Service Done Right #14: Troubleshooting Relay Controlled Circuits

[@mikechiodetti4482]

Make sure the battery is up to full potential first. Hearing a relay "click" does not guarantee current is flowing through the contacts. The worse relays are the ones internal to a "Smart Junction Box" or "Power Distribution Module."

[@deankay4434]

Pete, my favorite way of testing relays is using 4 jumper wires (w/fuse preferred) but carefully connect the relay back to the appropriate cavities in the relay center. Then, with the amp clamp, measure control side (85-86) then device side (30-87 or 87a). With each jumper 6-8" long, it allows the relay to safely sit on a fender cover. Switch the DMM to Volts DC, allows visual confirmation while commanded on or turned on, but disconnect 85 by removal, off then on then off. Watching for a voltage drop on the contacts side while connected in series or simply to ground. 5-9 times of "On-Off" operation should warm the contacts or cause movement as some do. One bad connection on high resistance is all you need. "It is like, how flat lobes on a camshaft does it take, to make a bad cam? Once! Just a tech tip. DK. ASE Master, retired.

[@MotorAgeMagazine]

Great tip! Thanks for sharing your experience with us.

[@nickayivor8432]

ACCOMPLISHED Sir Pete Meier Motor My teacher Thanks for sharing it 👍 loved it From the start to finish tutorial brilliant thank you very much. Take care and have a great Evening with all your family around you ACCOMPLISHED Pete Meier Motor Age From Nick Ayivor from London England UK 🇬🇧

[@Turts_McGurts]

great video!

[@californiadreamin8423]

Hi from across the pond. I’ve had 3 ecu’s fail on my French hdi diesel , always in the wet. It’s now a “project”. I’ve been researching the best I can “ecm failure modes” and at 2 min 37 seconds you mention voltage spikes up to 200 volts which can kill the ecu. Mechanical Mindset has a video called ECU Killer, and he demonstrates voltage spikes from faulty relays up to 400 volts !!! This has to a massive Achilles heal of modern systems. Surely there must be some additional protection available to stop this. ECU failure is very expensive….which is why my car is now a project until I’m absolutely sure I’ve found the source.

[@BMWQC]

I used to be a BMW tech and I can remember that the DME relay (engine control module), was always a different relay than others on the car. I would guess they would install a diode somewhere to prevent the voltage spike from damaging the ecu. Just a guess. I remember on some E-36 models we had to install diodes in the wiring for the ignition coils because they would short and send the ignition voltage back to the ecu.

[@johnmitchell8925]

Remember some relay's can control more then one component

[@MiguelMartinez-np8uo]

Hey Pete , thank you for sharing your knowledge with us.. question ,you mention that you shouldn’t use a relay just because it looks the same. I recently was working on a 2019 Chevy suburban and it was missing the fuel pump relay so I proceeded to look in my box where I keep fuses relays etc and found one that had the same exact pins , the truck did start but do you think that will be a problem in the future ?

[@MotorAgeMagazine]

Hard to say for sure. It's the possibility that we have to try and guard against. Does the relay you used have a spike suppression device? You can sometimes tell if there is a diagram on the relay body OR you can look it up at your local parts house. If it does, you're probably ok.

[@sherrodford955]

How to test starter if it is buried in behind the intake mainfold? Do the amp test on the relay controller side of control and load ? Or is it different way to test?

[@MotorAgeMagazine]

You can use this test to check the operation of the starter solenoid. Testing the control side of the circuit will tell you if the ECM has actuated the starter solenoid and testing the load side will tell you if the contacts did, indeed, close. Check current on the starter B+ cable to see if the motor itself is trying to turn. Hope this helps!

[@agboolanajeem5268]

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