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week 1 Quiz

  • Jonathan Haffer
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11 Feb 2020 20:37 - 11 Feb 2020 21:16 #37385 by Jonathan Haffer
week 1 Quiz was created by Jonathan Haffer
Need help on a few questions guys.

14.whats the typical current draw on the control side of a relay ?

16.Is the relay controlled by switching the power or the ground ? i thought it was either.?

Universal testing methods

13.What is a quick test used to find a felling head gasket or cracked head? is it the oil drop on hot manifold test ?

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Last edit: 11 Feb 2020 21:16 by Jonathan Haffer.
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12 Feb 2020 04:18 - 12 Feb 2020 05:14 #37410 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic week 1 Quiz
For a typical fan or fuel pump relay less than 0.25 amps d
The current on the control side of a relay to a large extent depends on the physical size of the relay a typical 12v Bosch pattern square 30 amp relay has a resistance of 50 ohms, the more modern smaller black rectangular relays commonly used by GM and others have a typical ressistance of 75 ohms.

A much larger relay for say controlling a motor on truck tail gate lift or a winch on a 4x4 will have a lower resistance and draw more current.

All the above applies to passenger cars and trucks with 12v electrical systems 24v systems on extra heavy duty vehicles will have higer ressistance relays and so draw 1/2 the current.

Most computer controlled systems on cars are now ground side switched, back in the 196Os power side controlled relays were more common. It usually only takes a minute or so with low amp test lamp connected figure out which you are dealing with and then if any doubt look up the wiring diagram.

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Last edit: 12 Feb 2020 05:14 by Andy.MacFadyen.
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12 Feb 2020 04:45 - 12 Feb 2020 05:05 #37411 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic week 1 Quiz
The oil drop test on a hot manifold is a useful test for head gasket failure BUT use with caution head gasket failure is propbably the most commonly misdiagnosed mechanical fault , Not all head gasket failures especially in the early stages mix coolant into the oil. Also in cold climates engines that are only used for short journeys can accumulate moisture from combustion in the engine oil. Also on engines with coolant passages in the inlet manifold it can indicate an inlet manifold leak ---in the UK this is a common cause of mis-diagnosed head gasket on the Rover/Land-Rover K engine.

So when using the oil drop test look for further evidence., cooling system presurising, gas test on header tank, cooling system pressurising before engine gets warm or just a strong ehaust fume smell from the coolant .

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



Last edit: 12 Feb 2020 05:05 by Andy.MacFadyen.
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14 Feb 2020 12:07 #37481 by ScannerDanner
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13. check for hydrocarbons in the coolant overflow bottle with a gas analyzer is what I was looking for. When I wrote that question, just about every shop in PA had a gas analyzer for our state emission tests, so it truly was a very quick test. Today, all of the tailpipe testing has been eliminated and all the shops let their machines die, so it really is no longer a "quick test". As Andy stated, the dipstick test is weak at best, so that leaves us with the chemical test kit (not mentioned in the lectures) and I find that test to be okay but definitely not as accurate as the gas analyzer.
14. around 100ma on the control side of a relay is what I have listed in the book (chapter 3 page 9)
16. a relay can be controlled by switching it's power or ground, so yes, you are correct, either side

BTW, nice job. This is what I have been wanting from my on-line students!! Keep it up

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19 Feb 2020 16:24 - 19 Feb 2020 16:25 #37650 by Jonathan Haffer
Replied by Jonathan Haffer on topic week 1 Quiz
Thanks Danner , I printed off all your test and I’ve been going through all your lectures, I’m really learning a lot, when I’m not sure of an answer I been posting it, I’m the only guy in the shop now that can diag with a scope, your classes has move me from the guy that pulls engines to the guy that can troubleshoot with accuracy. God bless

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Last edit: 19 Feb 2020 16:25 by Jonathan Haffer.
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