no rpm to tach

[napkinthief]

im currently using a 9964 ecu, and the tach is not showing rpm. at about 3k rpm, itll spike up a bit then settle back at 0

I have a chrome ecu, that isnt letting the car idle, but when i plug this ecu in, the tach works. (cluster is good, wasnt removed since i bought the car)

If i do an evoscan log using a 1.3 cable on the 9964, I get the rpm counts on the log but the tach isnt working.

any ideas?

[RealMcCoy]

What harness are you running? I'm not familiar with the pinouts for the JDM ECU, but I would assume from your symptoms that the tach wire(white wire, pin 101 on a 1G harness) is either being pulled to ground, or the ECU is not creating the required 5V reference voltage.

[futurevr4man]

odd. Your ECU is obviously reading the RPM which rules out a large chunk of harness. I would check your harness from the ECU to the actual tach itself... I'll bet you have something awry

[napkinthief]

the ecu is definetly reading the rpm. I dont think there is an issue at the harness to cluster, since if i use the chrome ecu, itll work properly. there is continuity from the white white at the ecu to the tach wire at the PTU. i stripped a bit of wire off that white wire pin, at the ecu and tapped a $25 autozone tachometer and it works, but its frickin tiny. i think the ecu pin out to the cluster doesnt match the 9964 ecu. im off until monday so this weekends going to be a whole lotta troubleshooting

[RealMcCoy]

I don't think the ECU scan data is being generated from that source... I'm willing to bet you could yank that wire out of the ECU connector, and it would still show you RPM in scan data.

What you aren't getting is the 5V reference voltage needed for the PTU to pull to ground in order to make the tach signal. Your cheap tach doesn't care because it makes it's own reference voltage.

[napkinthief]

I don't think the ECU scan data is being generated from that source... I'm willing to bet you could yank that wire out of the ECU connector, and it would still show you RPM in scan data.

What you aren't getting is the 5V reference voltage needed for the PTU to pull to ground in order to make the tach signal. Your cheap tach doesn't care because it makes it's own reference voltage.

im looking into this more now....

the tach is working iffy, sometimes sometimes not, over 3500 rpm, itll jump around all over. its making me think its a ground issue

how can i check the 5v to ground? are you refering to the plug on the PTU, and the wire labeled ground there? should i ground it direct to the motor, or the ecu ground pin?

[RealMcCoy]

im looking into this more now....

the tach is working iffy, sometimes sometimes not, over 3500 rpm, itll jump around all over. its making me think its a ground issue

how can i check the 5v to ground? are you refering to the plug on the PTU, and the wire labeled ground there? should i ground it direct to the motor, or the ecu ground pin?

You are just looking for 5v at the tach wire pin of the ECU... When the PTU closes the transistor to fire a coil, it also grounds that wire, which produces the waveform that is your tach signal... People think the PTU makes the tach signal, that's not technically true. It would be more accurate to say it takes it away to make the waveform.

[napkinthief]

You are just looking for 5v at the tach wire pin of the ECU... When the PTU closes the transistor to fire a coil, it also grounds that wire, which produces the waveform that is your tach signal... People think the PTU makes the tach signal, that's not technically true. It would be more accurate to say it takes it away to make the waveform.

ok so i am getting 5v at the PTU...this is the same 5v that comes from the ecu pin that powers the original maf etc. currently using it to feed power to my map sensor aswell.

is it safe, to ground the PTU wire directly to the PTU bracket, or does it need to run to the ground on the ecu pin?

[RealMcCoy]

Now you lost me... What pin on the PTU has 5V? Where is it coming from? What wire are you wanting to ground and why?

The map sensor, TPS, and any three wire sensor MAKE a signal, using the 5V reference voltage from the ECU. If that 5v gets grounded out it kills everything... A two wire sensor or trigger like a temp sensor or your tach signal has a 5v reference after a resistor, that is pulled down by the sensor or trigger by partially or completely grounding it.

[napkinthief]

I just checking again, now im even more confused...

I swear I had 5v on the wire labelleed tach yesterday. now im only seeing about 1v.

I was wanting to ground the PTU wire, that is labeled GND, directly to the engine/bracket.

Interestingly enough, its about 40 degrees today and the tachometer is working fine at idle. after about 15 minute warmup, its pinned at 0 and only spikes up when revving. really sounds like a ground problem

I have a couple spare PTUs, I may swap one in to see if it fixes it, without messing with the harness.