What a Di Endorsement Lets You Drive

A Di endorsement on a Michigan CDL permits you to operate a tractor pulling two or three trailers — called doubles or triples. Without this endorsement, your CDL restricts you to a single tractor-trailer combination. The Di endorsement is specific to Michigan and is required before you can legally haul doubles or triples on Michigan roads.

The endorsement applies only to the vehicle configuration itself, not to cargo type. You may still need additional endorsements — such as Hazmat (H) or Tank (N) — depending on what you are hauling, but the Di endorsement is what authorizes the multi-trailer setup.

Key Takeaways

  • You must hold a valid Michigan CDL before you can add a Di endorsement; you cannot test for Di without an existing CDL.
  • The Di endorsement requires passing a written knowledge test at a Secretary of State office; there is no behind-the-wheel skills test for this endorsement alone.
  • You must be at least 18 years old and have a valid medical certificate on file with the Michigan Secretary of State.
  • The fee for adding a Di endorsement is a flat amount set by Michigan and does not vary by renewal cycle.
  • Once you pass the knowledge test, the endorsement is added to your CDL when ready or at your next renewal, depending on your current license status.

Requirements Before You Test for Di

You must already hold a valid Michigan CDL in Class A or Class B. If you do not yet have a CDL, you will need to obtain one first through the standard CDL testing process — written knowledge test, medical certification, and skills test — before you can add the Di endorsement.

You must be at least 18 years old. Michigan does not waive this age requirement for the Di endorsement, even if you are younger and hold a CDL for intrastate-only operation.

You must have a current medical certificate on file with the Michigan Secretary of State. This is the same medical certificate required for your base CDL. If your medical certificate has expired, you will need to renew it before testing for Di.

The Di Knowledge Test: What to Expect

The Di endorsement test is a written examination administered at any Secretary of State office that offers CDL testing. The test covers the operation, safety, and handling of doubles and triples — including coupling and uncoupling procedures, weight distribution, braking differences, and how the longer wheelbase affects turning and backing.

Michigan does not publish the exact number of questions on the Di test, but it is shorter than the general CDL knowledge test. You will be asked to answer questions on a computer terminal at the Secretary of State office. You must score at least 80 percent to pass.

There is no behind-the-wheel skills test for the Di endorsement. Once you pass the knowledge test, you are done with testing. You do not need to demonstrate doubles or triples operation on the road.

How to Schedule and Take the Test

Contact your local Michigan Secretary of State office to schedule a CDL knowledge test appointment. You can find office locations and phone numbers on the Michigan Secretary of State website. Some offices allow online scheduling; others require a phone call. Availability varies by location, and wait times can range from a few days to several weeks depending on demand.

Bring your current Michigan CDL, a valid photo ID, and your medical certificate. Arrive at least 15 minutes early. You will be asked to sign a statement confirming you understand the test rules and that you are taking the test for the correct endorsement.

The test is administered on a computer. You will have a set amount of time to complete it — typically 30 to 45 minutes, though Michigan does not publish a strict time limit. Once you submit your answers, you will receive your score when ready. If you pass, the endorsement is added to your record that day.

What Happens After You Pass

If you pass the Di knowledge test, the endorsement is recorded in the Michigan Secretary of State database when ready. Your physical CDL card will show the Di endorsement on your next renewal or replacement. If your CDL is not due for renewal soon, you can request an updated card for a fee, or you can wait until your regular renewal date.

Some employers require proof of the endorsement before you start driving doubles or triples. You can print a record of your driving record from the Secretary of State website, which will show all endorsements on file. This document is often accepted as proof until your new card arrives.

Fees and Renewal

Michigan charges a flat fee to add a Di endorsement to your CDL. This fee is separate from your CDL renewal fee and does not change based on when you add it during your license cycle. The exact amount varies and should be confirmed with your local Secretary of State office or the state's CDL information line.

Once you have the Di endorsement, it remains on your CDL through every renewal cycle as long as you maintain a valid CDL. You do not need to retake the Di knowledge test at renewal. The endorsement renews automatically with your base license.

Common Mistakes and How to Avoid Them

The most common error is attempting to test for Di without an active CDL on file. You cannot add Di to a learner's permit or a regular Class D driver's license. Your base CDL must be valid and current.

Another frequent mistake is letting your medical certificate expire before testing. If your medical certificate is not current, the Secretary of State will not allow you to test, even if your CDL itself is valid. Check your medical certificate expiration date before scheduling your test appointment.

Some drivers assume they need a skills test for Di. Michigan does not require one. The knowledge test is the only testing requirement. If someone tells you that you need to demonstrate doubles operation on the road, they are mistaken.

Frequently Asked Questions

Can I test for Di if my CDL is about to expire?

Yes. As long as your CDL is currently valid on the day you test, you can add the Di endorsement. The endorsement will be added to your record and will appear on your renewed CDL when you renew your base license.

What if I fail the Di knowledge test?

You can retake the test. Michigan allows you to schedule another appointment and test again. There is typically a waiting period between attempts — check with your local Secretary of State office for the exact policy. Each attempt requires paying the test fee again.

Do I need a Di endorsement to tow a single trailer with a dolly?

No. A dolly is not a trailer in the CDL sense. A Di endorsement is required only when you are towing two or more full trailers behind a single tractor. A tractor with one trailer and a dolly does not require Di.

Can I add Di to a Class B CDL?

Yes. Michigan allows Di endorsements on both Class A and Class B CDLs. The endorsement rules are the same regardless of which class you hold.

How long does it take to get the endorsement added to my physical card?

The endorsement is added to your driving record when ready after you pass the test. Your physical CDL card will show it at your next renewal. If you need proof before then, you can print your driving record from the Secretary of State website or request an expedited card replacement for a fee.