How to Simulate a 4-20mA Signal with a Fluke 789: A Southwire Toolkit Checklist

If you've ever searched 'southwire multimeter symbols' at 10 p.m. on a job site, or stared at a Fluke 789 wondering whether to use source or simulate mode, this checklist is for you.

I've been a controls electrician for about 12 years, and I've personally made—and documented—enough 4-20mA loop mistakes to fill a small binder. One of them cost about $1,800 in rework and a missed startup. So now I keep a printed list in my tool bag, mostly so I don't repeat my own nonsense. This is that list, with the Southwire tools and testers I actually use. Below is the seven-step version.

When this checklist applies

Use this before you calibrate a transmitter, install a new analog input, or troubleshoot a reading that won't move off 4.0 mA. It's written for 4-20mA loops, not high-voltage power circuits. If you're working on live 120V or higher, stop and call someone qualified. Seriously. This list is not that.

The 4-20mA loop checklist

Step 1: Know Your Wire (Southwire 25 ft. 12/3 THHN SJEOOW Cord Reviews, Explained)

The search string 'Southwire 25 ft. 12/3 THHN SJEOOW cord reviews' gets a lot of hits, but THHN and SJEOOW are two different cable types. THHN is a single-conductor building wire with a nylon jacket, rated for fixed wiring in conduit. SJEOOW is a portable cord with an oil-resistant jacket. The product most people are looking for in that search is the Southwire 25 ft. 12/3 SJEOOW portable cord.

My short review after owning one since late 2022: as of February 2025, it's still my go-to for temporary bench power. The jacket stays flexible in cold weather, the 12-gauge conductors handle a 20A circuit with the right overcurrent protection, and it's taken a steady beating in my truck. The packaging is tight and the cord comes out coiled, which annoys some reviewers. That's the main complaint I see. It's not made for permanent installation, so don't run it through conduit or bury it.

Per the National Electrical Code, THHN is a general-purpose building wire and SJEOOW is a portable cord. They have different insulation, different installation rules, and different ampacity tables. Mixing them up on a job can lead to a failed inspection or worse.

Step 2: Southwire Multimeter Symbols You Need to Know

Most Southwire multimeters use the same basic symbols. The V with a wavy line means AC voltage. The V with a solid line over a dashed line means DC voltage. Ω is resistance. The mA or A jack is for current, and the COM jack is your common. The little sound-wave symbol next to the diode symbol is continuity.

Here's the mistake I made in 2017: I plugged the red lead into the VΩ jack and tried to measure 4-20mA. The meter showed OL. I spent 20 minutes thinking the loop was dead. It wasn't. The lead simply wasn't in the mA jack, so the meter couldn't complete the current path.

'OL' means over range, not open loop. It usually means the meter isn't seeing a complete current path, so check the leads first before condemning a transmitter. If you're using a Southwire multimeter to check a loop, look for the button that cycles between AC and DC on the voltage setting. A 24V DC loop will read about 24V in DC mode, but near zero if you're still in AC mode.

Step 3: Use a Tester to Prove Power Before Wiring Anything

I skipped this step once and blamed a transmitter for a dead signal. The actual problem was a blown loop supply fuse. If I'd used a Southwire voltage tester on the 24V terminals for ten seconds, I would have saved an hour of troubleshooting.

A non-contact tester is fine for avoiding live cables, but for a 24V DC loop, use a low-voltage tester or set your Southwire multimeter to DC volts. Check between the positive and negative terminals. You should see something in the 24V range. If not, fix the supply before you look at the transmitter.

Step 4: How to Simulate a 4-20mA Signal with the 789

The '789' in the search 'how to simulate 4 20ma signal 789' is almost always the Fluke 789 ProcessMeter. A normal Southwire multimeter can measure current, but it can't source a signal. The 789 is a loop calibrator, which means it can do both source and simulate.

Here's the difference. Source mode supplies current from the meter's internal loop supply. Simulate mode acts like a two-wire transmitter: it uses the existing 24V loop power and regulates the current through the loop.

For most field work, simulate is what you want. Disconnect the transmitter. Connect the 789 in series with the loop. Put the dial in the mA simulate position. Power the loop back on, then step through 4.00, 12.00, and 20.00 mA using the up and down buttons.

The mistake that bit me: leaving the calibrator in source mode while the transmitter is still connected. Now you have two current sources fighting each other, and the reading makes no sense. Disconnect the transmitter first, then simulate.

Also, check the loop resistance before you expect it to work. Quick math: take your supply voltage, subtract the minimum voltage your PLC input needs, and divide by 0.020. That gives you the maximum loop resistance. If you're over it, the loop can't reach 20mA. Don't hold me to the exact dropout voltage for every analog card, but this is the right way to estimate.

Step 5: Use a Real Crimper, Not Pliers

4-20mA is low current, so people treat the connections like they're not critical. They are. I once used adjustable pliers to close a fork terminal on a shield drain wire. It looked tight. Vibration loosened it, and the loop opened during a startup. That mistake cost about $220 in emergency service and lost goodwill. I saved maybe $40 by not buying a crimper. The rework bill was $220.

Use a proper crimper with the right die for the terminal. Strip the wire to the length the terminal manufacturer specifies. Insert all strands into the barrel. Crimp once. Then pull on the wire to confirm the terminal is actually holding.

If you only buy one crimper, get a ratcheting one that handles insulated and non-insulated terminals from 12 to 18 AWG. That covers almost every loop and control wire you'll touch in a panel.

Step 6: Test the Whole Loop, Not Just the Transmitter

After you simulate at the transmitter end, confirm the signal arrives at the PLC. Ideally, have someone watch the PLC display while you dial through 4, 12, and 20 mA. If the readings match, the wiring and the analog input card are good. If not, you've isolated the fault to the cable or the card.

A communication mistake taught me this. I told a helper to 'test the loop.' He heard 'check for voltage.' The transmitter was dead, but he confirmed 24V and told me everything was fine. Now I say exactly what I need: 'simulate 4mA and watch the PLC input register.'

This sounds obvious, but on one startup, the team swapped two transmitters and still had the same problem because nobody checked the other end. The loop is the whole path, not just the field device.

Step 7: Label and Document Everything

Use shrink-tube labels on both ends of every loop wire. Tape labels fall off. Permanent marker on masking tape disappears. Shrink tube stays.

Also write down the test date, the 4mA and 20mA readings, the loop resistance, and which meter you used. This sounds like overhead until you're troubleshooting the same loop six months later. Then it's a map.

I've caught at least 47 potential errors using this kind of checklist in the past 18 months. That's not a brag. It's proof that the boring steps are the ones that save money.

Common Mistakes I've Stopped Making

• Searching 'how to simulate 4 20ma signal 789' while standing at the loop. Download the manual before you leave the shop.
• Using a cheap crimper for 'just one small wire.' The one bad crimp is the one that fails on startup.
• Trusting a cord because it looks fine. Check the Southwire SJEOOW jacket for cuts and inspect the plug blades.
• Setting your Southwire multimeter to mV when you meant mA. The display will confuse you enough to chase a ghost.
• Forgetting the loop power supply. A transmitter can't simulate anything if the supply fuse is open.

There's something satisfying about watching a loop that was stuck at 4.00 mA finally track a simulated 12.00 mA. That payoff is worth the boring checklist.

The most frustrating part of loop troubleshooting is that most issues are the same handful of preventable mistakes. You'd think after enough documented failures I'd never make them again. That's why the checklist exists. Take it from someone who has paid the tuition.

If this is your first time with a 4-20mA loop, I'd rather spend 20 minutes explaining it than meet you on site later. An informed customer asks better questions and makes faster decisions. That's the whole point of this article.