Epson L1800/1390 Printhead Multimeter Test: What Does "OL" on Every Pin Mean?

Question

I watched the "Test a Printhead with Multimeter: Epson1800/1390 - MEMBERS ONLY" video and tried testing my printhead. I set my multimeter to what I believe is diode mode, placed the red probe on pin 1 of the narrow cable, and tested the remaining pins with the other probe. However, every pin showed "OL." Does that mean the printhead is good, or am I doing something incorrectly?

Watch the Epson L1800/1390 printhead multimeter testing video [https://www.youtube.com/watch?v=G0BnM7UWQ6Y]

Answer

In this case, if every pin is displaying OL, I would not immediately conclude that the printhead is good. Something is probably wrong with the measurement setup, the cable connection, the meter setting, or the contact between the probes and the FFC cable.

For an Epson L1800/1390-style printhead, a normal test does not consist entirely of OL readings. Some pin combinations are expected to display OL, while others should produce measurable values. In our L1800/1390 testing method, the red probe is placed on pin 1 and the black probe is moved through the appropriate pins. For example, the sample reference information shows a measurable value of approximately 875 mV between certain test points such as pins 1 and 2, rather than OL. Other designated positions are supposed to be OL. The important part is that the readings follow the expected pattern rather than simply showing OL everywhere. (BCH Technologies)

First, Check Whether the Meter Is Really in Diode Mode

One thing in your description stands out: you mentioned "diode mode 2M."

On many multimeters, 2M means the 2-megaohm resistance range, which is an ohms setting rather than diode-test mode. The diode-test position normally has a diode symbol on the selector dial. Depending on the meter, diode testing may share a selector position with continuity testing and require a button to switch functions.

So before testing the printhead again, verify the exact setting on your particular multimeter.

In the testing procedure shown in our guide, the meter is placed in diode mode, the red probe remains on pin 1, and the black probe is moved from pin to pin while the readings are recorded. A properly connected head should produce a combination of measurable readings and designated OL readings according to the reference chart. (BCH Technologies)

What Does "OL" Actually Mean?

"OL" generally means Open Line, Open Loop, or Over Limit, depending on how the meter manufacturer describes it.

In practical terms, your meter is telling you that it cannot detect the expected electrical path in that direction.

However, OL is not automatically good or bad.

If the reference chart says a particular pin should read OL and your meter shows OL, that is normally what you want.

If the chart says a pin should produce a measurable diode value and your meter instead shows OL, then something is wrong with either the measurement or the circuit being tested.

That distinction is very important when testing an Epson printhead. The BCH L1800/1390 reference information specifically contains both numerical values and positions where OL is expected. (BCH Technologies)

If Every Pin Says OL, Check These Areas

1. Confirm the probe connections on the multimeter.

Make sure the black probe is connected to COM and the red probe is connected to the meter's voltage/resistance/diode input. If the red probe is accidentally plugged into the current or amperage socket, you may not get the expected measurements.

2. Verify diode-test mode.

Do not rely only on where the selector knob appears to be pointing. Look for the diode symbol and check the meter's display. If the meter says "2M," there is a good possibility that you are actually measuring resistance rather than using the diode-test function.

3. Check the orientation of the FFC cable.

This is one of the easiest mistakes to make. The Epson L1800/1390 uses a narrow cable and a wider cable, and identifying the true first conductive pin is important. Some portions of an FFC can look like pin positions even though they are not actual electrical contacts. Our testing instructions therefore emphasize carefully identifying pin 1 before taking measurements. (BCH Technologies)

4. Make sure you are touching the exposed conductive surface.

FFC cables have exposed metallic contacts on a particular side. If the probe is touching the insulated side of the cable instead of the metal contact, the meter will naturally display OL.

Even when you are on the correct side, the tip of a standard multimeter probe can be relatively large compared with an FFC contact. It is easy to think that you are touching the pin when the probe is actually resting on the plastic between two contacts.

5. Verify that the test cable is actually connected to the printhead.

If you are using an extension FFC or separate test cable, make sure it is fully seated in the printhead connector and that its exposed contacts are facing the correct direction. A cable that is inserted backward, partially inserted, damaged, or not making contact can cause every test point to appear open.

6. Check the multimeter itself.

Touch the two probes together.

Your meter should respond immediately. The exact response depends on the mode and meter design, but there should be a clear change from an open-circuit OL indication.

If touching the probes together still gives you OL, troubleshoot the meter, probe cables, input sockets, or meter setting before testing the printhead.

Probe Polarity Is Important

Diode testing is directional. Reversing the red and black probes can produce a completely different result.

For the specific test demonstrated in our Epson L1800/1390 procedure, the red probe is placed on pin 1 and the black probe is used to test the remaining specified pins. (BCH Technologies)

Therefore, if you have already confirmed that the red probe is on the correct pin 1, make sure the red and black leads are also plugged into the correct meter sockets and that you have correctly identified the orientation of the narrow FFC cable.

Do Not Assume "All OL" Means the Head Is Safe

This is especially important.

The purpose of this test is not merely to find OL. It is to compare each electrical path with the expected reference value.

A healthy printhead will normally give you a pattern:

• Some pins have measurable diode readings.

• Certain designated pins show OL.

• The measurable values should be reasonably close to the reference values.

A printhead can be questionable if a pin that is supposed to produce a value shows OL. Conversely, a pin that is supposed to remain electrically isolated can indicate a serious problem if it suddenly produces continuity or an unexpected low reading. Our reference material is designed around comparing those individual pin readings rather than treating OL by itself as a pass condition. (BCH Technologies)

Be Careful Before Reconnecting a Questionable Printhead

I would not reconnect the printhead to a known-good mainboard simply because all of your measurements currently display OL.

First determine why you are not getting the expected numerical readings.

A printhead with an electrical short can potentially damage components in the printer's mainboard or printhead-driving circuitry. That is one of the main reasons for performing this electrical test before installing an unknown or questionable head. (BCH Technologies)

Also make sure the printhead and FFC connections are completely dry before electrical testing or installation, particularly if the printhead was recently cleaned, flushed, soaked, or exposed to cleaning solution. Liquid contamination around the contacts can make electrical diagnosis unreliable and may create conductive paths when power is applied.

Based on what you described, I would start with the meter setting and cable contact, rather than assuming that every circuit inside the printhead has suddenly become open. Confirm that you are truly in diode mode, verify that the probes work by touching them together, confirm the red probe is on the actual pin 1, make sure you are contacting the metallic side of the FFC, and then repeat the measurements. You should normally see a mixture of measurable values and OL readings rather than OL on every test point.

Addressing printer problems can be complicated because so much of the diagnosis depends on hands-on inspection and measurement. For that reason, we're not able to provide ongoing remote troubleshooting, repair suggestions, or individualized repair support. We do offer an in-person evaluation and repair service through our BCH Technologies printer diagnostic and repair facility [https://bchtechnologies.com/printer-repair-service]. Because demand is high, repairs are handled on a first-come, first-served basis, and it may take several weeks before we are able to accommodate a printer for drop-off. Our service options cover either complete printers or certain individual components, with instructions available on how to proceed. We also recognize that professional repair service may not be the most economical choice for every situation, so we strongly encourage self-help and independent online research whenever practical. A good place to begin is YouTube or the homepage of our BCH Technologies YouTube channel [https://youtube.com/@bchtechnologies]. On our channel, use the search icon near the "About" section to look for the specific printer model, component, or symptom you are working on. I receive dozens of questions asking whether we have a video about a particular repair, and after producing videos for more than nine years, it is difficult to remember every individual topic we have covered. YouTube's search function is therefore usually the fastest way to locate the relevant material, and it may also recommend useful videos from other creators.

Thank you again for reaching out, for watching our videos, and for supporting BCH Technologies. We truly appreciate you taking the time to ask the question, and I hope this explanation helps clarify why an OL reading on every pin should be investigated before deciding whether the printhead is good or bad.