2G Tachometer No RPM Reading

[cswindel]

Okay so I know this is a covered topic but here are the parameters I am dealing with:

• Tachometer worked ~2 years ago when I first got the car
• Tore out full interior and replaced
• Installed OHM Racing Engine and Fusebox harnesses
• Using ECMLink (So still stock ECU)
• Stock ignition coils
• Car now runs fine and the rest of the dash gauge cluster gets power
• ECMLink is reading RPMs while pulling a log with car running

I think that covers the important components...here is the testing I have done (continuity testing that is):

• Did continuity test of PTU per FSM Got continuity
• Tested continuity between PTU connector signal wire and C-22 female connector signal node Got continuity
• Tested C-22 male connector (signal wire to Tach) and C-22 female connector NO CONTINUITY
• Did continuity test of C-04 pin 44 to a known ground Got Continuity
• Did continuity test of C-05 Pin 24 and C-22 Male connector (tach signal wire) Got continuity
• Re-did test between C-22 Male and Female connector and still no continuity

Now the course of action I want to clarify before I dive into the weeds more is:

• Would this be a continuity issue between C-22 and pin 58 of the ECU
• Would this be a continuity issue of, I believe pin 3, the coil pack connector

Thank you for your consideration of this even though it may be a simple answer and has been beaten to death but I am not so super-duper with wiring and all the diagrams

 

[steve]

You thinking this is another problem with the wiring of your Ohm Racing harness?

There should be continuity from ECU Pin 58 to C-05 pin 24 and to PTU pin 4. At C-22 pin 6 one side goes to C-05 pin 24 and the other side to the ECU (B-55 pin 58) and PTU (A-92 pin 4).

I don't understand what your third test really is.

Pin 3 of the coil pack is the switched power for the coils. Since the car runs you're getting power to coils and to pin 6 of the PTU. Hopefully you still have the A-64 capacitor filtering the voltage at the coil pin 3.

Remember the ECU doesn't use the PTU signal for RPM, that comes from the crank/cam signals.

This is a good place to use a cheap Oscilloscope to see if you have the tach output reaching C-05 pin 24.

 

[19Eclipse90]

Nice investigative work thus far!

It sounds to me that the 3rd test listed is checking the continuity through the connector, i.e., that the pins are actually making contact.

If the C-22 male and female ends are joined and by probing pin 6 through the backside of each connector half you do not see continuity, that would be a problem. Broken pins, poorly seated, loose, etc., could cause this.

Please clarify for us.

 

[steve]

If the C-22 male and female ends are joined and by probing pin 6 through the backside of each connector half you do not see continuity, that would be a problem. Broken pins, poorly seated, loose, etc., could cause this.

As can not accounting for the directional swap that goes on when you change perspective. But you would think that's obvious if your probes don't line up when you check both sides at once. Same for the wire colors being a clue.

I couldn't tell if the C-22 connector were connected or not for the test. If they were and only one side was checked at a time it's easy to loose track of pin numbering.

 

[cswindel]

I think we are on the brink of greatness here.

So to clarify what exactly I did for test 3 (C-22 Male and Female connectors):

• I had the male and female connectors disconnected from one another
• I then identified the white tach signal wire on the male side (as this portion of the harness is still OEM)
• Then I identified the corresponding pin of the C-22 female connector (This is where the OHM harness begins and trails down to the ECU, PTU, and all that good stuff.)
• I inserted a probe into each of these pin locations on the face of each connector with then still disconnected from one another (so basically the probes were bridging the gap between the male and female C-22 connectors)

From the sounds of it, I will have to investigate further starting with pin 58 of the ECU and probing 2 sites for continuity with pin 58. That is:

• Pin 24 of C-05 to pin 58 of B-55 (ECU)
• Pin 4 of PTU to Pin 58 of B-55

Hopefully I was understanding the directions correctly based on your guys' posts above

 

[19Eclipse90]

What I’m gathering from that, you would not and should not have continuity between the corresponding pins on separate halves of C-22. The wire run on the male side is completely separate from the run on the female side until the two halves are joined together.

Yes, run those checks and let us know what you get.

 

[cswindel]

What I’m gathering from that, you would not and should not have continuity between the corresponding pins on separate halves of C-22. The wire run on the male side is completely separate from the run on the female side until the two halves are joined together.

Yes, run those checks and let us know what you get.

Okay so if I connect the male and female of C-22 and probe from the back end of them i should get continuity?

 

[19Eclipse90]

Yes.

 

[steve]

If you don't then you’re either on the wrong pin on one side or the connector is broken.

 

[cswindel]

Okay so with C-22 connected there is continuity so good start. Now to get access to the ecu...

Alright so pin 58 to pin 24 of C-05 got continuity and same with pin 58 to pin 4 of PTU

 

[cswindel]

Only test i forgot to do was C-22 pin 6 to ECU pin 58. Now the question is, if there is a continuity reading there should I just chalk it up as the gauge has just mysteriously gone bad 2 years later?

 

[19Eclipse90]

That test would be redundant at this point. In order for C-05 pin 24 to have continuity with PTU or ECU, the connection at C-22 pin 6 must be made (good). Continuity is made through that connection, just as Steve wrote in post #2.

Hoping that I’ve tracked everything well enough but it sounds like wiring is okay.

You never mentioned whether the noise capacitor at A-64 was still present or not.

I’m not positive what the figurative TACHO box inside the PTU consists of, but it wouldn’t hurt trying another PTU. Since the car runs and drives perhaps the output from that box simply can’t make it out of the PTU to pin 4. The tach going out is not impossible, either.

 

[steve]

If you have continuity from ECU pin 58 to C-05 pin 24 and same with PTU pin 4. you don't need to check C-22 because it can't get from the ECU to C-05 without going through C-22.

There are four unanswered questions.

1) Are these wires somehow shorted to ground or another wire, like maybe something get pinched in the work.

2) Is the PTU actually creating the pulses.

3) Is the Tach responsive. Somewhere I had a scope trace. It wouldn't be difficult to whip up a circuit to test it works. A 555 or any microcontroller should be able to generate the pulses.)

4) I forgot the fourth while writing the other three. I'll edit when it comes back to me. (Senior Issues and not enough Coffee)

 

[cswindel]

Starting with one piece at a time, and doing the easiest trouble shoot (from my understanding), could I pull out the PTU and hook up a 1.5v power source as called for with the continuity test I did to cause the PTU to send the signal out to pin 4? Essentially what I am conceptualizing is, I would feed power to the PTU and hook up the voltmeter to pin 4 and check for a voltage reading. If this is the correct approach, would this then at least rule out if the PTU is faulty or not? Or does the car have to actively be running? Which if it does, could i run the car with the gauge cluster still removed and just probe C-05 for a voltage/signal reading?

 

[steve]

I don't know what electrical tools you have at your disposal but I don't think this test is going to tell you much that you wouldn't find out by starting the car and looking.

 

[cswindel]

@19Eclipse90 if im looking at the location of A-64 correctly it looks like it sits on the intake manifold. Now with the car not having a stock manifold since I got it, I can imagine that little silver cylinder piece is probably long gone and I dont believe my OHM harness has any connectors that are hooked up to something

@steve I have minimal electrical testing tools. All i have right now is pretty much just that $5 multimeter from Harbor Freight and some probes to make my life easier while using it. Would I need additional tools to answer/test the three questions you laid out above (Post #13)? I guess my train of thought with hooking up the power source to the PTU would be to verify its kicking out the correct voltage for the pulse its supposed to generate (even though I already did the continuity test on the PTU, I figured testing voltage range would verify what its for the signal is in spec)

 

[steve]

I doubt the $5 one would but the Hz measurement on the $20 one would give you the basic indication.

https://www.harborfreight.com/electrical/electrician-s-tools/multimeters-testers/dm300-pocket-sized-digital-multimeter-64018.html

I might buy one of these just to see how useful it is. I already have several real oscilloscopes but none are battery operated. If this one works it would be killer in the garage.

FNIRSI DSO-510 Handheld Oscilloscope/DDS Signal Generator 2 in 1, Digital Portable Automotive Oscilloscope 2.8 inch TFT, 10MHz Bandwidth, 48MS/s Sampling Rate, Trigger Function Auto/Nomal/Single: Amazon.com: Industrial & Scientific

FNIRSI DSO-510 Handheld Oscilloscope/DDS Signal Generator 2 in 1, Digital Portable Automotive Oscilloscope 2.8 inch TFT, 10MHz Bandwidth, 48MS/s Sampling Rate, Trigger Function Auto/Nomal/Single: Amazon.com: Industrial & Scientific

www.amazon.com

 

[cswindel]

Well in true Amazon product fashion, most of the cheap ones are frequently returned so I may just buy that $20 or the $40 multimeter harbor freight has (since they have a sale this weekend anyway) and get the hertz reading from pin 24. Thats all I'm really verifying right, is that there is a varying hertz signal? If thats the case then the multimeter would do fine for me. I just dont even use a multimeter enough let alone have enough use for the scope to put down the money on the $89+ ones

 

[steve]

I suspect the cheap ones get returned by the people to buy them expecting a $300 device or by the people that get them and don't know what to do with it.

Yes, you're looking for a pulsing output based on engine RPMs. Right now it doesn't matter the actual frequency because you tach is showing nothing. If it was doind something but not actual on the tach I'd be more interested in the pulse rate.

 

[cswindel]

Okay well for the time being i can just run down the street today, get that multimeter, fire up the car and at least test pin 24 for a Hz reading. I'm assuming I just stick the positive lead in pin 24 and ground the negative one to a known ground.

But if the test shows a pulse reading at the pin then I'm assuming the verdict will be the tach/gauge cluster is bad

But if there is no pulse reading then its the PTU?

Just want to make sure i have all my ducks in a row

 

[steve]

No pulse at C-05 24, then you check at PTU. whole nother diag tree in that case.

 

[cswindel]

Well I managed to get my hands on a multimeter with a Hz setting but haven't had time yet to do the test on the pin. Really just wanted to verify I don't need to run anymore tests on the ECU side because, until I find a better way to do it, my kick panel is a PITA to remove. Dont was to throw it back on and have to take it off again. Sorry its not really related directly to resolving the issue

 

[cswindel]

Well as of right now the tach is getting a Hz reading of 13.6 on average. So now its either the PTU or the gauges themselves I suppose. Not sure what the diags for those look like at the moment

 

[steve]

My brain is still overflowing with CAN-Bus info but this sounds both right and wrong. 800 rpm is 13.333 Hz (800/60) but I expect you get more than 1 pulse per rev, so it seems off.

 

[luv2rallye]

this sounds both right and wrong. 800 rpm is 13.333 Hz (800/60) but I expect you get more than 1 pulse per rev, so it seems off.

Ya doesn't the "tacho" part of the PTU "OR" the 2 primary coil signals together so you should get 2 pulses/rev (1 for cyl 1/4 pair and 1 for cyl 2/3 pair)?