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  #1  
Old 05-15-2010, 04:18 PM
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Testing the tach

Hi, per my other thread, just verified that my car has a 3.07, so this means my tach is out of alignment. So, a few questions:

-85 cars still had the hall effect sensor, though it didn't feed the tach - was used for troubleshooting. Is there a way to read the signal right from the tach amp connector?

- if I just pull my cluster partway and attach a good tach, do I need to add any other connections for power, etc., or do I just need to attach the lead going into the tach?

Thanks very much!

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #2  
Old 05-15-2010, 06:47 PM
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Quote:
Originally Posted by JHZR2 View Post
.... just verified that my car has a 3.07, so this means my tach is out of alignment
This is a mind numbingly simple system. The tach amp reads a pulse from the VR sensor at the balancer, and sends a pulse to the tach. The tach counts pulses. There is no conversion to a voltage - read a pulse, send a pulse. There is nothing to go "out of alignment".


Quote:
Is there a way to read the signal right from the tach amp connector?
Not easily. You need an oscilloscope and a constant current source of 50mA or so. The pulses will be very small - less than half a volt.


Quote:
do I need to add any other connections for power, etc., or do I just need to attach the lead going into the tach?
The two wires in the connector should be +12V and signal. You would need to supply ground to the backing plate of the tach.

All that being said, there is a trimpot on the back of the tach. I've never messed with it, but it might be an adjustment point.....
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'82 300D Petrol B-G Metallic
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  #3  
Old 05-15-2010, 06:56 PM
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Well the thought is to remove the needle and then re-press on so it reads correctly. There is a pretty much constant error across speeds. I'll just alligator clip a ground to the. Other tach, take a reading, and then fix the one in the cluster... Simple.
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #4  
Old 05-15-2010, 07:12 PM
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Quote:
Originally Posted by JHZR2 View Post
Well the thought is to remove the needle and then re-press on so it reads correctly. There is a pretty much constant error across speeds. I'll just alligator clip a ground to the. Other tach, take a reading, and then fix the one in the cluster... Simple.
That would probably work fine. Assuming your "other tach" is reading correctly....

I'd be tempted to try the trimpot thing first. Pressing on a needle with the engine idling and not disturbing the position of the shaft while doing so sounds like more trouble than it's worth....
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'82 300D Petrol B-G Metallic
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  #5  
Old 05-15-2010, 07:47 PM
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I Will play with that first... My thought was to fix the needle on the original one on the bench, not in operating mode...
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #6  
Old 05-15-2010, 08:45 PM
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Posts: 5,265
Can you advise which to try to adjust? This was the tach from my 83, a 2-prong type just like my 82 has.



thanks!
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #7  
Old 05-15-2010, 10:23 PM
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Originally Posted by JHZR2 View Post
Can you advise which to try to adjust? This was the tach from my 83, a 2-prong type just like my 82 has.


thanks!
I'll see if I can take a photo tomorrow. It's on the other side, between the faceplate and the backplate.
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'82 300D Petrol B-G Metallic
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  #8  
Old 05-15-2010, 11:20 PM
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so then you have to remove the faceplate to access?
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #9  
Old 05-16-2010, 09:43 AM
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Location: Austin, TX
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Quote:
Originally Posted by JHZR2 View Post
so then you have to remove the faceplate to access?
No. Look at the tach edgewise. You can see the electronics, and have direct access to the trimpot with a jewler's screwdriver.
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Bob
'82 300D Petrol B-G Metallic
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  #10  
Old 05-16-2010, 10:25 AM
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how do you know your tach is out of alignment? what rpm's are you reading at 60-70mph?
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  #11  
Old 05-16-2010, 10:40 AM
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vs, been through it here:
http://www.peachparts.com/shopforum/showthread.php?t=276535

Always have said the idle is low, sits just off the 500rpm point.

Verified that my speedometer is accurate via GPS. It is in fact spot on to the GPS value.

Driving along, at 60 MPH, I get 2100, and at 70 I get 2600 RPM.

Verified that I have a 3.07 rear, so something is off.
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #12  
Old 05-16-2010, 10:50 AM
vstech's Avatar
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Location: Mount Holly, NC
Posts: 26,835
ahh. ok I thought it sounded familiar.
needle being off makes sense, you have another tach to toss in?
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  #13  
Old 05-16-2010, 11:55 AM
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Location: New Jersey
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Dont want to toss another in as the tach on there has nice orange needles that all match. My old 83 tach has yellowed. So Id like to try to adjust the 82 to read correctly, using the known good 83 as a reference. So long as I can get the 83 unit to connect to the harness, with enough slack to get an idle reading, I should be able ot start to adjust from there.

We will see...

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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