Teddy952

I was able to borrow a +12v dc source and would like to use it to test my tachometer. I have removed the entire gauge cluster and verified continuity between the 8 pin connecter in the passenger foot well and the associated pin outs in the A, B and C connectors on the cluster harness.
I mapped out terminals 2, 3 and 4 in the clusters 'B' ribbon PCB connection. The 2, 3 and 4 terminals show continuity to the back of the tach's connections so there are no breaks in the ribbon PCB as far as I can tell.
I as understand things the tach works buy getting a +12v square wave from the ECU. I want to confirm that the tach is working so wouldn't a constant +12v just peg the tach? Since I replaced my wiring harness this tach needle has yet to move. I also did some testing with the stock boost gauge and that hasn't budged either. Getting the tach working is more important at the moment.

Also... has anyone removed the stock boost gauge's electric guts and replaced it with the internals of an aftermarket air pressure boost gauge? After swapping gauge internals and the needle you need to connect it to a boost source. The gauges faceplate will need to be changed to reflect the sweep of the new boost gauges needle. Instead of .4 - 1 - 2 bar you could create a more circular and accurate PSI sweep similar to what the aftermarket gauge came with. I know I have seen replacement gauge faces with just the tach reading and nothing where the boost or MPG gauge would be.

Thanks

Scott H

I can't help with your first question but Tom M'Guinn built a circuit to control the boost gauge, seems much easier than hacking a mechanical gauge in there: https://rennlist.com/forums/944-turb...e-control.html

Teddy952

I chatted with Tom earlier about my tach issues and controlling the boost gauge. I was more curious than anything else about replacing the stock boost gauge with an air pressure gauge. Does not seem very hard to do now that the cluster is on my bench.

Tom M'Guinn

Do you have someone who can just plug your tach into their car?

Teddy952

I do now. Shipping address please? ;-D


Just kidding... I don't know of anyone local that I could test it with. Good idea though.

Tom M'Guinn

I'll plug it in my car if you want. Might take a weekend or two and I'm in ca, but pm if you want....

Btw, I assume the tach is looking for a pulse and not a steady voltage, so a straight 12v probably won't work. Never tested to know for sure however...

Teddy952

Hey Tom,
Thanks for your very generous offer.
I hope that I can either test it myself or maybe a local shop can. But you never know.....

Im probably talking out of my @ss here but I thought pulsed +12v dc signal is nothing more than an on | off cycle similar to a strobe light. The +12v is delivered whenever a coil is fired. Example would be if this tach was connected to a household light switch. Switch on and the tach will peg, switch off the needle drops. If I could toggle the light switch fast enough, milliseconds, I could get the needle to hover at a particular rpm....?

or maybe not.

nofalls

I did a 555 timer circuit to initially set up my speedo and I have an old tach sitting around. I'll see what I can do with it this weekend and report back.

Tom M'Guinn

Would probably cost more to ship back and forth than it's worth...

I suspect that it will not peg with 12volts because it is looking for pulses (unlike the boost gauge which is looking for voltages)-- the faster the pulses the higher the rpm. If you just feed it 12 v, the tach will probably see that as one pulse. That one pulse will likely translate to no rpms on the tach. Just my guess. Nofalls will likely beat me to it, but I'll dig out an old tach and see if I can drive it with a microcontroller pulse. Or you could try to cook up some creative low tech approach to making a 12v pulse....

Teddy952

Thanks nofalls... let me know what you find.

Ill see what I can find for making a pulsed 12v source. A quick search showed a couple options I can try.

Last night I verified the +12v, ground and tach pulse signal in the gauge cluster's B connection on the back.
The B connection pin outs to the tach are the following..
Term - 2 = ground
Term - 3 = pulse signal
Term - 4 = switched +12v

I also checked the wiring harness's B connector that plugs into the gauge cluster. Having the voltmeters ground probe in term 2 and positive probe in term 4, I was seeing +12v with key on. Assuming the B connector was making good contact, the gauge should be getting 12v. I can replicate this circuit easily with a DC source but generating the pulsed 12v signal that term 3 needs is the tricky part.

Tom M'Guinn

Are those terminal references for the cluster connector or the tachometer itself?

Teddy952

Here is how the terms on the back of the cluster correlate to the tach connections
I found an a photo of the back of the 951 tach. I added a section of the cluster electrical diagram for reference.
On the B connection
Term 2 (gnd) connects to term B in the tach picture
Term 3 (signal) connects to term A in the tach picture
Term 4 (+12v) connects to term C in the tach picture

I believe term D and E are for the boost gauge but I haven't checked.

Adker

Try this, run it in astable mode. Circuit will run and output a 12V signal using a 12VDC power supply.
http://www.ebay.com/itm/Configurable...item43b89ac23f

Tach = Revs per MINUTE
Freq = Cycles per SECOND

nofalls

Sorry. I got caught up in a couple of other projects over the weekend but will try to get to this during the evenings this week.

I got to thinking about the signal and relaized the 50% duty cycle of the 555 circuit is proably not going to work. I suspect the tach is looking for four spikes (or grounds) per rev and to sit at the opposite voltage in between. I'll fire up the jimstim to simulate a four tooth wheel and see if that works. The signal may need to be inverted if my guess about pulling ground is correct. I'd just scope the DME signal but I traded my 951 for my dad's 911 for a couple of weeks under the guise of fixing some of his electrical woes.

nofalls

Just read Josh's ignition thread and realized there are only two ignition events per rev.