Class1 “EVEN Series” Load Manager Flickering Problem

I’ll be the first to admit that the number of people on this planet who have this problem is pretty small. And even smaller is the number of those Googling this problem and finding this page. But if I can save even one fire department from the hell of this nasty little issue, it’s worth it. And if you’re a troubleshooter, you could learn something here too.


This is the story of an electrical gremlin that flummoxed our local fire department for — I kid you not — 25 years. For that entire time, the emergency lights on their first-out engine had been randomly cutting out. And they could see and hear the relays on this board rapidly opening and closing (“flickering”) while this happened.

The department sent the truck to the dealer countless times, and each time the dealer piled ineffective fix on top of ineffective fix. They rewired the switches. They added auxiliary relays. They replaced the entire board. They added extra grounds. And none of it made a damn bit of difference because they didn’t have the skill and/or patience to find the root cause.

At the department, no one knew what the problem was so mythology started to take over. “It’s got too much load on it.” “We need to run bigger power wires.” “It’s worse when it rains.” “We should switch to LED lights.” “Replacing the switch helped.” Finding the problem turned out to be as much about dealing with theories as it was doing the troubleshooting.

The Method

Since this was a known tricky problem that had been looked at by several at-least-somewhat-competent people, I didn’t spend a lot of time on the easy stuff. Instead I focused on listing very clearly all of the hard evidence we had.

Mostly this was in the form of several cell-phone videos taken of the board while it was misbehaving. The indicator lights could be seen to flicker in seemingly random patterns, and relays could be heard rapidly cycling at several Hz. I studied these videos carefully, and wrote down exactly what I could see in each.

The second thing I did was research the board itself to understand how it actually works. This helped me dismiss several myths. For example, the board actually has no way to detect current flow. So being “overloaded” couldn’t be the cause. What it does detect is the battery voltage — if that voltage sags, it turns up the idle (to increase alternator output), and if that doesn’t work, it starts turning off loads (and an LED comes on saying so). But in the videos, that LED never came on. Further, the board displays the battery voltage on a display — and that voltage never fell during the videos. So the load manager wasn’t shedding loads.

Although I rarely do this, as part of my research I called Tech Support at Class1 (now owned by IDEX). As expected, they had no clue — they couldn’t even find a manual. But I read their tech support rep every part number I found on the board, and to his credit he looked them up. One of them came up in his database as the circuit board itself, and he actually sent me the schematic! (I’d call him out by name for praise here, but they’d probably discipline him.) This saved me a ton of time.

The Insights

Although the flickering in the videos was mostly random, it wasn’t completely so. Specifically, some channels that were ‘on’ flickered ‘off’, but channels that were ‘off’ never flickered ‘on’. Also some channels stayed on and never flickered — and these were always the same channels in every video.

The schematic gave me more insights:

  • Several relays (#3, #4, #7, #10) have no electrical connection to the LM logic board.  They are controlled only by their cab switches.  The LM does not know if they’re on, and it cannot turn them off.  
  • Some relays (#1, #2, and #5) turn on with the MASTER switch, but have no individual switches.  (I believe that #1 and #2 were intended for ‘white’ (aka ‘scene’) and ‘amber’ (aka ‘respond’) lights.  #5 was intended to be used for A/C and is always on unless it needs to be shed.) 
  • The remaining relays (#6, #8, #9, #11, #12, #13, #14) come on with the Master, but also have individual switch inputs, meaning that they also need to be turned on with their individual cab switches.

It was this last group that was flickering. And that group uses different circuitry than the rest of the channels. Specifically these channels have 74HC125 tri-state buffers (U40 and U41) which “mix” the signals from the load manager logic and the switches on the driver’s control panel. Further, the panel switch inputs are active low (i.e. grounded to trigger), and pass through 1N4008 diodes which serve as protection against applied voltage.

The specified input voltages for the 74HC125 are 0-1.4v (low)  or 3.2-5v (high).  Any voltage between 1.4 and 3.2v results in ‘undefined’ output behavior.  This can include being “on” or “off” randomly, or rapidly oscillating between on and off.

To be safely “low”, the voltage has to be below 1.4v. But recall that the pulldown current passes through a 1N4008, so there’s a .7v diode drop.

Finally, this board was made as a drop-in replacement to fit on a relay board made by another manufacturer (Federal-Signal). That board has its own diode protection on the inputs, adding further diode drops and effectively eliminating the already small .7v margin.

The Root Cause

So the root cause of the problem was that the input channels on the board, combined with the relay board it’s attached to, created a system that’s extremely sensitive to the quality of the switch wiring, to the point that even reasonable imperfections in switch contacts or grounding would put the logic devices into their unstable regions. In other words, there is no right way to install this thing and have it work reliably, unless you have absolutely perfect grounding of the switched-on inputs.

Now What?

There’s a lot more to this story, but Mr. Determined Enthusiast is a busy guy and this is already pretty long. If you’re struggling with this problem, hit me up in the comments and I’ll be happy to help.

2 thoughts on “Class1 “EVEN Series” Load Manager Flickering Problem

  1. Jeremy

    I am having a problem with a truck and I believe it is the load manager causing it. In this truck if the parking brake is on and you put the truck into gear it goes to a high idle at 1200rpm. If the parking brake is off it does not go into high idle when putting it in gear. If the high idle switch is on the truck preforms as it should. Could this load manager be causing my issue with the high idle while my brake is on and truck in gear? I have tested brake switches, batteries, and charging system. Thank you for any help.

    1. Mr. Determined Enthusiast Post author

      Hi Jeremy…as you might already know, a load manager engages high idle if it believes your system voltage is low. The hope is that higher RPM at the alternator will bring the voltage back up. The LM in my post has an LED that turns on whenever the LM requests a high idle. If your LM doesn’t have an LED, you can try disconnecting the high idle wire from the LM and see if the behavior goes away.

      Keep in mind that some high-idle circuits are powered and others are grounded, to engage the high idle. If the parking brake switch or gear switch is wired incorrectly (or the load manager), you could get some “opposite from what you want” behaviors.

      BTW, apparently a common non-electrical reason for high idle in gear is a transmission torque converter clutch failing to unlock. The engine throttles up when stopped to avoid stalling. When you release the brake and start rolling, the idle returns to normal.


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