weird electrical problem

[RedD250]

Good morning all,

Recently, my truck has developed an intermittent problem where the volt gauge will go from 10 to 14 volts. It seems to have a certain pattern, it'll go to 10 and stay for a few seconds, then go to 12 for a few seconds. When this happens, my dash lights will dim, but nothing else seems to be impacted. Headlights don't dim, blower motor doesn't slow down, radio doesn't cut in/out. It'll do this for maybe a couple of minutes, then stabilize at the normal 14 range. Could this be a voltage regulator going bad? Not sure if these trucks even have one. I have LED headlights and dash lights, could that be an issue? I know on some older vehicles LEDs don't play nice with the wiring. Thanks!

 

[petersonj]

Sounds like normal post-cycling operation of grid heaters if this is happening when the truck is first started.

How long have you had this truck?

- John

 

[RedD250]

Yeah, it happens when I first start it. I've had it since April, so almost 7 months now. It's normal for it to do that when the grid heaters cycle? If so, I may need to get rid of the grid heaters, because this behavior makes my aftermarket gauges go haywire. Either all of them remain blank, or only 1 or 2 of them will come on, and I've got to disconnect the battery to reset them, then reprogram all 4 of them for the high/low warnings and backlight settings. Something like that would never fly in a new truck haha.

 

[AnonEd]

Thinking like petersonj I would like to know when this happens...only at certain time/temp/conditions or randomly anytime. Doubt LED headlights are causing an issue, especially if they are off or wired through a relay. Were my voltmeter doing that with no other indication that voltage was lower (I can see it in my headlights but they are not LED...the radio never changes) I'd first be verifying my voltmeter is accurate.

 

[RedD250]

It just started doing this. I first noticed it a couple weeks ago, then nothing until today. It was cold out that first time, then the weather warmed up and now it's cold again. It was 30º outside this morning when it did it. I do get right around 14v when the truck is idling normally though, according to my volt meter. 12.2 when the truck is off. When the volt meter says 14, the needle on the stock volt gauge is one line over the center, which would be 14 if the lines go in 2v increments.

 

[petersonj]

It's normal for it to do that when the grid heaters cycle?

Yes, it is normal for the voltage to drop when the grid heaters post-cycle. The colder the ambient temperature, the more and longer it happens. Once the truck has reached approximately 20 mph or the engine warms to a certain operating temperature, the post-cycling stops. I believe you live in Georgia, so owning the truck since April falls right in line with the grid heater behavior you are experiencing.

If you have an aged battery, the voltage drop can be magnified during post-cycling.

I have never been a fan of automatic operation of grid heaters. This is why I use a momentary switch inside the cab for grid heater operation. The circuit is low amperage because it only triggers the coil portion of the grid heater relay. I determine when and how long to use the grid heaters.

At 08:20, I edited the first sentence in this post to answer the question.

- John

 

[RedD250]

How did you wire up the mementary switch? Is that like a spring button that only stays engaged when you hold it? I may do that. I know the grid heater is in the intake manifold somewhere, is it that thing right where the intercooler pipe goes in? I live in GA, so it really doesn't get that cold here, usually the upper 20s. Maybe a couple days it'll dip to single digits. From what I have heard, the Cummins will fire right up no matter how cold it gets haha.

 

[petersonj]

Is that like a spring button that only stays engaged when you hold it?

Yes.

How did you wire up the mementary switch?

I think this is the relay used for your truck - don't know if one or two is used. You will leave the heavy wires intact. Each relay uses two small terminals - this is the coil portion of the relay. I believe one of the wires goes to ground and the other wire is supply voltage from the PCM. You will have to verify. If it is wired this way, you will need to install a fuse from a switched 12 volt source and route a wire to a momentary switch inside the cab. From the switch, run a wire to the small positive terminal of the relay. Make sure your momentary switch is positioned in a manner as to not being accidentally activated.

If you are successful, from now on you will be the boss.

- John

 

[RedD250]

I'll check it out next chance I get. Just to clarify, I am assuming I'd disconnect the PCM signal wire and just replace it with the hot wire from the new switch, while leaving all other wires as they were? Any particular amperage fuse that I should use for the switch, and what amperage rating switch do I need? Thanks!

 

[BigPapa]

Sounds like normal post-cycling operation of grid heaters if this is happening when the truck is first started.

This is why they went to two batteries in the 2nd Gen’s. If you had two batteries, you probably wouldn’t have this problem.

 

[RedD250]

I've been thinking about getting dual batteries. Is it just a matter of hooking them up in parallel, or are there other modifications/parts I'd need? I've heard tell of people mounting dual batteries sideways in the stock tray. There appears to be plenty of room for 2 in there.

 

[petersonj]

Just to clarify, I am assuming I'd disconnect the PCM signal wire and just replace it with the hot wire from the new switch, while leaving all other wires as they were?

Without a wiring diagram I cannot tell you for sure - this is why I said you will have to verify. I would use a test lamp on a cold morning when the grid heaters will operate. Before you start the engine, clamp the wire end of the test lamp to a good ground. Maker sure the test lamp works. Start the engine and probe each of the two small wires connected to one grid heater relay while the heaters are activated. If the test lamp lights when touching one of the connections, then this connection would be the 12 volt signal from the PCM. The other wire is ground. If this is the case, then you can wire the relay in the manner you assumed (from your copied post).

Any particular amperage fuse that I should use for the switch, and what amperage rating switch do I need?

The coil of the relay doesn't draw much current. On the high side I would say no more than 1 amp each (more likely about half that). A 5 amp fuse / switch and 16 gauge wiring should work fine. You probably can't buy a switch with less than a 10 amp rating, which would be fine as well.

When I did the modification on my '02 truck, two codes were set (one for each intake heater), but it did not set the "Check Engine" light, so I was fine with that.

- John

 

[RedD250]

Cool, good to know. I don't think the first gens computer is that smart, so I should be fine. Gonna try dual batteries first, and if it still interferes with my gauges then I'll go the switch route. I figure a pair of 1,100 CCA batteries will get me straight.

 

[BigPapa]

Is it just a matter of hooking them up in parallel, or are there other modifications/parts I'd need?

Yes, parallel. Connect the positive posts together and the negative posts together or just ground both negative posts.

 

[AnonEd]

In the 92 FSM intake heater relay #1 is connected by 18 YL BK from pin 16 on the SBECII (PCM of the day) and relay #2 by 18 OR BK from pin 15. The other smaller wires (16 DB) go to splices, A/C WOT relay, auto shutdown relay, ign switch, etc. and pin 9 "IGN FEED" on the SBECII.
I would guess the dark blue wires supply 12V+ trigger power to the relays and the SBECII grounds them via pin 15 and 16 to activate them, but I don't know this.

 

[petersonj]

I would guess the dark blue wires supply 12V+ trigger power to the relays and the SBECII grounds them via pin 15 and 16 to activate them, but I don't know this.

Of course, it would have to be more complex. If I were to add the momentary control switch, I think I would just provide fused switch power and ground to the coil portion of the relay in a stand alone circuit. This way the "SBECII (PCM of the day)" would be out of the loop.

@AnonEd , thanks for the information.

- John

 

[RedD250]

Cool, thanks for the info. I'll look into it this weekend.