RPM with automotive multimeter

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Cisco82
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RPM with automotive multimeter

Unread post by Cisco82 » January 16th, 2021, 2:50 pm

Hi guys,

im going to do engine tuneup tomorrow (i have installed intake valve rubber cap as per your suggestions , thx again) and i was thinking how i can check RPM and dwell.

I have an automotive multimeter that need to be connected to the negative of the coil but the distributor is closed. I know there is a pulg on the top cover of distributore , it is designed for a special electric tool that touch teh middle top of the coil, is it connected to the negative of the coil?

In this case i could use simply a little round that touch the top centre of the coil and come out ov the distribitur cover , i d add an isulation in corrispondence of the distributor hole in order to avoid contacts.

Could it work?

Thanks

Francesco
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M151-A1 1969
Alfa Romeo AR-51 1952

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Re: RPM with automotive multimeter

Unread post by rickf » January 16th, 2021, 4:58 pm

Francesco, remember that you are working with 24 volts and all of the automotive equipment I know of is 12 volt. You can do timing by hooking the light to a separate battery in another car or a lawn tractor and using an old auto spark plug wire with the ends cut off and jammed into the distributor and spark plug to hook the timing light pickup on. But don't you have electronic ignition? If you do then there is no dwell setting.
1964 M151A1
1984 M1008
1967 M416
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12/1952 M100- Departed
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Cisco82
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Re: RPM with automotive multimeter

Unread post by Cisco82 » January 16th, 2021, 5:11 pm

Thx rick but i think there is a misunderstanding ... i m not tryng to use strobo lamp , o only want use my automotive multimeter to find RPM.

The user manual says it has to be attached to the negative of the coil , i thought i could use the distributor cap hole (the one used for electric adapter) if the top centre of the coil is connected to the negative of the coil, thats all.

I m aware about the power supply of the strobo and no problem about it
Francesco

M151-A1 1969
Alfa Romeo AR-51 1952

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Re: RPM with automotive multimeter

Unread post by Mark » January 16th, 2021, 6:06 pm

You could use a tach meter that clamps over the old park plug wire rickf was talking about to show rpm.Some timing lights has a tach with them
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Re: RPM with automotive multimeter

Unread post by rickf » January 16th, 2021, 7:19 pm

Your automotive tach/dwell meter is 12 volts I am sure. You can take that nut off of the distributor cap and find a suitable probe to go down there and touch the coil post. But you will be zapping it with 24 volts. I don't think the meter is going to like that. If you really want to know the RPM exactly then get a handheld tach that uses reflective tape stuck on the crank pulley.
This is an example of what I am talking about, It uses a piece of reflective tape that you put on whatever is turning and shine the tool on it and it will tell you the RPM. Personally I would not buy the cheapest one out there but that is just me, years of making a living with tools has taught me that they are super cheap for a reason.

https://www.ebay.com/itm/Cen-Tech-Digit ... 4235895907

Or, like Mark says, some higher end timing lights have tachometers built into them.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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Re: RPM with automotive multimeter

Unread post by Mark » January 16th, 2021, 8:31 pm

I seem to remember clamp the wires to the grounded battery for my timing light(12v),
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Re: RPM with automotive multimeter

Unread post by rickf » January 16th, 2021, 8:38 pm

You can use either battery as long as you go between the posts on that battery only. Never go from battery positive to body ground to get 12 volts.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
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Re: RPM with automotive multimeter

Unread post by Horst » January 17th, 2021, 8:01 am

Francesco,

to answer your original question. Yes, that cover in the top gives access to the negative connection of the coil. Now if your multimeter can handle that yes or no is unknown without knowing what meter it is.
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Re: RPM with automotive multimeter

Unread post by Cisco82 » January 17th, 2021, 9:59 am

Thx guys , i understood. to be honest i thought multimeter usually has a protection in order to avoid overtensin / current ...

However i have the multimeter in the link (it is not a pro ones lol)

https://www.amazon.co.uk/US-PRO-DIGITAL ... B013JYKHQA

I see no reference rgarding to the max voltage for tach function .....
Francesco

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Alfa Romeo AR-51 1952

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Re: RPM with automotive multimeter

Unread post by Horst » January 17th, 2021, 10:03 am

max voltage is written on the bottom at the terminal sockets. I see no problem using it
Horst

1972 USMC M151A2 w/ROPS (ex Barstow) and M416
1962 M201 and trailer
1966 GTO,1982 E350 Skoolie, 1987 SJ413, 1987 911
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Re: RPM with automotive multimeter

Unread post by rickf » January 17th, 2021, 11:31 am

You are going to use a multimeter to get rpm or dwell? I am a bit confused.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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Re: RPM with automotive multimeter

Unread post by Hambone » January 17th, 2021, 11:38 am

rickf wrote:
January 17th, 2021, 11:31 am
You are going to use a multimeter to get rpm or dwell? I am a bit confused.
Looks like the meter has a tach mode.

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Re: RPM with automotive multimeter

Unread post by Cisco82 » January 17th, 2021, 2:06 pm

It hase dwell and tach mode.
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Re: RPM with automotive multimeter

Unread post by rickf » January 17th, 2021, 2:54 pm

Gotcha. I am used to dedicated tach/dwell meters.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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Re: RPM with automotive multimeter

Unread post by Mark » January 17th, 2021, 4:08 pm

I also took a spark plug wire(NOS) removed waterproofing jacket for about 1/2" near spark plug boot, used that space for clamp/induction tach/timing light.The timing light is from the 70's, R&R #1 spark plug wire with it.Maybe too much detailed procedure?Rickf's way is better cuz he didn't ruin NOS spark plug wire, but this was back in the day when the wires were cheap.
mark


1968 m274A5
1960 m151
1981 m151A2
1964 m416
1971 m416

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