I’ve driven in the rain without wipers, and I won’t forget the experience. The wiper rod failed on the way home from a trip to the city, arm out the window working it Fred Flintstone style… for 2 hours, anyway here’s the fix.
Front wipers work on a separate wiring circuit to the rear wiper. Common front wiper problems include:
- Blown fuse
- Faulty relay
- Faulty Switch
- Faulty motor
- Faulty wiring
- Faulty Control module
By the end of this post, you’ll understand how to diagnose all the most common causes of an inoperable front wiper. You’ll also learn what you can do to fix it.
Wiper System Overview
Your wiper system is a very important kit that you rely on for your safety, especially in the winter and that’s usually when they cause the most trouble. As you now know, your front wiper system works independently to the rear, they are not connected in any way other than they may share control from the same driver stalk switch.
I won’t go too deep into how your wipers work, but a working knowledge of how the system operates will help your diagnosis journey. Your car’s wiper system has evolved along with car technology. Wiper systems are no longer hard-wired; they are like most electrical components on your car, controlled by a control module (computer).
This has facilitated advancements such as auto wipers, which employ rain sensors to detect windshield moisture. These advances have made the system more complex and at the same time in theory, easier to diagnose.
When the computer tasked with controlling your wipers detects a problem, it sets a unique code that indicates the type of fault identified. The technician scans the vehicle, reads the codes, checks the database for known problems or recalls, and sets about verifying the fault reported.
Fault codes help a ton when fault finding; however, the level of detail is usually proportional to the spec level of the car.
It’s therefore not uncommon to have a problem with the wipers or other components and the computer not set a code at all. This is where the control module is no longer helpful and becomes an obstacle to diagnoses.
A basic wiper system comprises of:
- Wiper assembly (superstructure and wiper arms and blades)
- Wiper motor
- Relays (commonly 2)
- Wiper control modules
- Wiper switch
How Does It Work?
Here’s a brief outline of operation flow:
- Wipers turned on at the steering wheel and signal a is sent to the wiper control module
- Control module receives the request and processes. An output is sent to the wiper on/off relay
- On/off relay receives the output (could be ground or power, ground is now more common)
- On/off relay completes the load circuit and power is sent to the slow speed of the wiper motor
- When a higher wiper speed is requested the second relay ( low/high speed relay) is activated and wiper speed increases
Diagnosing The Fault
With the correct test equipment, the job is easier, a pro shop will use a diagnostic code reader with an integrated command function. That means the diagnostic tool has the ability to turn wipers on and off through the tool.
This shortens diagnostic time as it indicates if the problem is on the load side of the circuit or the control side. You can check out the scan tools I use here on the Auto electrical repair tools page.
You won’t have a dealer-level scan tool, so we’ll do it old-school style. If you have a code reader and you should consider buying one, they aren’t expensive.
To begin our diagnoses, we’ll use our ears. Have a helper turn on the wipers while you listen for some sounds of life; you’ll need to turn on the ignition.
Hearing some noise from the general wiper area tells us the switch and power supply is likely not the issue. The issue could be a mechanical failure of the wiper assembly or a failing motor. I wrote a post about mechanical wiper fault here “Wipers not working but motor is”
Hearing no noise at all tells us you may have a power issue, as I expect this will be the root cause of most problems we’ll dig deeper on this subject.
A blown fuse is first on our list of components to check. It’s easy and is the most common cause of inoperable wipers. But it may not be the root cause of the problem. We may find replacing the fuse works for a short while and then blows again (more on this later).
The wiper fuse is usually located under the hood, but it will be listed inside the fuse cover or certainly in your driver’s manual. Most late-model cars use mini fuses, so use the tool often supplied in the box cover or grab small pointy nose pliers to remove the fuse.
Fuses are rated by amp (A); each circuit is designed and rated at different amps relative to the size of the load (light, motor, etc.) and the wiring size. Using a fuse that is too small will cause the fuse element to heat and blow.
Conversely, using a fuse too large will prevent the fuse from blowing if (when) needed and instead causes a fire, damaging components or destroying the vehicle completely. An insurance claim on such a vehicle, indecently, if discovered by an insurance forensic investigator, would likely be rejected.
A blown fuse will display a broken element. Check the fuse size in the fuse guide; I never assume the fuse removed is the correct rating.
A relay is a simple electro-mechanical device used to power a higher amp load circuit using a lower amp control circuit. The circuit’s load and control are independent.
Your wiper circuit may have two relays, one is the on/off relay, and the other is the low/high-speed relay. You’ll likely find them in the engine bay fuse box, but check the location in the driver’s manual.
A couple of simple tests get the ball rolling. Have your helper turn on the ignition and operate the wiper switch on and off repeatedly. Listen carefully for the audible click of the relays.
No sound at all means any of the components could be at fault, including the relay. A clicking sound tells us the switch control module and wiring are likely good, the problem could be a load side wiring, motor, or faulty relay. You are correct to think a clicking relay is a good sign, but it isn’t conclusive that the relay is good.
Here’s a quick hack for eliminating the relay as the fault: if the fuse box contains a second relay that’s identical (usually does), swap it out and test the wipers again. Do be careful that the connections are identical.
Other relay checks include:
Next, go ahead and pull the relay, shake it, if it rattles, it’s faulty, and if it doesn’t, it could still be faulty. So, we’ll need to hotwire the relay and use a DVOM to check continuity, see the infographic.
Now we’ll turn our attention to the wiper motor itself. Accessing the motor can be a pain in the ass. So instead of stripping lots of components to access the motor, which after all could be OK, we’ll hotwire the motor from the fuse box.
Remember the relay has a control side and a load side, the wiper motor is the load. Go ahead and remove the fuse and identify the load side, (pin 87 and 30) the load side is the power to the motor.
Using a jumper wire, connect 87 to 30 momentarily.
If the wipers moved, we know the motor is good and we can move on to the next test. If wiper didn’t move, we’ll suspect the motor has failed. But we’ll need to run a few more checks first. Go ahead and check for power at pin 30, if you don’t have power here, you’ll need to trace the wiring for a fault.
Continuity testing between motor and pin 87 of the relay board and the motor will require access to the motor.
Faulty Control Module
So the motor tested good, now we’ll turn our attention to the control module. The module could be hidden in lots of places and for now, we don’t care where it is because we’ll check its output at the wiper relay. This time we’ll test the control side of the relay circuit.
Many late model cars control relays by sending a ground to the relay, but yours could be different. You can tell which type you have by testing for power at pin 86 with the ignition on and wiper off. The relay board will be marked with the pin id numbers, as will the relay.
If power is present in the relay board at 86 with wipers off, your wiper relay is ground controlled by the control module.
Use a DVOM or test light to probe both sides of the relay while a helper turns on the wipers with the ignition on.
- If no power is found, use an external ground and test for power at 86 with wipers on
- Now swap your test light to a power source. Check board pin 85 for ground with wipers on (test light lights when gets a good ground)
- If your control supply is missing, move on and check the wiper switch out put
- If your power/ground is missing, you’ll need to trace wiring for open
The switch isn’t as easy to access as the relay, the lower steering wheel column will need to be removed to access wiring. However, at this point, the relevant wiring diagram will need to be referenced, without the proper wiring diagram we’ll be shooting in the dark.
The switch likely operates using reference voltage (Logic). To test the output of the switch check for voltage changes as the wiper switch is operated. You will need to disconnect it from the controller before testing.
Wiring faults are the toughest to find, if you like a good puzzle you’ll just love fault-finding wiring circuits. Truthfully I like wiring problems, it’s a pleasant change from the mundane brake and suspension jobs that are more brawn than brains.
A mechanics hack for finding wiring faults, while not guaranteed, it does often show you the local. Called the wiggle test – With the wipers and ignition on, move the wiring loom around the wiper motor and rear of the fuse box, this often causes the wipers to work momentarily.
Check the circuit in a systematic way, but obviously, check all the easy-to-access points of the circuit first. Check for voltage, or run a voltage drop test.
The volt drop test is the best test for a circuit and it’s simple to execute and interpret. The test will only work when the circuit is live (wipers on), and a DVOM is required. The test involves checking the voltage difference between two points in the circuit. A large difference, (more than .3v on the power side and .2v on the ground side) indicates high resistance.
The high resistance may be caused by any of the following:
- Loose wiring
- Broken wiring
- Dirty wiring
- Faulty motor
- Bad block connector pin tension
- Corroded wiring
- Corroded pin connectors
Rodent damage is common, if you see evidence, suspect open wiring. Rodents love to eat wiring insulation. Other problems areas include the rear of the fuse board connection, rear of relay board connection, control module connector, loom turning points on the chassis, especially where there’s vibration.
Tools I use
Check out all the tools I use on the Auto electrical repair tools page. The tools I used here include a:
- Volt meter
- Test light
- Battery maintainer
- Banana jacks
- Back probes
How long should a wiper motor last? Wiper motors will typically last the working life of a car, in excess of 150k miles and 15 years. However, poor maintenance and operator misuses, such as bad wiper blades and uncleared windshield snowfall, will shorten its life expectancy.
- About the Author
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.