycling when you felt the heater blowing out hot air.

Are the voltage readings that you are reporting coming from dash voltmeter, or are you using a multimeter and checking the voltage at the battery? Or, are you doing both?

I am asking these questions because small details matter - especially when trying to diagnose difficult electrical problems.

- John

 

[RTRAM]

I am using a Fluke multimeter for the readings under the hood. The dash meter reflects the same basic values but not as accurate.

"before the alternator kicked in" was my thought at that time that the alternator was not working at all but in hind sight it appears that the alternator has some output but just not 14.4 until something gets warm enough. I have seen the normal grid heater cycling over the years but this time there is not cycling. The needle on the dash just stays buried around 8 volts. Poor verbage on my part.

Let me know if you need any more info. Thanks

 

[petersonj]

have seen the normal grid heater cycling over the years but this time there is not cycling.

How are you determining that the grid heaters are not cycling or being stuck on? When I troubleshoot difficult problems, I like to remove all of the variables that may interfere with troubleshooting. That is why I would disconnect the grid heaters until this problem is resolved.

With the grid heaters disconnected, you will be absolutely sure that they will not be affecting the voltage readings at the battery before and after the truck is running. After starting the engine, voltage at the battery should approach 14 volts fairly quickly, unless the battery is deeply discharged.

Do you know if there is a battery temperature sensor under the battery? A battery temperature sensor provides input (regarding ambient temperature) to help the voltage regulator in the PCM control battery upper limit voltage - typically between 13.5 - 14.5 volts (lower voltage with high ambient temperature, higher voltage with lower ambient temperature).

Can you perform the above test? If so, check the voltage reading at the alternator output terminal as well. There should be no more than .1 volt difference between the alternator output terminal and the positive battery terminal while the engine is running.

You may be sabotaging your troubleshooting if you are allowing the battery to get to a low discharge state while this ongoing charging system problem exists. Leaving the grid heaters connected while troubleshooting will contribute to excessive discharging of the battery.

- John

 

[RWherley]

There is no battery temp sensor in these trucks, or ecms aren't that smart, where did your new alternator come from? I've had many issues with parts store garbage over the years, so a relatively new part failing would not surprise me, while I haven't experienced it my self I have heard of issues with the voltage regulator in the ecm failing as well, if your unlucky enough that the regulator is failing you have 2 choices, roll the dice on a "rebuild" ecm or by pass it and install a external voltage regulator from 90 or older truck

 

[RTRAM]

If the ECM is going bad, how do I install an external voltage regulator? Are there other ECM provisions that I can work around? Thanks

 

[Timd32]

I read the ASD relay also controls the feed to the Field Control for the Generator on a '93.

Sounds like it uses the Crankshaft sensor to determine minimum RPM and closes the ground to pick up the ASD relay then send that voltage to the generator field terminal.

That might be a pretty easy check to see that voltage working.

Also some Grid Heater info is attached.

The old manuals were pretty useful and had some really nice trouble shooting trees, some require the old school DRBII deal.

 

[NIsaacs]

It might be the IAT sensor. I am not sure if it controls the grid or not? I know it does control the KSB.

https://www.genosgarage.com/product/sensor-3918461/engine-sensors-1

POST-HEAT CYCLE The air intake post-heat cycle will begin if the following conditions exist: ^ The engine has been started and is running. ^ The air intake temperature is at or below 59°F (15°C) when the ignition switch was turned ON. The intake air heaters have two different modes of operation depending upon the intake manifold air temperature, either both heaters are ENERGIZED or they are cycled ON and OFF individually (while one is ENERGIZED the other is not and vice versa). The amount of time that the air intake heaters remain ENERGIZED depends upon the intake manifold air temperature.

 

[RTRAM]

I went out today and the ambient temp is around 45 F. The battery voltage from overnight is 12.4. I checked the resistance on both grid relays and both were about 17 ohms measured at the two small posts. I disconnected the relays and started the truck. I was grumpy but started and the inside volt meter read around 14 and the battery was 14.3. I tested the alternator and the top field stud was 10 and the bottom stud was 14.3. When I connected the grid heaters, the batgtery voltage dropped to 11.7, the alternator top field stud went down to 1.1. and bottom field stud went down to 11.1. As the engine warmed up, I watched the top alternator field stud climb up and as it did, the battery voltage went up also. Does this help??

 

[petersonj]

The battery voltage from overnight is 12.4

I disconnected the relays and started the truck

I was grumpy but started and the inside volt meter read around 14 and the battery was 14.3.

At this point the charging system seems to be operating properly. The alternator recharged the battery quickly after using the starter for a bit. Leave the grid heaters disconnected! Turn on the headlights and set the blower fan on high. Does battery voltage still remain around 14 volts? Take the truck for a drive long enough to reach engine operating operating temperature. Does the battery voltage still remain around 14 volts?

After your test drive, reconnect the grid heaters. Restart the engine - the grid heaters should not activate. Is battery voltage still around 14 volts?

I tested the alternator and the top field stud was 10 and the bottom stud was 14.3.

I am not sure which studs you are referring to since I do not know what the back of that alternator looks like. I am only interested in the voltage of the large alternator output terminal when the engine is running. It should be within .1 volts of battery voltage.

You have titled this post, "It is not the grid heaters". I asked, "How are you determining that the grid heaters are not cycling or being stuck on?" You did not answer that question. I am not saying that your grid heaters are the problem, but they could be the problem. An perfectly good working alternator cannot provide the amount of current that grid heaters draw, consequently voltage drops and continues to drop while grid heaters are activated. This is normal. If they are post cycling longer than they should or if they are not cycling at all, but one or both are staying activated, this could make it seem that you have a charging system problem. It is for this reason that I would check the charging system operation with grid heaters disconnected.

- John

 

[RTRAM]

The large alternator output was hidden at the bottom and I had not tested it before. It is running about .8 to 1.0 volt more than batter voltage. Once it gets hot, it runs great. The air temp today was around 45. As it was warming up today, the battery was at about 9 while it was warming up. The inside meter looked about right for the grid heater to be on. Then it took a turn down to 6 att he battery on my multimeter and the inside dask voltmeter was way to the left. The voltage on the alternator stud was also down in the 6 volt area. Once it got warm, everything came back up to 13.5 to 14.5. Is it the grid heaters?
Why did I title this "its not the grid heaters"? When I bought the truck back in 93, everytime I had an electrrical problem the standard answer was about the grid heaters. I was living in the desert, so the truck was already over 60 most of the time, even in the winter days when I driove it. Since that time, I had grandchildren that took up a lot of my time and I did not drive the truck, I was driving a minivan. I retired to snow cournty and have only driven the truck 1,000 miles in the past six years. But, I want to start driving it more in the winter but the heavy battery drain is hard on one battery normally but with this problem, i really need to figure it out if I want to take this somewhere remote overnight. I was going down the road after dark and the headlights kept getting dimmer until it warmed up and thd alternator kicked in.
Thanks for continuing to help me with this. I have neglectedthe truck over the past 20 years and need to get more familiar with it.

 

[petersonj]

When I troubleshoot difficult problems, I like to remove all of the variables that may interfere with troubleshooting. That is why I would disconnect the grid heaters until this problem is resolved.

With the grid heaters disconnected, you will be absolutely sure that they will not be affecting the voltage readings at the battery before and after the truck is running. After starting the engine, voltage at the battery should approach 14 volts fairly quickly, unless the battery is deeply discharged.

Can you perform the above test?

Leave the grid heaters disconnected! Turn on the headlights and set the blower fan on high. Does battery voltage still remain around 14 volts? Take the truck for a drive long enough to reach engine operating operating temperature. Does the battery voltage still remain around 14 volts?

After your test drive, reconnect the grid heaters. Restart the engine - the grid heaters should not activate. Is battery voltage still around 14 volts?

I would disconnect the grid heaters and measure voltage at each battery with headlights on and heater fan on high while the engine is running. Battery voltage should remain near 14 volts. Drive the truck. Again, battery voltage should remain near 14 volts.

I will make one last attempt. Disconnect the grid heaters and leave them disconnected! From what I have read so far, you have not disconnected the grid heaters and driven the truck. That is what I want you to do. My guess is that the charging system will function normally without the grid heaters connected. If it does not, then you will know that you have a problem with the charging system.

I am not saying that the grid heaters are the problem, but if you don't disconnect them and leave them disconnected, you are going to find it very difficult to diagnose whatever problem you have, especially if it turns out the grid heater operation is the problem.

The grid heater control circuitry could be telling the grid heaters to post cycle for too long, or could be telling them to post cycle together instead of alternately, or you could have a battery in such poor condition that there is no reserve capacity while the grid heaters are post cycling. Remember, a perfectly operating charging system cannot produce the current flow that grid heaters draw while they are activated.

I was going down the road after dark and the headlights kept getting dimmer until it warmed up and thd alternator kicked in.

The alternator does not "kick in". It begins charging immediately when the engine is running. This sounds more likely that a heavy electrical load (possibly the grid heater operation) has just been removed, which now would allow for the alternator to direct its output to the lighter electrical load and to recharge the battery, consequently battery voltage would rise to the 14 volt level.

I disconnected the relays and started the truck. I was grumpy but started and the inside volt meter read around 14 and the battery was 14.3.

This information indicates that the charging system is functioning properly. The alternator quickly brought up the battery voltage immediately after starter motor operation (a heavy current draw).

So, please disconnect the grid heaters and leave them disconnected. Drive the truck and report back.

- John

 

[RTRAM]

Thanks for you instructions and I will try to get the results in a day or two. Right now the wife has me splitting and stacking two cords of maple in the barn and we have a snow storm approaching so the truck is getting put on the back burner. I have printed out your instructions and will get to it ASAP. Again, thank you for your logical diagnosis.

 

[petersonj]

You are welcome. We expect to hear from after you get shoveled out.

- John