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1972 350SL
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441 Posts
Discussion Starter #1
A couple of weeks ago, the transistorized switchgear unit on my 350SL failed. Fortunately, it was a short ride home on the flatbed. Rather than spend large $$ on the switchgear/ignition, my son (1972350SL) and I decided to replace the ignition with the Pertronix 1885 unit and the Flamethrower coil and eliminate the switchgear altogether. Works great!! Runs stronger than ever.

The only problem is that the tach reads about 1500 RPM high after the switch. We think we may have solved that by placing a resistor in series with the green/black trace lead that runs from the negative on the coil to the tach. We used an 8.2Kohm 10W resistor. Brings the tack to approximately the right idle speed. I would like to try a 7.5K ohm, when I get one. May be more precise.

Will dial it in closely when we get the meters on it.

Anyone else tried this fix?

MG
 

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Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,338 Posts
A couple of weeks ago, the transistorized switchgear unit on my 350SL failed. Fortunately, it was a short ride home on the flatbed. Rather than spend large $$ on the switchgear/ignition, my son (1972350SL) and I decided to replace the ignition with the Pertronix 1885 unit and the Flamethrower coil and eliminate the switchgear altogether. Works great!! Runs stronger than ever.

The only problem is that the tach reads about 1500 RPM high after the switch. We think we may have solved that by placing a resistor in series with the green/black trace lead that runs from the negative on the coil to the tach. We used an 8.2Kohm 10W resistor. Brings the tack to approximately the right idle speed. I would like to try a 7.5K ohm, when I get one. May be more precise.

Will dial it in closely when we get the meters on it.

Anyone else tried this fix?

MG
I have heard that a resistor will work, but never seen anything about just what it's value should be. I don't imagine the current would be high, but still it would be nice to know what power rating the resistors should have. 10W is probably way more than needed.

Resistors

One of those old variable resistors (Rheostats), but maybe a cheap Radio Shack potentiometer could be used? Potentiometer as a rheostat : DC CIRCUITS

Is Radiotek here? He should be able to advise!

BTW - Thanks for posting progress - I want to go to a Flamethrower too but tach problem has held me back.
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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Just a little FYI. The Crane unit has the same problem. There was one installed (incorrectly) on my '73 when I bought it. Fixed the install but burned out the tach. Not the greatest with electronics.
 

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One of the BW Old Guard/R129, W204 Moderator
1997 SL500- 40th Anniversary
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7,166 Posts
A 10W may be overkill, but the cement wire wound was in the parts box and they cost <$1. If we can confirm that a smaller, maybe 2W metal oxide would work, I'll dial in a value and use that. Will work in line easier than the heavier wire wound.
As I recall, there is a small pot on the Tach for adjustment as well... I have no idea what range of adjustment is available. It was mentioned in the thread on rebuilding the speedo and painting the needles, which I believe is in the EGv107...

I found the reference...
http://www.benzworld.org/forums/r-c107-sl-slc-class/1495136-odometer-repair-tutorial-2.html#post3997914
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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As I recall, there is a small pot on the Tach for adjustment as well... I have no idea what range of adjustment is available. It was mentioned in the thread on rebuilding the speedo and painting the needles, which I believe is in the EGv107...

I found the reference...
http://www.benzworld.org/forums/r-c107-sl-slc-class/1495136-odometer-repair-tutorial-2.html#post3997914
The shop manual (job 54-259) on this subject is confusing. It seems that what MB calls a tachometer, is actually the speedometer! There is a mechanical cable driven "tachometer" and an electronic "tachometer" that picks up a signal from the transmission. They say to take out the tacmometer before removing the rev counter!

What we call a tachometer, is referred to as a revolution counter.

The revolution counters in the two versions look different. Which version are the adjustments in the above link based on?
 

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Premium Member
1975 450SL
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2,440 Posts
I had the same problem when I went from points to an S&S (Crane) unit. I was able to get the tach 'sort of' working with approx 10Kohm of 1 Watt resistor in the circuit, but it stops working when the headlights are on and the brake or turn signals are in use.

1 Watt should be more than enough, even in the engine bay, especially if you use metal film resistors. I had to put two in series, and then slid some heat shrink tubing over them for protection.

I was just able to verify the grounds two weeksa ago, so my next step is to dig out a potentiometer and try to dial in the resistance, then get appropriate permanent resistors.

The instructions with the S&S unit said 7 to 11 Kohm, but I just 'got close' with what I had easily to hand. I was pressed for time, and things haven't been all that much better since. If I didn't need to sleep, things would work out SO much better.:D

I think that I'll put that on my list for this weekend, I'm being forced to take Friday off, so I may try to get something done on the SL.

Scott
 

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Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,338 Posts
I had the same problem when I went from points to an S&S (Crane) unit. I was able to get the tach 'sort of' working with approx 10Kohm of 1 Watt resistor in the circuit, but it stops working when the headlights are on and the brake or turn signals are in use.

1 Watt should be more than enough, even in the engine bay, especially if you use metal film resistors. I had to put two in series, and then slid some heat shrink tubing over them for protection.

I was just able to verify the grounds two weeksa ago, so my next step is to dig out a potentiometer and try to dial in the resistance, then get appropriate permanent resistors.

The instructions with the S&S unit said 7 to 11 Kohm, but I just 'got close' with what I had easily to hand. I was pressed for time, and things haven't been all that much better since. If I didn't need to sleep, things would work out SO much better.:D

I think that I'll put that on my list for this weekend, I'm being forced to take Friday off, so I may try to get something done on the SL.

Scott
Wonder if there will be any difference in resistances needed between a Pertronix and a Crane?

I read on an early Porsche site that their tachs needed an 11v square wave. (probably same VDO technology). I guess it would count pulses? I could see turn signals affecting it, but headlights? They were talking about adding a large resistance, maybe 100kohm to dampen out fluctuations. Everything I read made me think - leave well enough alone - at least until the switchgear gives up!

Glad to hear you are inspired to solve this :) We will all benefit.

I think I should get that Flamethrower on order! Hmm, maybe not quite yet - car probably won't be drive again until next April :(
 

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1975 450SL
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Damn! Now you've got me thinking about the waveform.:eek: I may have to snag a scope from work to see what the rig is putting out.

From what I'm just thinking of 'off the top of my head' it's pulsed DC that hits the coil, and MB had a separate wire to go to the tach. I wonder if they had a particular voltage and/or waveform the tach needed. Maybe an inductor in addition to the resistor, to smooth out the voltage spikes in the waveform.....(That smell of smoke??? Just me thinking;))

The instructions were to tie the tach wire to a lug of the coil, so the signal to the coil may be a bit 'hot'. I need to pad it down to the right level for the tach: not too hot to overdrive the electronics, and not too low, failing to get enough signal to the tach to get the needle moving correctly.

I hope I can find the envelope with the good probes for the 'scope, we just had to move the shop from one room to the next, because the Real Estate group wanted the room with windows.:mad: They had contractors move things while we were elsewhere, doing real work. I may never find some stuff.
 

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1987 Mercedes-Benz 560 SL
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973 Posts
Decades ago when I put a brand new VDO tach in my VW bug, the green cable to the tach input connected to the same coil terminal that the breaker points plugged into. The VDO tach had a three-position switch on the back for 4, 6, or 8 cylinder engines. I surmise that the tach counts 2 revolutions (a 4-cycle motor thing) for every 4, 6, or 8 pulses it senses from the breaker points and compares the pulse count to a clock to calculate--and display--the engine speed.

Thus I have exhausted my lifetime knowledge on the subject of VDO tachometers. Except that they do look reaallly cool in a '68 VW bug dash.

Good road,
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,338 Posts
Damn! Now you've got me thinking about the waveform.:eek: I may have to snag a scope from work to see what the rig is putting out.

From what I'm just thinking of 'off the top of my head' it's pulsed DC that hits the coil, and MB had a separate wire to go to the tach. I wonder if they had a particular voltage and/or waveform the tach needed. Maybe an inductor in addition to the resistor, to smooth out the voltage spikes in the waveform.....(That smell of smoke??? Just me thinking;))
I'm just thinking too!

In the original config, the green/yellow wire from the points connected to the tach wire (and also to the switchgear). I believe that the voltage that the points saw was 3V. With the Pertonrix/Flamethrower setup wouldn't the voltage be 12v? Maybe the resistor is needed to drop it to 3V, but a current would be needed to determine the size of the resistor. Maybe your scope could measure that?

BTW, I don't really know what I'm talking about ;)
 

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Always Remembered RIP
1979 280sl 4 sp w/ac 1957 MGA 1998 volvo xc/70 2004 F150
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I'm not nearly savy as you guys are as to the resistors etc, but when I was testing the old (non functioning) tach in the 280, my test tach/dwell meter read differently when hooked up to te coil vs the tach feed wire going to the cluster. Isn't there a way you can measure that difference?
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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Scott,
I received a reply from Pertronix. Possibly same solution might work with Crane?
But if not, a 6.9 owner on another MB list says he has a mod that Crane sent him. He said he could FAX it to me, but I no longer have a fax! Maybe you could ask S&S/Crane if they have those instructions?

I have had many different people make this work. It can be a little bit of work but you can make the tachometer work with the Ignitor.

First the tachometer wire will hook to coil negative. Sometimes you can just hook up the tachometer wire to coil negative and it works. But most of the time you are going to need to install a resister in the tachometer wire that is connected to coil negative. This is the tricky part of the install. For years I just recommend an 11.5K ½ watt resister and that seems to work. However recently I have had to use different size one’s to get the tachometer to work. I have used a 10K ½ watt and a 7K ½ watt and a 9K ½ watt. So, as long as you’re willing to work normally you can make it work.

If you have any further questions feel free to contact me.
 

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Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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I have settled on 8.2K for Pertronix. 1972 350SL 4.5L
Great!

Is resistor installed and tach working well?
Did you buy a new resistor of that value or is that from a pot?

I have Flamethrower on it's way. Might get it installed over winter, but otherwise first thing in Spring.

I went to buy some resistors today, but our only electronics store only had 1/4 watt units in stock.

Once we all get these things working, maybe a sticky will be in order. It's been like pulling teeth to get answers.
 

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Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,338 Posts
As I am electronics hobbyist, I had many resistors in stock. Installed - working fine.
Not exactly a hobbyist, but I have a box full of resistors too. But no idea of their wattage. Don't want them to blow so will try and get the 1/2watt or higher units first.

Congrats on getting a working system! Hope to be there too next Spring!
 

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1975 450SL
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2,440 Posts
Bleah!!!:mad:

Operation tach is postponed a week or so.

Came down with some sort of creeping crud. Trying to cough up my lungs half the time.

I hate getting sick!!!
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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32,345 Posts
Scott,
I received a reply from Pertronix. Possibly same solution might work with Crane?
But if not, a 6.9 owner on another MB list says he has a mod that Crane sent him. He said he could FAX it to me, but I no longer have a fax! Maybe you could ask S&S/Crane if they have those instructions?
If it is the motor cycle adaptor mentioned in the Crane instructions, it doesn't work.
 

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Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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10,338 Posts
If it is the motor cycle adaptor mentioned in the Crane instructions, it doesn't work.
It's a filter circuit that creates a square wave output for use on the tach. It has in use on owner's 6.3 Benz for some time. Mind you, I am not sure that the 6.3 had the same ignition system as the M116/7s. Over on Peaches, the Aussy guy posted a site also for M100 engines and they were using a 50Kohm resistor with the Pertronix.

I received the diagram, but part of it was missing. It is a lot more complex than adding a resistor, but Radiotek might need it with his Crane.

But it seems for our M116/7s with a Pertronix, we only need a resistor.

Attached Crane supplied Tach Filter - seems a bit complex. Diagram is cut-off but I guess it just fits between coil negative and tach. It is in use on an MB 6.3 which may be different from our cars.

A search for tach filters brings up a ton of stuff including some more simple circuits. It also brings up ready made filters like this one:
Tachometer Signal Filter but no idea if it would work on our cars.
 

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