RV Engine Stalling & Transmission Issues - Diagnosis Help

What's Going On

Tim, your RV's engine stalling when rolling to a stop at intersections, combined with the erratic transmission downshift behavior you're describing, points to a classic torque converter clutch (TCC) issue or a transmission control module problem. The fact that this happens intermittently (about 2 times per 1000 km as you mentioned) and that your engine RPM sometimes drops quickly when letting off the gas while other times stays constant suggests your transmission isn't properly managing the TCC disengagement during deceleration.

What's likely happening is that your torque converter lockup clutch is sticking in the engaged position during deceleration. In normal operation, when you lift off the accelerator, the transmission should disengage this lockup clutch to allow the engine to idle independently of the transmission. When it fails to disengage, the transmission essentially tries to keep the engine rotating at road speed, and when you slow down enough, the engine stalls because it can't maintain idle RPM while still mechanically connected to the drivetrain.

This problem is particularly common in RVs because of the additional weight and stress on the transmission system, and the fact that many RV owners don't drive their vehicles frequently enough to keep all the transmission components properly exercised. The intermittent nature you're experiencing is typical of a torque converter clutch that's beginning to fail but hasn't completely stuck yet.

Why This Happens

The root cause of your stalling and erratic transmission behavior stems from the torque converter lockup system, which is designed to improve fuel economy by mechanically connecting the engine to the transmission at highway speeds. This system relies on several components working in harmony, and when any one fails, you get the symptoms you're describing.

The most common culprits include:

• Worn torque converter lockup clutch: The friction material inside the torque converter can wear down or become glazed, causing it to stick in the engaged position
• Faulty lockup solenoid: This electronic valve controls hydraulic pressure to engage and disengage the lockup clutch, and when it sticks or fails electrically, the clutch won't release properly
• Transmission Control Module (TCM) issues: The computer that manages transmission operation may have corrupted programming or failing sensors that prevent proper lockup clutch timing
• Contaminated transmission fluid: Old or burnt fluid can cause solenoids to stick and clutch surfaces to grab inconsistently
• Vacuum line problems: On older RVs (pre-1990s) with vacuum-controlled transmissions, cracked or disconnected vacuum lines can cause erratic lockup behavior
• Engine idle speed issues: If your engine's base idle speed is set too low, it may not have enough RPM to stay running when the lockup clutch doesn't disengage properly

The intermittent nature of your problem suggests the issue is progressive rather than a complete failure. Components like the lockup solenoid may be sticking occasionally due to varnish buildup from old transmission fluid, or the clutch material may be at the point where it grabs inconsistently depending on temperature and hydraulic pressure conditions.

Environmental factors also play a role, especially in Germany's climate. Cold weather can make transmission fluid thicker, causing solenoids to react more slowly. Conversely, hot weather after long drives can cause components to expand and stick when they should release.

Getting Ready

Before diving into the diagnostic process, you'll need to gather some specific tools and information to properly troubleshoot this transmission issue. SAFETY WARNING: Always ensure the RV is on level ground and properly supported with wheel chocks before working underneath. Transmission fluid operates at high temperatures and can cause severe burns - allow the system to cool before checking fluid or performing maintenance. Never work under a vehicle supported only by a jack. The good news is that much of the initial diagnosis can be done with basic tools, though you may need access to a scan tool for deeper investigation.

Essential tools and supplies you'll need:

• OBD-II scanner or professional scan tool: Critical for reading transmission-specific trouble codes that may not show up on basic code readers. Note that many Allison transmission codes require manufacturer-specific scan tools
• Digital multimeter: For testing electrical circuits and solenoid resistance
• Transmission fluid sample containers: Clean containers to collect fluid samples for inspection
• Basic hand tools: Socket set, wrenches, and screwdrivers for accessing transmission components
• Vacuum gauge and pump: If your RV has a vacuum-controlled transmission (more common on older models)
• Fresh transmission fluid: Get the exact specification for your RV's transmission - this varies significantly by manufacturer and year
• Transmission filter: Have one on hand in case a fluid change is needed during diagnosis

You'll also want to gather some important information about your RV before starting. Document the exact year, make, and model of both your RV and the chassis it's built on, as transmission specifications can vary even within the same RV model year. Note the transmission type - this is usually stamped on the transmission pan or listed in your owner's manual. Common RV transmissions include the Allison 1000/2000 series, Ford TorqShift, and GM 6L80E/6L90E variants (with 4L80E found in older models).

Create a driving log for the next few days to better understand the pattern of your stalling issue. Note the outside temperature, how long you've been driving, whether the engine is warm or cold, and exactly what driving conditions trigger the problem. This information will be invaluable for diagnosis, especially since the issue is intermittent.

Walking Through the Fix

Start your diagnosis with the simplest and most common solutions before moving to more complex repairs. The systematic approach will save you time and money, and many transmission issues can be resolved with basic maintenance that's often overlooked.

Begin with a comprehensive transmission fluid inspection:

1. Check fluid level and condition: With the engine running and transmission warm, check the fluid level using the dipstick. The fluid should be bright red or amber (depending on type) and should not smell burnt or have a dark, thick consistency
2. Look for metal particles: Wipe the dipstick with a clean white cloth and examine for metallic particles, which indicate internal wear
3. Test fluid viscosity: Good transmission fluid should flow smoothly off the dipstick without forming thick drops or strings
4. Document fluid age: If you don't know when it was last changed, plan on a complete fluid and filter service

Next, perform a basic electrical diagnosis of the lockup system. Connect your OBD-II scanner and look for any stored codes related to the transmission, particularly P0741 (torque converter clutch circuit), P0742 (torque converter clutch stuck on), or P0743 (torque converter clutch electrical). Even if no codes are present, many scanners can display live data showing lockup clutch commanded vs. actual status.

Test the lockup solenoid operation by accessing live data while driving. You should see the lockup clutch engage at highway speeds (usually above 45-50 mph) and disengage when you lift off the accelerator or apply the brakes. If the data shows the PCM commanding disengagement but the clutch stays locked, you've confirmed a mechanical problem with either the solenoid or the clutch itself.

If electrical diagnosis points to the lockup solenoid, you can often test this component without removing the transmission pan. Locate the transmission solenoid connector and test the solenoid's electrical resistance according to manufacturer specifications. The connector (usually on the side or bottom of the transmission case) and test the lockup solenoid's resistance with your multimeter. Typical resistance values range from 20-50 ohms, but check your service manual for exact specifications.

For intermittent problems like yours, consider performing a transmission adaptation reset if your scan tool has this capability. Many modern transmissions learn driving patterns over time, and corrupted adaptation values can cause erratic shift behavior. This procedure varies by manufacturer but typically involves following a specific sequence with the scan tool while the engine is running.

If basic electrical and fluid checks don't reveal the issue, you may need to perform a more detailed hydraulic test. This involves connecting a pressure gauge to the transmission's test ports and monitoring lockup clutch apply and release pressures during the problem conditions. This test typically requires lifting the RV and may need professional equipment.

Beyond DIY Territory

While many transmission issues can be diagnosed and even repaired by experienced DIY mechanics, certain aspects of your problem may require professional intervention, especially given the intermittent nature and the complexity of modern transmission control systems.

Consider seeking professional help if you encounter any of these situations during your diagnosis:

• Internal transmission damage: If your fluid inspection reveals significant metal particles, burnt fluid, or if pressure tests show internal leakage, the transmission likely needs internal repair or rebuilding
• Complex electrical issues: Problems with the Transmission Control Module or engine PCM communication often require dealer-level scan tools and software updates that aren't available to DIY mechanics
• Torque converter replacement: If the lockup clutch inside the torque converter has failed, this requires transmission removal and is typically a job for a transmission shop
• Valve body problems: Issues with the transmission's internal hydraulic control circuits require specialized knowledge and tools to repair properly

When selecting a repair shop in Germany, look for facilities that specialize in RV or heavy-duty transmissions rather than standard automotive shops. RV transmissions often have different specifications and operating requirements than car transmissions, and experience with these systems is crucial for proper diagnosis and repair.

Get quotes from multiple shops, but be wary of places that quote repairs without thorough diagnosis. A reputable transmission shop should want to road test your RV, perform computer diagnosis, and possibly do internal pressure tests before recommending major repairs. Expect diagnostic fees of 100-200 euros, which should be applied toward repair costs if you proceed with the work.

For your specific intermittent stalling issue, a good transmission shop should be able to duplicate the problem using their scan tools and road testing procedures. They can also access manufacturer technical service bulletins that may address known issues with your specific transmission model.

If major transmission work is needed, consider the age and overall condition of your RV when making repair decisions. A complete transmission rebuild typically costs 2000-4000 euros depending on the transmission type and local labor rates. Sometimes a quality remanufactured transmission can be more cost-effective than rebuilding your existing unit, especially if multiple internal components have failed.

Keep in mind that transmission problems can sometimes be symptoms of other issues, such as engine performance problems, cooling system issues, or even problems with the RV's electrical charging system. A comprehensive diagnostic approach will ensure you're addressing the root cause rather than just the symptoms you're experiencing.