high voltage tacho output relay coil parallel resistor/diode

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weeblebiker
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high voltage tacho output relay coil parallel resistor/diode

Post by weeblebiker »

Im trying to drive a mid 80's vw tachometer originally driven by the coil - terminal.

I am looking at using the relay coil circuit as listed in the ms2 extra hardware manual.
I have read mixed results using this circuit.

all the relays I have opened(mid '80-early '90s vw relays) have an internal resistor of 400-570 ohms in parallel to the coil.

does this resistor need to be cut and removed?
will cutting cause a voltage spike greater than the 160v the 2n5551 and more importantly the Tachometer can handle?
should I go ahead and use something like a 3A 300v ZTX857 NPN instead of the 2n5551?
do newer relays have a internal freewheeling diode?
is this the "issue" that causes some relays to not work?

I have an LCR meter at work and have tested some relays @ 100Hz
v2.2 box, ms2, dual wheel, sequential ignition, staged injection, boost control, maf
'87 VW scirocco, 2L16V, TD0-14T turbo, itbs
slow_hemi6
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Re: high voltage tacho output relay coil parallel resistor/d

Post by slow_hemi6 »

In this case you just want the coil, no resistor or diode. The flyback voltage from the coil is what triggers the tach. A flyback diode will snub out the signal. The tach will be fine. The small relay coil won't generate as much back emf as an ignition coil does. The 160V 2n5551 is enough to handle the spike.
Find the Manuals up top under Quick links: Manuals. :RTFM:
Cheers Luke
DaveEFI
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Re: high voltage tacho output relay coil parallel resistor/d

Post by DaveEFI »

I'm wondering what the purpose of a resistor across the VW relay coil is? I've never seen that.
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slow_hemi6
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Re: high voltage tacho output relay coil parallel resistor/d

Post by slow_hemi6 »

It does the same job as a diode but not quite as well. Here is a great article on pretty much all things relay. http://www.autoshop101.com/forms/hweb2.pdf
Find the Manuals up top under Quick links: Manuals. :RTFM:
Cheers Luke
DaveEFI
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Re: high voltage tacho output relay coil parallel resistor/d

Post by DaveEFI »

slow_hemi6 wrote:It does the same job as a diode but not quite as well. Here is a great article on pretty much all things relay. http://www.autoshop101.com/forms/hweb2.pdf
Not too impressed with that article at a glance - early on it shows the same fuse feeding the entire circuit. Perhaps the main purpose of a relay is to save on wiring etc costs on the low current side.
That wouldn't be possible if it uses the same fused feed for both - the purpose of a fuse is to protect the wiring.

When such a fundamental is wrong I can't be bothered reading further. :D
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nismoautoxr
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Re: high voltage tacho output relay coil parallel resistor/d

Post by nismoautoxr »

DaveEFI wrote:
slow_hemi6 wrote:It does the same job as a diode but not quite as well. Here is a great article on pretty much all things relay. http://www.autoshop101.com/forms/hweb2.pdf
Not too impressed with that article at a glance - early on it shows the same fuse feeding the entire circuit. Perhaps the main purpose of a relay is to save on wiring etc costs on the low current side.
That wouldn't be possible if it uses the same fused feed for both - the purpose of a fuse is to protect the wiring.

When such a fundamental is wrong I can't be bothered reading further. :D
That cracks me up!!! I work with relay circuits in the automotive repair industry on a daily basis. What you call fundamentally wrong is done on a large scale by the OEMs. What exactly is fundamentally wrong with supplying the field and the load with the same wire being protected by the same fuse? Or maybe I am misunderstanding your post....?
Ricky-
1995 Nissan 240sx, 403 LS strokerv8 N/A 6speed trans
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sequential fuel and sequential spark
LS3/LSA card MAF sensor
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DaveEFI
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Re: high voltage tacho output relay coil parallel resistor/d

Post by DaveEFI »

I explained why.
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