Posted: 3/21/2015 10:25:33 AM EDT
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Hi guys...trying to track down a no start issue. Simple question, I'm learning as I go and can't seem to find this answer by googling..
On a (British) vehicle that requires *perfect* voltage from its electrical system, could a drop in voltage caused by weak battery/charging system cause a relay to not operate? I'm trying to learn how relays work but have failed to understand this part of it. My knowledge of a relay- when power is applied it clicks. Could low voltage cause a failure of this particular relay to not operate? This particular relay has been tested and is okay. However it conclusively links every failure I'm getting (intermittent failure) |
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Yes it is possible. Relays are simple, it's nothing more than an electromagnet that flips a switch for you. If the voltage falls, the magnet may not have the power to click the switch. Perfect, that's exactly the answer I was looking for and helps me to diagnose. Is there any particular reason one relay would fail to operate due to low voltage over and over while the other relays are okay with it? Should I be looking at the ground for that relay? |
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Yes it is possible. Relays are simple, it's nothing more than an electromagnet that flips a switch for you. If the voltage falls, the magnet may not have the power to click the switch. This is true, but in my experience its more likely the relay has a bad coil ground. Relays are designed to operate over a pretty wide voltage range, generally speaking. |
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This is true, but in my experience its more likely the relay has a bad coil ground. Relays are designed to operate over a pretty wide voltage range, generally speaking. Quoted:
Quoted:
Yes it is possible. Relays are simple, it's nothing more than an electromagnet that flips a switch for you. If the voltage falls, the magnet may not have the power to click the switch. This is true, but in my experience its more likely the relay has a bad coil ground. Relays are designed to operate over a pretty wide voltage range, generally speaking. Yup It's british you have bad(dirty) grounds |
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Yup It's british you have bad(dirty) grounds Quoted:
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Yes it is possible. Relays are simple, it's nothing more than an electromagnet that flips a switch for you. If the voltage falls, the magnet may not have the power to click the switch. This is true, but in my experience its more likely the relay has a bad coil ground. Relays are designed to operate over a pretty wide voltage range, generally speaking. Yup It's british you have bad(dirty) grounds This. But since you say its British, and you can even afford a team membership, I'm guessing its also old. And since its British, it isn't just the grounds that are dirty and corroded. I think there is something about the English climate that causes all wiring done there to start aging faster than a prom queen with a meth habit. Ohm out your wires and I'll bet you will find they are all nasty as fuck. Best way to fix Brit electrical work, is to buy a couple spools of wire and get to soldering. |
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what kind of vehicle? what are the symptoms and what relay specifically do you suspect? 2002 Range Rover 4.6 (p38) Intermittant no start. On message display says fuse 6, 7 and 17 failure. Engine cranks very healthy, but no start. Also no shift interlock, and the LED is out, no power. Can't move shifter out of park. Tracing those fuses, they control Engine and Gearbox ECU, shift interlock, etc. Explains no start. All of those fuses are controlled by Relay 10 in the Body Control Module (main computer). That relay has been tested by the "BECM Doctor" (the expert in Range Rover electrical) and deemed working. I just had sort of a break through though. I remembered I hardwired my Valentine One using the power supply wire to the sun visor lights, which are controlled by fuse 6. Fuse 6 is directly connected (according to the workshop manual) to fuse 7, which controls engine/gearbox ECU. Could a draw/short/etc downstream of fuse 6 cause a voltage situation on relay 10, which controls the other fuses? I know that's getting complicated!! |
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Quoted: 2002 Range Rover 4.6 (p38) Intermittant no start. On message display says fuse 6, 7 and 17 failure. Engine cranks very healthy, but no start. Also no shift interlock, and the LED is out, no power. Can't move shifter out of park. Tracing those fuses, they control Engine and Gearbox ECU, shift interlock, etc. Explains no start. All of those fuses are controlled by Relay 10 in the Body Control Module (main computer). That relay has been tested by the "BECM Doctor" (the expert in Range Rover electrical) and deemed working. I just had sort of a break through though. I remembered I hardwired my Valentine One using the power supply wire to the sun visor lights, which are controlled by fuse 6. Fuse 6 is directly connected (according to the workshop manual) to fuse 7, which controls engine/gearbox ECU. Could a draw/short/etc downstream of fuse 6 cause a voltage situation on relay 10, which controls the other fuses? I know that's getting complicated!! Quoted: Quoted: what kind of vehicle? what are the symptoms and what relay specifically do you suspect? 2002 Range Rover 4.6 (p38) Intermittant no start. On message display says fuse 6, 7 and 17 failure. Engine cranks very healthy, but no start. Also no shift interlock, and the LED is out, no power. Can't move shifter out of park. Tracing those fuses, they control Engine and Gearbox ECU, shift interlock, etc. Explains no start. All of those fuses are controlled by Relay 10 in the Body Control Module (main computer). That relay has been tested by the "BECM Doctor" (the expert in Range Rover electrical) and deemed working. I just had sort of a break through though. I remembered I hardwired my Valentine One using the power supply wire to the sun visor lights, which are controlled by fuse 6. Fuse 6 is directly connected (according to the workshop manual) to fuse 7, which controls engine/gearbox ECU. Could a draw/short/etc downstream of fuse 6 cause a voltage situation on relay 10, which controls the other fuses? I know that's getting complicated!! Try removing your add-in (Valentine One) from the equation. If that doesn't change the situation, I'd be looking at your grounds. I've wasted so much time tracking electrical problems, that now I routinely just add some extra grounds (battery, chassis & engine block) to every vehicle I own that is more than 5 or so years old. Since doing that, I've had zero electrical issues. |
| Check your fuse block for corrosion. Very important check the backside of it. As a precaution buy a 1 or 2 foot battery cable with a single eye lug at both ends. bolt one end to the engine block and the other to the frame or chassis. Best place to do it is around an engine mount. Older vehicles and especially British ones seem to develop bad grounds over time. |
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1. Relay contacts degrade with use. The contact resistance increases over time such that, at some point, they cause a significant voltage drop in the line they are switching, even if the relay is fully in the "on" state.
2. Wire harnesses can degrade over time. If the wires are flexing or vibrating, they may break strands inside the insulation, etc. If the connectors/connections go unused, corrosion builds up at the mating surfaces and increases contact resistance or causes an open circuit. I had an electrical gremlin that was remedied by replacing a $5 relay. Open relay box, pull out the old relay and plug in the new = end of problem. I also had to replace the main power cable coming off the battery because, after 20 years, the resistance of the high current cable leading to the starter motor had grown too high and even a brand new starter motor was not working properly. It cost $20 and took ten minutes to do. |