John Deere T105S Trimmer (Troubleshooting & Parts)
The John Deere T105S is a gasoline, two-cycle string trimmer for grass and light weeds. Start troubleshooting by checking the model label and exact manual, then verify fuel mix, starting steps, air filter, cutting head, and safety parts. Do not guess the oil ratio, line size, or carburetor settings; these can vary by version.
A trimmer that will not start can feel like a dead link in a chain of yard jobs: one small fault delays everything. I work through the T105S in a set order, from fuel and controls to airflow and parts. That approach can help you find simple problems without replacing good components or risking engine damage.
Identify your T105S and its correct specifications
The model name is a starting point, not enough to identify every replacement part or service setting. Versions can differ, so use the product label and the operator’s manual for your exact unit before mixing fuel, buying line, or ordering parts. If a marking is worn, record the model and serial details before seeking help.
Find the label on the trimmer and note the model and serial number. Then use those details to locate the matching operator’s manual and parts diagram. Check the manual for the fuel-to-oil ratio, starting procedure, allowed line diameter, maximum cutting swath, and any service limits. Do not fill gaps with a value from another trimmer.
The T105S is a straight-shaft, two-cycle gasoline string trimmer. It is intended for line trimming, not chainsaw work. Chain pitch, guide-bar length, bar oil, and drive links do not apply. Do not fit a saw blade or another attachment unless the manual for your exact unit expressly approves it.
| Item to confirm | Where to check | Why it matters |
|---|---|---|
| Fuel and oil ratio | Emissions label or exact manual | A wrong mix can damage a two-cycle engine |
| Line diameter and swath | Head marking or manual | Oversize line may overload the head or engine |
| Replacement part number | Exact parts diagram | Similar-looking parts may not fit |
| Idle and maximum speed | Exact service information | Incorrect settings can cause unsafe operation or damage |
Keep a photo of the label and the relevant manual pages with your maintenance records. That small step makes later troubleshooting and parts ordering more reliable.
Prepare the trimmer safely before testing it
A safe inspection means checking the parts that control the cutting head, engine, and operator before starting. Wear eye or face protection, hearing protection, gloves, long trousers, and sturdy footwear. Work outdoors, away from people, pets, and ignition sources, and stop if you find a leak or damaged control.
With the engine off and cool, inspect the guard, handle, cutting head, line-retaining parts, and fasteners. Check that the throttle trigger and lockout move and return as intended, and that the stop switch works. Look over the fuel cap and lines for cracks, wet spots, or a strong fuel smell.
Do not operate the trimmer with a missing or damaged guard, loose head, fuel leak, or defective throttle control. Do not bypass a safety feature to keep the unit running. If a part is damaged, match the replacement to the exact model and serial version.
Before starting, move the head away from the ground, people, and objects. Follow the manual’s cold- or warm-start steps rather than using a generic choke routine. When the engine runs, confirm the head stays still at idle and the stop switch shuts the engine off.
Troubleshoot a T105S that will not start
A no-start check begins with the simplest causes: fuel, switch position, starting method, and airflow. Change one thing at a time so you can tell what helped. If the engine smells strongly of fuel, leaks fuel, or shows damage, stop and have it checked rather than continuing to pull the starter.
Use this sequence:
- Confirm the stop switch is set to run.
- Check that the fuel is fresh and mixed at the ratio printed on the unit or stated in its manual. Never use straight gasoline in this two-cycle engine.
- Follow the manual’s primer, choke, and starting steps for a cold or warm engine.
- Inspect the air filter. Clean or replace it only as the manual directs.
- Check the spark plug for visible damage or heavy deposits, using the manual’s removal and service instructions.
- Look for fuel leaks, damaged fuel lines, or a primer that appears cracked or loose.
If it still does not start, avoid repeated adjustments to the carburetor. A qualified service shop can test ignition and fuel delivery, then check the carburetor using the correct service data. Do not set mixture screws or maximum engine speed by ear.
| What you notice | First checks | Next step |
|---|---|---|
| No response after pulling | Switch, fuel, starting steps | Recheck manual procedure |
| Starts, then stops | Fuel condition, filter, lines, air filter | Stop if it surges or runs hot |
| Wet plug or strong fuel smell | Starting procedure, possible flooding or leak | Let it cool; inspect for leaks |
| Head turns at idle | Idle setting or clutch issue | Stop use and seek qualified service |
Diagnose stalling, weak power, or a hot-running engine
Poor acceleration and stalling can come from restricted fuel or air, excess drag at the cutting head, or a service issue. First inspect the easy-to-reach causes with the engine off and cool. If power loss comes with surging or unusual heat, stop the engine; continued running may cause severe damage.
Check that the fuel is fresh and correctly mixed. Inspect the air filter, visible fuel lines, filter condition where accessible, and cooling-air passages. Clear grass and debris from the head and guard, and check whether line is tangled or wound in a way the manual does not allow.
A fuel-starved or lean-running two-cycle engine can overheat and seize. Stale fuel, a restricted fuel line or filter, or a blocked carburetor may contribute to fuel starvation. If the trimmer surges, loses power, or runs unusually hot, shut it down. Correct the fuel supply and have the fuel system inspected before using it again.
Do not force the head through dense growth to make up for weak cutting. Use the line tip for controlled trimming, reduce throttle, or reposition the tool. If the engine still bogs with fresh, correctly mixed fuel and a clear head, arrange service rather than turning carburetor screws without the exact procedure and a tachometer.
Check the cutting head and choose the right line
The cutting head is the line-holding assembly at the end of the shaft. Its approved line size and cutting swath depend on the head fitted to your unit. Read the head marking and manual before buying line; do not assume that a line size used on another trimmer is suitable.
With the engine off, switch set to stop, and head fully still, check for tangled line, cracks, wear, and loose parts. Reload the head as shown in the manual. Too much or too-large line can add drag and reduce performance, while damaged retaining parts can let line come loose.
Use the correct guard and keep it in place. The guard helps block thrown debris and may also trim line to the intended length. Never remove it to gain a wider cut. Do not install a blade or other attachment unless the exact T105S manual approves that setup.
Find and order the right replacement parts
A parts diagram is an exploded view that shows how components fit and identifies their part numbers. Use the diagram for the exact model and serial range when possible. Before ordering, compare the part name, number, and fit details; a general listing for a similar John Deere trimmer is not enough to confirm compatibility.
Parts commonly checked during service include the cutting head, guard, air filter, fuel lines, primer, spark plug, and ignition parts. The diagram may list revisions or separate versions. If your label or diagram does not settle the fit, ask the parts supplier to confirm using the serial number.
| Part or area | Check before ordering | Do not do this |
|---|---|---|
| Cutting head | Head type, attachment, model fit | Guess from appearance alone |
| Guard | Exact fit and mounting points | Run without an approved guard |
| Air filter | Shape and part number in diagram | Use a loose or poorly fitting filter |
| Fuel line or primer | Routing, connectors, and version | Substitute a line that kinks or leaks |
| Ignition part | Exact model and service listing | Replace parts without checking basics |
When the fault is unclear, diagnose first and order second. A clogged filter and stale fuel can look like a carburetor problem, but replacing a carburetor will not fix the underlying fuel or air restriction.
Use, clean, and store the trimmer between seasons
Routine care helps prevent starting problems and catches wear before the next job. After trimming, release the throttle, switch the engine off, and wait for the head to stop. Let the engine cool before inspection, cleaning, or service, and follow the manual for any fuel left in the tank.
Keep the guard, head, and cooling-air inlets clear of grass and debris. Check for tangled line, loose fasteners, damaged parts, and fuel leaks. Store the trimmer in a cool, dry place away from flames or other ignition sources. Follow the manual’s directions to drain or stabilize fuel; do not assume one storage method suits every version.
Before the next season, inspect fuel lines and seals for cracks or leaks, check the air filter, and confirm the head and guard are secure. Mix fresh fuel at the specified ratio when needed. If the unit has been stored with old fuel and now runs poorly, do not try to solve the problem by changing the mixture or adjusting the carburetor at random.
Field troubleshooting examples
These examples show a diagnostic method, not a claim that every T105S has the same fault. I use the same principle in service: confirm the unit’s version, inspect basic causes, and change one factor at a time. That makes it easier to avoid unnecessary parts and unsafe work.
Example: starts, then stalls. Check the fuel age and mixture against the label or manual. Then inspect the filter, visible fuel lines, and air filter. If the engine surges or becomes unusually hot, stop and arrange fuel-system inspection before running it again.
Example: cuts poorly and bogs down. Turn the trimmer off and let it cool. Check for tangled line, a damaged head, debris, or line beyond the permitted size. Reload with the specified line and clear the guard. If power remains weak, inspect airflow and fuel supply before seeking carburetor service.
Example: replacement head does not fit. Do not force it or modify the shaft. Compare the old head with the parts diagram for the serial range and confirm the attachment details with a supplier. A head that appears close may still be the wrong part.
Frequently asked questions
What fuel mix does the John Deere T105S use?
Use the ratio printed on your unit’s emissions label or specified in the operator’s manual for its exact version. Do not assume 50:1, and never run straight gasoline in this two-cycle engine.
What size trimmer line fits the T105S?
Check the marking on the installed head or the exact manual. The approved line diameter and maximum swath should not be guessed or copied from another model.
Why does my T105S start and then die?
Check fresh, correctly mixed fuel, the air filter, visible fuel lines, and the starting procedure. A restricted fuel system can also cause stalling. Stop using it if it surges or runs unusually hot.
Why does the trimmer head spin at idle?
The idle setting or clutch may need attention. Stop using the trimmer until the head stays still at idle; have a qualified service shop inspect it if the manual does not give a safe adjustment procedure.
Can I adjust the carburetor by ear?
No. Use only the exact service procedure and specifications. If adjustment is allowed, a qualified technician should use a tachometer and the correct data.
Can I put a saw blade on the T105S?
Only if the operator’s manual for your exact unit expressly approves that attachment. Otherwise, use the specified line-trimming head.
Where do I find the right T105S parts?
Use the model and serial number on the product label with the matching parts diagram. Confirm the part number and fit before ordering.
What should I check if the T105S will not start?
Check the stop switch, fresh correctly mixed fuel, the manual’s primer and choke steps, air filter, and spark plug condition. If it still will not start, have the ignition and fuel system checked.
How should I store the trimmer over winter?
Follow the exact manual’s fuel-storage instructions. Clean the head and cooling-air inlets, inspect for leaks or damage, and store the unit in a cool, dry place away from ignition sources.
Is it safe to keep running the trimmer when it loses power?
No. Shut it down if it surges, loses power, or runs unusually hot. Fuel starvation can overheat and seize a two-cycle engine, so correct the fuel supply and have the system inspected before continued use.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
