# best way to drive the tachometer in my '87 racco from ms2 and coil on plugs



## weeblebiker (Apr 3, 2003)

I've seen the diagram that mimics a ignition coil with a relay to drive the tach and the diagram using diodes directly connected to the coil secondary ground. I'm not digging the diode route with individual coils

Anyone have experience with the relay set up? Does a relay really have enough inductance to generate a big enough flyback spike to drive the tach? I've read some have had trouble with the relay.

If you had success or failure with the relay, what relay did you use?


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## need_a_VR6 (May 19, 1999)

Two things:

The drive transistor has to have a high breakdown voltage. There is a new one listed in the extra manual that works but isnt common. You can use a bip in its place id you have spares (or old vb's as they are paperweights otherwise!)

The relay needs to have a fairly high coil impedance. 40a ones work pretty readily, 30a are hit and miss depending on the tach used. Even different year mk2s seem to have different requirements. Might have to experiment a bit. The right size inductor is the "right" part but I have t put the time in to id a range that works.


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## weeblebiker (Apr 3, 2003)

Maybe I'll just ground the HV out of the old ignition coil and jumper the tacho out to the old ignition out or use the vw coil driver and wind a coil of similar inductance as the ignition coil primary or buy one from digikey for now. This is one more reason to just stick a 7 inch tablet in place of the gauge cluster


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## bonesaw (Aug 8, 2004)

MSD 8920 tach adapter should do it.


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## weeblebiker (Apr 3, 2003)

bonesaw said:


> MSD 8920 tach adapter should do it.


$60 is a bit steep for me.that would be at least the ancillary cost of embedding a tablet in the dash if not a bit toward the tablet itself. I try to use my brain more than my wallet:thumbup:

I may just do the ear tach gauge till I embed a tablet. I have an extra gauge cluster and plastic visor peace to play with so,,,,,


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## bonesaw (Aug 8, 2004)

Yeah but it works. And you dont spend hours ****ing with something that might not work or kill you. What do you value your time at?


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## weeblebiker (Apr 3, 2003)

bonesaw said:


> spend hours ****ing with something that might not work or kill you.


I play with 300 kV pulsed power generators at work, induction coil systems are fun to play with the fun is doing the theory work, figuring out the simplest,cheapest,most durable design and testing it.

The Rocco doesn't have to be drivable, infact it's been a year since it's been on the road(aba bottom end put in, 2 stagefuel injection, etc).at this point it's mostly a electrical engineering and combine systems tinkertoy.


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## weeblebiker (Apr 3, 2003)

Taking a coil in to work to measure on a LCR meter monday


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## weeblebiker (Apr 3, 2003)

Taking a coil and a couple relays in to work to measure on a LCR meter monday


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## need_a_VR6 (May 19, 1999)

Anxiously waiting results, the L on my meter hasn't worked in years!


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## weeblebiker (Apr 3, 2003)

will try to get it done this week. totally busy at work right now, been working through lunch.


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## weeblebiker (Apr 3, 2003)

ignition coil
freq Ls Z
1kHz 3.6mH 23 Ohm
10kHz -495 uH 32 Ohm
100kHz 250uH 168 Ohm

suspect some resonance action with the secondary around 10kHz


vw 40 Amp relay pn 171919505A

freq Ls Z
1kHz 67mH 566 Ohm
10kHz 24mH 2kOhm
100kHz 12mH 8.5kOhm


so the relay has like 20 times the inductance ish as the ignition coil


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## weeblebiker (Apr 3, 2003)

so I'm making a digikey order and thought I might as well figure this out. below are a selection of fixed inductors. I'll use the oe bosch ignition control unit first since I know it will drive the ignition coil and tachometer. once I find the proper inductor, I will try it with the 2n5551 and see if I can build a proper circuit to drive the vdo tach with known parts. it will be a couple weeks, this is one of many small projects to be done to get the car running.


1	

RL875S-682K-RC-ND	CHOKE RF 6800UH 10% RADIAL LEAD	

1
Immediate	0
1.30000	$1.30
2	

811-2090-ND	INDUCTOR RAD 4.7MH .18A T/H 10X8	

1
Immediate	0
0.71000	$0.71
3	

811-2076-ND	INDUCTOR RAD 10MH 0.12A T/H 10X8	

1
Immediate	0
0.71000	$0.71
4	

811-2084-ND	INDUCTOR RAD 22MH 73MA T/H 10X8	

1
Immediate	0
0.71000	$0.71
5	

2N5551TAFSCT-ND	AMP GP NPN 160V 600MA TO-92	

3
Immediate	0
0.21000	$0.63
Subtotal	$4.06
Shipping	Estimate
Sales Tax	unknown
Total	unknown


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## weeblebiker (Apr 3, 2003)

testing a hand full of relays today
I think the big issue with relays not working is with the resistor in parallel with the coil inside most relays; acts as a cheap circuit like a free wheeling diode to damp the fly back spike. 
will post up #'s this afternoon. will also post up to the EEs on the ms boards on this issue.


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## weeblebiker (Apr 3, 2003)

I suspect newer relays may have a internal free wheeling diode in parallel with the coil as this is the best place for the diode and these are ones that don't work at all.

I posted my Q up on the ms boards


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## need_a_VR6 (May 19, 1999)

The resistors and diodes are only part of it. I have cut those out of a few relays with no change. The ones that I have gotten to work are 40A (larger coil) and free of both the resistor and the diode.


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## weeblebiker (Apr 3, 2003)

tested @ 100 Hz
current series parallel DCR Z resistor size
20A 97 mH 252 mH 61 98 562
25A 124 mH 296 mH 61 120 560
40A 107 mH 347 mH 70 121 400
40A 114 mH 346mH 71 125 470

the 40A has a more inductive coil, probably to get it open quicker and help prevent bounce and arcing. cutting the resistor increases the DCR to 86 from 71 Ohms. I got confirmation from the ms board to cut ant resistor or diode out of the relay.

so since the relay has for spade connectors I thought of doing this


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## weeblebiker (Apr 3, 2003)

BTW
it works!:wave:

but I'm only getting half rpm:banghead:


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## weeblebiker (Apr 3, 2003)

the rpm being off was a ground gremlin. it works as it is supposed to


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