BOOM PUMP: Having E-Stop Issues? Simple Things to Check.

This relates to standard 12V units as well as early European 24V units.

Every component in your Control (aka Combi) Box should be labeled; age and changing parts without putting a sticker on the new part can lead to problems. In the image the decal shows 10A17, yours might say 6A17.


Deciphering the Number

The first number, 10 or 6, is the page number of the schematic that you will find this component on. All components in the Combi box work this way; for example, 6F76 is the fuse for e-stop on page 6. The page number of the schematic varies with the options a unit has, or the amount of revisions to a particular unit. We can go into revisions later.

The letter, A, is the code for the part, A= Assembly, F=Fuse, S=switch, K=relay. Notice a German unit’s code letters are the same as English.

The second number, 17, is the assigned number for the device. Notice 10A17 and 6A17 are the same part just different pages in the schematic.

So You Are Having Issues With The E-Stop? A Quick Test Will Reveal Why.

Look at top left and find Terminal A1 (+), also in the lower right find A2 (-), putting a voltage tester at these 2 points will tell you if you have voltage. We NEED to use a voltage tester not a test light to see how much voltage we have here. Note: The red locking paint might need to be cleaned a little at the screw to get a good test.

For this relay to reset and hold you need to maintain 12 VDC, these 2 pins are powering a coil that is rated for 12 VDC. If you hit an E-stop button on the unit or turn off the remote, the voltage disappears and the relay unlocks, the 2 green lights go out and nothing works. So if you have no voltage here check the E-Stop buttons on your remote and if the local/remote switch is in the center position. A quick test for checking the remote is to set the local/remote switch to local and check for voltage. If the E-Stop resets and you have 12(+- 2 VDC), you have a remote issue, try your hard wire remote.

As previously, mentioned, always test with a voltage tester not a light, also test using the A2 (-) pin for ground, this is the ground the relay is using, If the wire from A2 to ground is bad the relay won’t set either. A quick check for this is: positive lead on A1, negative lead on A2 and look at the result, then move the negative lead to main ground in the Combi Box and look at the result. No or low voltage at A1 and A2, and 13.6 at A1 and the main ground point tells you that your problem is a bad ground from A2 to the main ground, follow that part of the circuit.

Voltage Drop

Most of the time the issue is voltage drop. I get a comment like “I get the E-stop to clear and start pumping then I hit 2 or 3 boom functions and it goes back into E-stop. If I do one function at a time it stays on.”  To avoid this or find the issue you need to test for voltage drop at A1. You can do this with the boom closed just don’t put the transmission in gear; have someone assist you with the remote, make sure the engine is running, PTO ON, not in gear on the transmission, this way you can activate the electrical components and not bend boom arms. That’s a different Tricks of the Trade post.

Check for the voltage at A1 then clear the E-stop by honking the horn, turn on the pump and see the voltage drop a little, next hit A arm, it dropped a little more didn’t it,“ then B, then slewing, then tip. Each function you activate is more draw on the circuit and at some point the E-stop relay will drop out. Not from a bad E-stop button but from voltage drop.

The problem is current flow. Look for loose or corroded wires in the E-stop switches or in the cable powering the remote receiver. Open the boxes and look at the wires, are they loose? Give them a little yank did it pull out of the terminal or sleeve? One common source for resistance issues is corrosion in the cable due to washing the pump with acid. Acid loves concrete, copper and chrome, it is not recommended to use acid to wash pumps. I have seen it turn the wires green as far as 3 feet inside the plastic coating, a major source for voltage drop. Poor grounds for the Combi Box and poor power from the truck are also big issues as the unit ages.

Take the time to do this easy test, if your voltage drops you can dig a little deeper to locate the problem and avoid future issues by resolving the problem now.

Do  Not Bypass E-Stop.

This is also the time to mention that bypassing the E-Stop is a serious problem, if the power is not at the A1 pin then even if bypassed the remote won’t work. This is due to the fact that the remote is being powered with this same circuit. So bypassing is not the best way out of problems it presents major safety issues and most likely won’t get you up and running on the remote.

As mentioned in the beginning, this is the basic start to troubleshooting the 12V Combi Box. If you have a 24 V ZMSK box the E-stop circuit is a little different, contact the Service Dept (800-890-0269), e-mail me ( or comment on this post and we can go deeper into the particulars.