HOW TO DO AN ADVANCED SEARCH.

Mon Jul 08, 2013 3:56 pm by Admin


For the benefit of 944Hybrids users there are two search functions available for you to use.
The purpose of this sticky is to explain the "Advanced Search" function because it is much more powerful and is the best choice when researching information.

When you log on to the site a list of options is shown in a line at the top of the page. One option is labelled "Search", use this option (NOT the search box lower down on the right).

After you click on the upper search option, a drop down box appears. At the bottom of this box is a radio button marked "Advanced …

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Tach not working

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Tach not working

Post  Hotrodz of Dallas on Mon Apr 18, 2016 1:39 pm

Hey all,
New to the forum. Been lurking for awhile. I own a shop in Dallas-Ft.worth area that builds hotrods of all types. I am a long time 944 guy and have a lot of experience on them. I have done some V8 swaps on them as well. All LS builds.
I have 86 944 with a 2003 vette LS1 engine and ecu that is giving me fits trying to get the tach working. I know the tach worked with the 4 cylinder engine before I started the swap. I know I have the tach wire from the ecu plugged into the red/green wire on the 4 pin red plug. I am using a brand new PSIconversions wire harness. I can read rpm in the ecu with my scanner. I have tried 3 different instrument clusters and none of the tachs worked. I have ohmed all of the wires to the tach and all are good.
I don't have an oscilliscope to check tach signal wire so I am not sure that the ecu is putting out.

If anyone has any ideas, i'm willing to try whatever at this point.

Thanks, Bob
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Re: Tach not working

Post  944-LT1 on Mon Apr 18, 2016 7:43 pm

Should be a green/black wire at pin 3 of plug B at the cluster. This wire goes to pin 2 of the 4 pin plug. From what I can see, you have it hooked into the boost pressure gauge.

You should have:

pin 1 - white/green - mpg gauge
pin 2 - green/black - tachometer signal
pin 3 - red/white - boost pressure gauge
pin 4 - red/green - boost pressure gauge

See if those colors match up and if they do, hook up to pin 2 green/black.


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Re: Tach not working

Post  944-LT1 on Mon Apr 18, 2016 7:58 pm

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Re: Tach not working

Post  Hotrodz of Dallas on Mon Apr 18, 2016 9:30 pm

My apologies. I looked again and it is the green/black wire that i'm using. I was typing this without verifying what colors the wire is.


I got a pm from someone else. I can't answer pm's yet.
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Re: Tach not working

Post  sharkey on Mon Apr 18, 2016 10:27 pm

the ecm needs to be set correctly for tach output. depending on what your ecm came out of originally it may be set to send the tach signal through data bus instead of the tach output

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Re: Tach not working

Post  Hotrodz of Dallas on Tue Apr 19, 2016 8:12 am

sharkey wrote:the ecm needs to be set correctly for tach output. depending on what your ecm came out of originally it may be set to send the tach signal through data bus instead of the tach output

I had that checked when I had it tuned on the dyno. My tuner tried different settings to see if anything changed. But nothing he did would made the tach work.

Both of the previous cars I did the LS swap on, the tach worked perfect.
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Re: Tach not working

Post  Techno Duck on Wed Apr 20, 2016 7:32 pm

The LS tach signal by default is set for a 4-cyl so it required no changes at all when it was hooked up on my car. Have you tested the output wire from the PCM to make sure its sending a signal? Borrow an aftermarket tach possibly and test? I would also suggest ohming out the tach signal wire on the engine harness to the PCM plug to make sure the harness is wired correctly. My new harness from PSI had the MAF signal wires reversed at the plug causing a CEL. I found this out by mistake almost!... dont trust everything to be perfect!

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Re: Tach not working

Post  Hotrodz of Dallas on Wed Apr 20, 2016 8:17 pm

I ordered an oscilloscope to read the pcm output signal.
I haved ohmed all wires from pcm to the tach.
I did verify all pin locations in the pcm plugs. It's definitely in the correct location. And I don't have a cel on nor any faults in the pcm.
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Re: Tach not working

Post  scryfst on Fri Apr 22, 2016 8:15 am

Making the Late model Tach work is much different than the early model Tach. Early model is a no brainer.

You need a tach conditioner.
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There is also info available on making your own, this is what I do. Cost is about 7 bucks.

Good luck.
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Re: Tach not working

Post  Hotrodz of Dallas on Fri Apr 22, 2016 8:53 am

This is an 86 which means late model. No tach adapter needed. It's direct wire.
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Re: Tach not working

Post  944convert on Sun Apr 24, 2016 5:33 pm

scryfst wrote:You need a tach conditioner.
huh?

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Re: Tach not working

Post  944LS4 on Fri Apr 29, 2016 3:46 pm

before you use a tach conditioner, try installing a pullup resistor. Sometimes the TTL levels from the ECM are too weak to be heard by the tach.
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Re: Tach not working

Post  Hotrodz of Dallas on Wed May 04, 2016 3:38 pm

944LS4 wrote:before you use a tach conditioner, try installing a pullup resistor.  Sometimes the TTL levels from the ECM are too weak to be heard by the tach.  

You were correct. Checked my output signal at the ecu with a scope and found it to have a signal but was very weak. Found a website that shows what pull up resistor to use on an LS ecu. Followed their instructions and now the tach works perfect.
Apparently all 2003 & up ecus have a much weaker signal than the early ecu's.

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Re: Tach not working

Post  944convert on Wed May 04, 2016 4:03 pm

yeah, it's a great website...tons of info:

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Re: Tach not working

Post  944LS4 on Thu May 05, 2016 12:20 pm

I'm glad to help.  Tachometers have evolved over time as in most devices.  The early tachs (60-70's) began listening to back EMF straight off the ignition coil, which is around 900 volts. Circuitry inside this style tachometer took this noisy ringing signal and conditioned it.  Later tachs in the mid 80s were developed to listen to 24volts signals, then the tach designed morphed to 12volt signal (very short lived).  

In the 90's, we started to see cars with no distributors, so the ECU would drive the tachometer which was now a 5volt signal.   This type of 5 volt system seemed to be the standard for many years, until the advent of the friction-less aircore motors. Aircore motors are driven by sin/cos signals and a digital driver that is integrated all within the tachometer.  TTL data sent by the ECM is still a 5volt signal, but only has a whisper fart worth of current behind the 5volt signal.  

Your LS ECM computer was sending this data to your tach, but your tach couldn't amplify something that's not existent. This is one consequence with trying to mix different era devices together but with alittle ingenuity, anything can be solved (except large prime numbers) Very Happy
Since I pointed you in the right direction, am I now nominated to be the Admin of this site? Laughing
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