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Old 07-30-2012, 04:24 PM
Rnk Amateur
Join Date: Feb 2012
Location: Georgia
Posts: 10
300D Tach Inop

1983 300D Turbo. Tach does not move at all.

1. pulled cluster verified it is connected in the back.
2. purchased new replacement Tach Amp installed.
3. inspected sensor and it looks as though it is actually touching the metal pin on the balancer. So it is surely close enough.

With Amp removed.
Pin 2 to Pin 6 reading 12 VDC
Pin 2 to pin 3 reading 10 VDC
* Reading 77ohms from sensor ground (pin 7) to *Sensor + (pin 9).
* Sensor shield (pin 8) to pin 9 is reading open.
* Pin 7 to pin 8 are reading open.

With Amp installed.
* Glow Position Pin 8 to Pin 9 reading 2.4 VDC
* Glow Position Pin 7 to Pin 9 reading 2.4 VDC

Assuming here that Pin 8 is a shielded ground (shorted to pin 7).

I believe all reading are as they should be for a working Tach. What have i failed to check other than the Tach itself failed internally?

Thank you for your time.

Update: Checked two pin connector of Tach at back of cluster with engine running and amp installed getting 13.4 VDC. Checked pins 2 and 3 with engine running: idle 44 mVAC. Voltage actually dropped with throttle advance. High RPM gave me about 24 mVAC.
Reinstalled Tach Amp and checked voltage cluster two pin connector. Here read 13 .4 VDC from fuse box. Also signal wire from Amp read 275 mVAC and increased with throttle advance as it should.

Although not positive i am almost convinced it must be the Tach itself. Just don't like swapping components without being more certain.

Update: scrounged up a cluster schematic for Tach. BK/RD wire is hot from fuse box and the other wire GN/BK is signal wire from Tach Amp. The signal wire is a straight run from Tach Amp pin 3.

Funola, i misinterpreted your above post and read pin 3 voltage at amp connector. I went back today and read at the cluster as i should have the first time. If i understand the pickup correctly then having this mVAC at cluster signal wire along with increase with throttle tells me the pickup and circuit through amp is acting correctly.

Looks like the tach body serves as ground to the cluster body then through the large multi pin cluster connector. So i either have a ground issue or dead Tach. Does anyone know what the actual Tach resistance should be reading at the back of the cluster(two pins)? Mine is reading .76 meg ohm. I would think a functioning tach would read higher than this.

Comments appreciated.

Last edited by mattrob; 07-30-2012 at 09:54 PM. Reason: additional Voltage readings
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Old 08-01-2012, 03:43 PM
Rnk Amateur
Join Date: Feb 2012
Location: Georgia
Posts: 10

I disassembled the Cluster and inspected board. Board looks great as all lights annunciators have been working correctly. I did notice that the solder joint from the speedometer metal housing to the square piece of metal(back of fuel gauge circuit board) did have a broken solder joint. Repaired that. I know for certain that i have a good ground, 13 VDC, and Sensor signal from Tach Amp to the Tach Amp itself. Deducing from this that i have a bad Tach? Something tells me that it's something else. Next logical thing to do would be replace the Tach so i guess i will start my search for one and give that a shot.
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Old 08-01-2012, 06:01 PM
Registered User
Join Date: Jul 2004
Posts: 101
Not sure if the 83 has it or not, but did you check the Overload Protection Relay/Fuse? On my '87 when the relay failed the tach did not work and the A/C did not work. The relay/fuse on the '87 is located behind the battery.
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Old 08-01-2012, 11:31 PM
Rnk Amateur
Join Date: Feb 2012
Location: Georgia
Posts: 10
From my understanding the O load protection started in 85'. My 83 doesn't have it. Gettting voltage. Must be a bad Tach.
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