It looks like I've joined a club I didn't know existed.
The first time I started my Excursion after installing a new L-N alternator, I was surprised by the fluctuating RPMs and headlights. It settled down after 30-45 seconds and everything else seemed well.
The next time I started it cold I remembered to data log the start up with my Dashboss. Ambient temperature was mid-30s.

I was going to ask here what was going on but remembered to search first and found that I am not alone.

I've been in touch with Jack (TooManyToys) already as he seemed to have spent the most time looking into this, and he explained to me what's going on between the regulator in the alt and the GPCM.
Edit to add: This was the explanation he gave me, shared with permission:

"Laurel-Anne,

That's the tug of war between the alternator and GPCM. The GPCM will shut down once voltage goes over 14.5 (L-N says it will never go over 14.2). With the glow plugs off, the voltage regulator backs off, GPCM kicks back in since voltage drops, volts drop more because of the glow plugs and the regulator kicks high again. It seems to continue until the GMCP times out, or amp demand lessens as the batteries fill. Behind the steering wheel it feels really wicked out.

I went nowhere with L-N as it was always my trucks fault. I tried every goofy thing they thought the problem was (wiring) but would never admit their voltage regulator would go over 14.2v."


I wanted to try to get a somewhat different graph, only showing the first little while after start up today, with a few different PIDS. The picture above was from a log that lasted about 5 minutes, and the relevant section lasted about 30 seconds. The voltage readings ranged between 11.75 and 14.5.

Today I sat with the truck KOEO a little longer while I rearranged my gauges to get the ones I wanted all on one screen. But when I actually started the truck, the fluctuations didn't happen. (I also forgot to hit "record" on the DB.)

Two things were different from the day before - it was colder out (15*) and I'd had the block heater plugged in, and I sat in the truck longer playing with my DB settings. Would that likely explain the lack of fluctuations, that the glow plugs had already been turned off?

 

Yes, there is a time out so if you left the key on then did not recycle the key the GPs may just have stayed off. However, in my case the surging can happen, or not. It depends on the current draw by the batteries taken a charge and other things turned on. For example, when my truck was actually running (g) I could turn on the lights or the HVAC fan to cause a higher current need and the voltage would not spike high enough to turn off the GPCM. So this setup seems to be right at the threshold. When I went back to a stock size pulley, where the L-N current output was limited to 160-180a at idle, the fluctuations would not occur.

 

I have the same problem, even with the stock size pulley. The only way I can somewhat control it is by turning on the blower to at least 3, headlights, and my light bar. Which is annoying to say the least every morning!!!

I can watch the voltage climb over the course of 10 seconds or so, hit over 14.5, glow plugs turn off. You can hear the change in rpm and watch the interior lights dim and brighten a dozen times before it gives up and the plugs stay off.

I spoke with L-N about it AGAIN and while they were “sympathetic” and spoke to other engineers, I was told it’s never seen before and I should check existing cabling and add additional to control voltage drop. I don’t understand how it’s never been heard of. I’m pretty sure I’m not the only one who has contacted them about it....

They did offer to replace it under warranty but I’m not at all convinced that wil make any difference. I also spoke with FICM repair since they promote and sell these. They mentioned they are working/talking with the engineers from L-N to come up with a solution 🤷🏽🤷🏽

 

Yeah, never been discussed before, same story, talking to Engineers, must be part of the customer training. 13 emails, a few phone conversations, and having me alter wiring all over my truck and videoing it.




It’s the damn voltage regulator. You built the damn thing to go on this truck and didn’t know it had a GPCM, thought it had a starter like relay.

Oh, and Feb 13, 2017

 

Mine does the same. It goes to 14.2 then every time the GPCM kicks in it dives to 13ish and back up repeatedly until the module kicks off. Nature of the beast as far as I'm concerned. No worries.

 

I always assumed the glow plugs on these trucks ran for a set time...(2mins., I think) when the key is turned on even if the truck is running and even when it's warm, I don't have a LN alternator, but my scangauge tells me this is the case. The voltage doesn't come back for that period of time anyway. If this is the case, why would that make the LN fluctuate in that time frame? I ask because I have had LN alternators in the past on other vehicles...tough alternators but very hard on regulators. The up side of that was the regulator was easily changed by the user and the alternator lasted for years. I was considering getting one for my 6.0 for that reason, but if they still have issues I think I'll stay with the Autozone I have now.

 

Nav had Beru design the GPCM with a set cutoff if the voltage exceeded 14.4-14.5v, so with a high enough alternator amp output unless the voltage regulator has tight control below that threshold the GPCM will shut down. Once that 85a load is removed the regulator backs the current output down, and voltage goes below the 14.4v threshold. The GPCM senses the acceptable voltage and reactivates the glow plugs, pulling 100a (GPs cooled a little) which drops voltage way down for an instant before the alternator regulator can react. When it does, up goes the current output and subsequent voltage, again over 14.4-14.5v. The sequence keeps repeating until the GPCM fully times out from Key On, or the batteries have increased resistance from filling up to a point where current draw is not as high.

You can have situations where the GPCM just completely stays off. Again it depends on many variables and most of the time during the warmer weather mine will do that with the overdrive pulley. Randy’s truck seems to do that always. Other alternators of this high of an output do not seem to have this issue so I believe it’s Prestolite’s choice of V regulator.

If you put this alternator on a 7.3L it would never go through this, but it could burn off the tips of the GPs due to keeping voltage up. 7.3L had a number of failures from GP tips which is why the GPCM was developed for the 6.0 and other applications, to protect the GPs and engine.

You stated the voltage doesn’t come back while the GPs are on, that because you have a lower output alternator. Voltage will not come up unless there is sufficient headroom of amps available over amps used. The 140a has more headroom then the 110a, a 160a has more then the 140, the 185 has more then the 160...... voltage steps up with alternator during the high demand GP on time.

 

One of the reasons I went with the LN was this, from ficmrepair.com "Use AGM type batteries or want to in the future? This unit is pre-set at a 14.2 volt regulation to keep those options open!"
Seeing that this unit does in fact go over 14.2 was a little alarming, as I do in fact use AGMs. I checked Optima's site and they recommend 15v as the upper limit, so that has eased my concerns some.

I'm glad to hear that ficmrepair is involved, and I think I will get in touch with them and with L-N, just to add another voice.

 

I think you’d get more traction with L-N.

There’s a lot of differing info on battery charging rates and your better off going to the battery manufacturing sites or generic info sites like Battery University. Here is a chart from one battery manufacturers site about charging thresholds.





L-N specs also say this alternator does not have a temperature compensating voltage regulator. It does. Once warm under the hood it will drop lower, more so during summer temps. I’ve tracked it, have the data. CS will say no.

So based on that battery manufacturers site the L-N 230a is fine. In fact I believe Randy (WilsonR) has been fine running his AGMs.

 

Part of me is considering this..... A local, highly reputable diesel shop near me that I've used in the past uses these alternators and starter. I've not heard anything bad as of yet

https://www.xtremediesel.com/meangre...nator8306.aspx

 

You may have missed this.

 

Wow what a timely thread! Like others I have the stock pulley and I am using AGM batteries (Advanced Auto branded Johnson Control ones) and have the same cycling when cold starting. I remembered TooManyToys experience with this and made a video of it with readings from TorquePro that I was going to post but I manager to delete it. I'll try to recapture it today.

 

Back in the Dec-Feb time frame I kept trying to check different things based on what the L-N CS was saying might be the problem. Then I realized no-one was looking the vids. I got to the point I knew I was getting the run-a-round, get the customer tired so he doesn't bother us anymore. It's a common practice, it must be your truck. Even well thought of vendors here do the same.

Some of the work and vids I did, some deleted. In the case of the first vid the instrument reads are alternator volts, glow plug amps, time, and alternator amps, the common way I checked different alternator setups, but not always for some of these "It must be your truck 'cause we never heard of ....". These are unlisted on my channel as they were for L-N, not public.










Running a direct cable from the alternator to the drivers battery, eliminating two connections despite the supplemental cables I'm using, did seem to calm the issue down by kicking the voltage up so the GPCM just stayed off, most of the time.

It's one of the reasons after I get my truck up and running again I'm redoing the entire cabling on my truck, for better conductivity and equal battery draw. In another post I mentioned that many people keep buying these large cables to improve voltage drop, but it's the connections that are more of a concern. You can spend hundreds of dollars on additional cables, but you're waisting dollars. The vendor is making money, peeking under the hood looks impressive, but the actual accomplishment is not there.

Scott noticed this when he added a cable and that got me to look at this more then what I had already done with the "Balancing the batteries" vid.

 

One good review cancel's out that negative review, which means it's perfect!!

None of your time or efforts have been wasted in our minds Jack. Your videos and testing have been TOP NOTCH!

We really should try and get a count of how many people are having this issue. Though it will be less than a dropplet in their minds it would be interesting to know....

 

As Pete alluded to, with the GPCM in control it's more of an annoyance then a worry.

But ..... when people like you also have GP or GPCM module issues I go back to my prior career and take notice. It doesn't mean there's an issue, but I keep track to see if there's a trend. In an R&D/Product Testing environment its a really good trait that many don't have, in a public situation it can cause worry when there shouldn't be. I don't have to good sense when to turn that off.