Using Crank position Sensor for tach signal??

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Ian30

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Has anyone hooked this up? I hooked up an Autometer tach to the signal wire from the CKP and it reads way high and is jumpy. I read somewhere that the CKP tone ring is 32 tooth. Can anyone verify this for me? Also I believe that it is a 5v signal and the autometer tach says it requires a 12v. I have come up with a small circuit that boosts the 5v signal to a 12v signal and divides it by 4. I had it working at work where I have an oscilloscope but I haven't been able to try it out in the car yet to see what happens. I am waiting for one more part to come in the mail before I can do that. Just wondering if anyone else has tried this and what the result was. I just don't want to cut and tie together all the coil wires and pay $70 dollars for the autometer adapter.

On another note, does anyone know if the speed signal from the rear differential that goes to the ABS controller and then to the PCM is changed by the ABS controller or is it the same signal passed right through?

Any input is appreciated.
 
No idea what you are working on. This an EFI? or just crank triggered ignition? You using 1, 2 8, or 16 coils?

If this is a factory EFI, (or maybe even some aftermarket) the cpu may have a tach output? You may need a trigger from there to boost the signal to the tach. What is the tach designed to be triggered by, that is, what sort of signal? What model (and link to the destructions) is the tach?
 
I do not have direct experience with the tone ring on a Hemi, but I have used micro-controllers to translate signals of other crank and cam signals.

A V8 ignition generates 4 pulses per revolution, so you might need to divide by 8. Most all tachs inputs require more than 12V, because they connect to the coil primary, and there is voltage boost when a coil circuit is interrupted for the spark event. I generate a 55V pulse by using a small inductor, and a mosfet with 55V avalanche protection, driven by the micro. You might be able to use a one-shot, with your divider circuit, to generate about a 50uS "ON" pulse to drive the mosfet. For the inductor, I have used an un-snubbed (without diode) Bosch relay coil, because they are easy to mount and easily available. The pulse is so fast, so no click of relay.

I can help with schematic of simple inductor drive if I have not been clear.

A logic level mosfet is IRFL024 in a SOT223 package.
 
For around 80 bucks why not just use this set up...
New Style HEMI Tach Installation
using MSD GMR Pickup (part# 8918)
and Dakota Digital SGI-8 Tach Signal Interface
 
Stock Harness, Stock PCM, Stock Fuel Injection from a 2005 Ram. The signal coming from the Stock Crank Position sensor which is a hall effect type is a 5V square wave(I Think - No O-Scope at home). The autometer gauge is looking for a 12v square wave. I have used a FET with a pull up resistor to get it to 12V and isolate the original signal going to the PCM. I used a Dual Flip Flop to divide by 4, If i need to divide by 8 I have more. I just want to find an easy solution to this as I see alot of people adding boxes and stuff I dont think they need. CHeaper than the MSD and DD SGI-8 is the box from Dakota you can plug directly into the OBD II port. But I just don't think I need all that. I have plenty of other stuff I need to spend money on to complete this project. I cant start the car right now because of something else, so I cant test this out. I was just hoping someone else might have tried this and have some input. It would be great to be able to find a cheap/free solution to this as i have seen plenty of posts about it.
 
you will need to use what i posted, no getting around as I and others have tried. It does cost some money to get the parts, but how much time do you have invested already trying to figure out what the aftermarket has done for you? Get the msd and dd and your done. The only other way around this is to use a hotwireauto harness for an 03 ram as it comes with a tach signal wire that goes straight to the tach. I have told Chris @ hotwireauto about this as I had an 03 computers and harness then switched to an 05 computer and was curious to see if the 03 harness would work and whadya know, it did, so I have told Chris what I have found and has does nadda to add the tach wire to the harnesses. So bottom line, get the msd and dd stuff when ya can and your golden.
 
I should have prefaced my post by stating that I am a cheap ***. Everyone told me to just buy a harness for the motor from Hotwire. I didn't, bought the schematics for $25 on Ebay and after a couple hours studying found out exactly how easy it was to get the motor to run. (would have been slightly harder if SKIM was enabled) There are really only a couple of wires that need to be hooked up and the rest can go. Lots of money saved and to be honest I find it all interesting. I have small kids, so i have to spend time inside with them sometimes just watching them with no interaction so I have the time that I can't spend in the shop. I don't believe any of this is rocket science and its just a matter of finding the information to get the right result. I have had great luck in this forum and on NADODGE getting most of the information that I need and I appreciate all the free sharing of information. I might end up buying a solution but I am no where near doing that right now. My next task is to get the cruise control to work hence my question about the ABS controller speed signal. I believe I can generate this from the transmission output sensor which I am also using for the speedometer. Anyway I appreciate you giving me a solution, but my budget is really tight (my wife keeps track of everything) and like I said, I find it interesting. Hopefully I will get it worked out and be able to share with other people so they can benefit from my time like I have from others.
 
Does the schematic show the tach signal wire??
 
I built the tach drive into my engine management system. It is there because coil on plug and waste-fire need an adapter, because coil signals are separate. In development a simple resistor pull up to 12V failed to work, on all the various tachs, mainly OEM VDO. The relay coil as an inductor made them all work. I am cheap too. My EMS also provides RPM via communications link to PC, the numbers agreed well for the numerous installs on various vehicles.

If the tone does not have equal spacing for the teeth, it may be necessary to use a micro to fix that, or the tach needle may waver.

If you have a picture of the tone wheel, I can write the code, program a chip and send a schematic for you to build. I will simulate the Hall signal, and pre test. The postage may cost more than the few dollars in parts. It will be simple, a $1 micro (ATtiny45 in 8 pin DIP), a few resistors, a few 0.1uF caps and the MOSFET. The circuit will only draw a few mA at 5V, so the
ECU sensor supply can be used.

The transmission sensor could be used. There are safety and liability issues related to ABS. I would stay away from that! Again a $1 micro can re-scale the frequency as desired, with just a few parts. I speak from experience.

If you want to get involved more, a USB chip programmer can be purchased for $20 that even has a 2 channel scope that will work for for development debug. It is from Pololu.com AVR USB programmer. The data sheets for the ATtiny45 can be found at Atmel.com. There is a C compiler that is available as evaluation, for small non commercial applications from HPinfotech CVAVR.
 
It does not show a wire in the schematic because all that info goes out over the can bus with all the other information to the gauges. Maybe if we can get the schematics for an 03 it will show where on the PCM connectors to hook up the wire. That seems a lot simpler than what I'm doing. Are you sure that the wire feeds a Tach directly? I was under the impression that some sort of adapter was still necessary but it sounds like you have first hand experience. This could be viable - What kind (brand) tach were you using?

KitCarlson -
I am worried that the teeth are not equally spaced but in groups of eight with a larger space between the groups, although I have not seen the tone wheel in a picture or in person so I am not 100% sure. I am hoping if this is the case that the autometer gauge will not be able to react fast enough to the gap. I will definately be contacting you if that is my only problem when I get this hooked up. I probably won't be able to try it until next wednesday. I will start looking for a picture of the tone ring so I can be ready if necessary.

The speed signal from the rear diff. and wheel sensors is routed through the ABS Controller then to the PCM. I do not have the ABS controller or ABS, but I have read that the signal that comes from the ABS controller is necessary to get the cruise to work. So no worries since it won't affect anything on the brake system.
 
It is likely low rpm will result in the most variation. With 32 teeth, and the damping of the mechanical movement there is a chance it will work.

The micro solution would be to count 32 and generate 4 equal pulses over the period. It is a simple problem with perhaps, less than 10 lines of C code.

Same for cruise speed, the hardest part may be to find the correct pulse rate. There is often a minimum speed to engage cruise. By using a signal generator, it might be possible to find the rate for that speed.
 
Yeah, the 03 harness from hotwireauto has a dedicated wire that goes straight to the tach. Might be worth a phone call to Chris?
 
Not like I thought it was going to look, but it is 32 tooth.
 

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Look at this!!!!, I found a 2003 manual online and low and behold there is a rpm signal wire in the PCM pinout. I havent checked to see if it is blank in my PCM connector as I have to open it up again but this is definaltely something I need to look into.
 

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here for contrast is the same pinout from the 2004 manual
 

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I think if you run a wire to that then your tach will work just fine.
 
And here is the 2005....not really sure what to think.
 

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This wire goes to the transmission and is the pressure control solenoid control.(As shown in the picture below) It is however a pulse width Modulation, a square wave which will trigger a tachometer. 5.7 - Did you just start the motor and verify, or did you rev it up and drive it? A little road testing would probably show it to not be accurate. Or maybe it is the original tach signal that they use to control the line pressure. Either way, I will check into it a little more and see what I can come up with.
 

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I am suggesting the obvious. If pin 11 is a rpm signal it will need to have both 4 pulses per revolution, and sufficient amplitude to drive the tach. I modified a clamp-on timing light by adding a 10k resistor and BNC connector, so a scope can be use to monitor ignition. By using one sparkplug wire as a reference, it is then possible to determine the pulses per revolution on the rpm signal however one ignition per revolution unless waste fire.

If the rpm signal is from the ecu, it is likely 5v and will not drive the Autometer tach without boosting to 12v square wave. The more recent Autometer tach work with 12v.
 
Kit, am I mistaken that I have been thinking it should be 8 pulses per revolution on a V8 motor? I start my workweek today - 12 hour shifts, so no work on the truck until wednesday when I am off, but I can then verify the pin 11 signal.
 
A 4 cycle engine makes 2 revolutions per combustion event. With 8 cylinders, it is 4 combustion events for one revolution. That is assuming non-waste fire ignition.

I also had the thought that if the ignition is coil on plug it is not possible to use a timing light, unless put on the coil primary. The clamp-on probe is a current transformer, so it should work.

It is possible to double fire coil on plug because it saves ignition drivers. I have no idea what the Hemi uses. If the timing light shows two blinks, 180 degrees apart it is waste fire.

More often a cam sensor is used to resolve the engine phase, and is crucial for efi emissions, idle quality, and low speed economy.
 
Thankyou for clearing that up - That is something I should have been able to figure out on my own - I feel like a complete idiot. I need to get another flip flop in my circuit now.
 
Sorry didn't read it all, but if you want simple, then just connect the 12v tach input to any one of the coils trigger wires.
 
If ya do that, then a Dakota Digital signal box along with the MSD that I mentioned earlier is required.
 
5.7 hemi said:
If ya do that, then a Dakota Digital signal box along with the MSD that I mentioned earlier is required.
Click to expand...

All I can tell you brother is it works on my 03 hemi Ram. I have several items on my truck requiring tack input and connecting to the COP coil works as well as connecting to my AutoMeter tach adaptor. I believe the setting was for something odd like two cylinder. Good luck..
 
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