Home » Public Forums » GMCnet » [GMCnet] Tach wire
[GMCnet] Tach wire [message #370065] |
Sun, 03 July 2022 12:49 ![Go to next message Go to next message](/GMCforum/theme/default/images/down.png) |
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![United States United States](/GMCforum/images/flags/us.png) Messages: 124 Registered: February 2019
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I know the tach wire goes to the end of the distributor but I cannot find it. Can someone tell me the color?
It is not shown in the wiring diagrams.
Emery Stora
emerystora@mac.com
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Re: [GMCnet] Tach wire [message #370068 is a reply to message #370065] |
Sun, 03 July 2022 19:01 ![Go to previous message Go to previous message](/GMCforum/theme/default/images/up.png) ![Go to next message Go to next message](/GMCforum/theme/default/images/down.png) |
midlf
![United States United States](/GMCforum/images/flags/us.png) Messages: 2212 Registered: July 2007 Location: SE Wisc. (Palmyra)
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Not sure this will help:
On my '75 Vette there is a pink wire that is the power feed to the HEI. The HEI cap has a hood over two connectors. HEI power plugs onto one. The tach to the other.
HEI wire is pink. Tach wire is brown. IIRC the two plugs for these are not interchangeable.
Steve Southworth
1974 Glacier TZE064V100150 (for workin on)
1975 Transmode TZE365V100394 (parts & spares)
Palmyra WI
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Re: [GMCnet] Tach wire [message #370069 is a reply to message #370065] |
Sun, 03 July 2022 20:45 ![Go to previous message Go to previous message](/GMCforum/theme/default/images/up.png) ![Go to next message Go to next message](/GMCforum/theme/default/images/down.png) |
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RF_Burns
![Canada Canada](/GMCforum/images/flags/ca.png) Messages: 2277 Registered: June 2008 Location: S. Ontario, Canada
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Now here is an example of where I think I'm missing the original question from the email side. I only use the web-based forum.
This is the first message in the thread that I see from Emery:
I know the tach wire goes to the end of the distributor but I cannot find it. Can someone tell me the color?
It is not shown in the wiring diagrams.
Emery Stora
emerystora@mac.com
So I'm not sure what Emery is hooking up... is it just a Tachometer, or is at an EFI system, or is this some piece of test equipment. Regardless, the tach connection is usually to the points side of the ignition coil. On HEI systems, the TACH connection is marked on the distributor cap and the connector is parallel with the HEI module's open collector output to the coil. OEM, the GMC had nothing connected to the TACH point.
Be careful that what you are hooking to the Tach connection is compatible with 12V switching to ground. In the case of a Howell EFI, there is a small circuit board in the wiring harness that level shifts the 12V Tach signal to 5V. This circuit board gets removed when a spark-controlled distributor is added because ESC output is 5V.
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC. 1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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Re: [GMCnet] Tach wire [message #370070 is a reply to message #370069] |
Mon, 04 July 2022 10:38 ![Go to previous message Go to previous message](/GMCforum/theme/default/images/up.png) ![Go to next message Go to next message](/GMCforum/theme/default/images/down.png) |
rjw
![United States United States](/GMCforum/images/flags/us.png) Messages: 697 Registered: September 2005
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RF_Burns wrote on Sun, 03 July 2022 21:45Now here is an example of where I think I'm missing the original question from the email side. I only use the web-based forum.
This is the first message in the thread that I see from Emery:
I know the tach wire goes to the end of the distributor but I cannot find it. Can someone tell me the color?
It is not shown in the wiring diagrams.
Emery Stora
emerystora@mac.com
So I'm not sure what Emery is hooking up... is it just a Tachometer, or is at an EFI system, or is this some piece of test equipment. Regardless, the tach connection is usually to the points side of the ignition coil. On HEI systems, the TACH connection is marked on the distributor cap and the connector is parallel with the HEI module's open collector output to the coil. OEM, the GMC had nothing connected to the TACH point.
Be careful that what you are hooking to the Tach connection is compatible with 12V switching to ground. In the case of a Howell EFI, there is a small circuit board in the wiring harness that level shifts the 12V Tach signal to 5V. This circuit board gets removed when a spark-controlled distributor is added because ESC output is 5V.
I didn't know that. I was using the HEI tach connection to my Custom Instrument Panels tachometer with no issues starting back in 1999. Worked fine. In 2012, I added to my Howell EFI (+EBL) a computer controlled distributor and found tachometer worked OK but sometimes acts weird. Like registering 4000 RPM when idling on occasion. I forgot about the PC board in the Howell wiring harness until I read this. I don't recall removing it when I went to CCD but then again that 10 years ago and I sometimes I don't remember what I had for lunch.
Richard
76 Palm Beach
SE Michigan
www.PalmBeachGMC.com
Roller Cam 455, TBI+EBL, 3.42 FD, 4 Bag, Macerator, Lenzi (brakes, vacuum system, front end stuff), Manny Tranny, vacuum step, Tankless + OEM water heaters.
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Re: [GMCnet] Tach wire [message #370071 is a reply to message #370065] |
Mon, 04 July 2022 11:20 ![Go to previous message Go to previous message](/GMCforum/theme/default/images/up.png) ![Go to next message Go to next message](/GMCforum/theme/default/images/down.png) |
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RF_Burns
![Canada Canada](/GMCforum/images/flags/ca.png) Messages: 2277 Registered: June 2008 Location: S. Ontario, Canada
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Richard,
The GM 7747 ECM used in the Howell (and EBL) was designed for use with a CCD for spark control. The output of the CCD module to the ECM is a 5V pulse so that is what the ECM is designed for. Howell designed a little board to reduce the 12V pulses from either the points coil or the HEI Tach connection terminal down to 5V. The Howell board might work with a 5V pulse input, but I would not think it would. Its just a Schmidt trigger NAND gate with a voltage divider at the input (if I recall). When you pulled the 4 wires required for the CCD, you likely pulled out the single Tach wire with the circuit board in heat-shrink from the wiring harness.
The Tach connection for the CCD distributor is the same for both standard and ESC distributors. Was it the CIP tach that was reading 4000RPM intermittently or the Tach on the EBL What's up Display (WUD)? If it was the WUD, there is a filter to smooth the electrical noise from the terminal. Look for "RPM-Filter for 12.5 RPM, mine is set to 81.57%
FYI, if you a Vehicle Speed Sensor (VSS), there is a noise smoothing filter for it as well. A couple of years ago I noticed my engine would randomly seem to cut out, so with the WUD I noticed the Speed indicator was jumping by 20-50MPH. Turns out the speed limiting was coming ON even though it was set to some high speed. I replaced the reed switch in the VSS, then Randy Van Winkle pointed out the filter setting called "VSS- Filter, Filter %" mine is set now to 75.29 .
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC. 1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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Re: [GMCnet] Tach wire [message #370072 is a reply to message #370071] |
Mon, 04 July 2022 12:10 ![Go to previous message Go to previous message](/GMCforum/theme/default/images/up.png) ![Go to next message Go to next message](/GMCforum/theme/default/images/down.png) |
rjw
![United States United States](/GMCforum/images/flags/us.png) Messages: 697 Registered: September 2005
Karma: 4
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RF_Burns wrote on Mon, 04 July 2022 12:20Richard,
The GM 7747 ECM used in the Howell (and EBL) was designed for use with a CCD for spark control. The output of the CCD module to the ECM is a 5V pulse so that is what the ECM is designed for. Howell designed a little board to reduce the 12V pulses from either the points coil or the HEI Tach connection terminal down to 5V. The Howell board might work with a 5V pulse input, but I would not think it would. Its just a Schmidt trigger NAND gate with a voltage divider at the input (if I recall). When you pulled the 4 wires required for the CCD, you likely pulled out the single Tach wire with the circuit board in heat-shrink from the wiring harness.
The Tach connection for the CCD distributor is the same for both standard and ESC distributors. Was it the CIP tach that was reading 4000RPM intermittently or the Tach on the EBL What's up Display (WUD)? If it was the WUD, there is a filter to smooth the electrical noise from the terminal. Look for "RPM-Filter for 12.5 RPM, mine is set to 81.57%
FYI, if you a Vehicle Speed Sensor (VSS), there is a noise smoothing filter for it as well. A couple of years ago I noticed my engine would randomly seem to cut out, so with the WUD I noticed the Speed indicator was jumping by 20-50MPH. Turns out the speed limiting was coming ON even though it was set to some high speed. I replaced the reed switch in the VSS, then Randy Van Winkle pointed out the filter setting called "VSS- Filter, Filter %" mine is set now to 75.29 .
Bruce,
It was the CIP tach reading wrong intermittently. Never checked it with WUD. It happens maybe a few times in a couple of hours and lasts 20 seconds or so. And yes, I do have VSS to the ECM + EBL. I think I better check this out before ordering a new Tach.
So much to do and so little time.
Richard
76 Palm Beach
SE Michigan
www.PalmBeachGMC.com
Roller Cam 455, TBI+EBL, 3.42 FD, 4 Bag, Macerator, Lenzi (brakes, vacuum system, front end stuff), Manny Tranny, vacuum step, Tankless + OEM water heaters.
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[GMCnet] Re: Tach wire [message #370137 is a reply to message #370067] |
Sat, 16 July 2022 10:51 ![Go to previous message Go to previous message](/GMCforum/theme/default/images/up.png) ![Go to next message Go to next message](/GMCforum/theme/default/images/down.png) |
gmc-email-list
![United States United States](/GMCforum/images/flags/us.png) Messages: 124 Registered: February 2019
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Thanks, Nelson.
I forgot that. I have a replacement MAC dash.
Emery Stora
emerystora@mac.com
> On Jul 3, 2022, at 12:04 PM, Nelson Wright wrote:
>
> Emory
> I may be wrong, but I don’t remember any factory GMC MH tachometer , so there wouldn’t be a wire.
> Nelson
> 78 Royal rear bath
> Belle isle Fl & Nantahala NC
>
> Sent from my iPad
>
>> On Jul 3, 2022, at 1:49 PM, Emery Stora via Gmclist wrote:
>>
>> I know the tach wire goes to the end of the distributor but I cannot find it. Can someone tell me the color?
>>
>> It is not shown in the wiring diagrams.
>>
>> Emery Stora
>> emerystora@mac.com
>>
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
> _______________________________________________
> GMCnet mailing list
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[GMCnet] Re: Tach wire [message #370138 is a reply to message #370137] |
Sat, 16 July 2022 11:48 ![Go to previous message Go to previous message](/GMCforum/theme/default/images/up.png) |
Nelson
![United States United States](/GMCforum/images/flags/us.png) Messages: 120 Registered: August 2014
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Emory,
Thanks for the reply, as it’s the only message I have received from the net.
Wonder what happened, is just me or has the net been down?
Sent from my iPhone
Nelson Wright
Orlando Fl.
> On Jul 16, 2022, at 11:58 AM, Emery Stora via Gmclist wrote:
>
> Thanks, Nelson.
> I forgot that. I have a replacement MAC dash.
>
>
> Emery Stora
> emerystora@mac.com
>
>
>
>> On Jul 3, 2022, at 12:04 PM, Nelson Wright wrote:
>>
>> Emory
>> I may be wrong, but I don’t remember any factory GMC MH tachometer , so there wouldn’t be a wire.
>> Nelson
>> 78 Royal rear bath
>> Belle isle Fl & Nantahala NC
>>
>> Sent from my iPad
>>
>>>> On Jul 3, 2022, at 1:49 PM, Emery Stora via Gmclist wrote:
>>>
>>> I know the tach wire goes to the end of the distributor but I cannot find it. Can someone tell me the color?
>>>
>>> It is not shown in the wiring diagrams.
>>>
>>> Emery Stora
>>> emerystora@mac.com
>>>
>>>
>>> _______________________________________________
>>> GMCnet mailing list
>>> Unsubscribe or Change List Options:
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
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