Thetastycat

Hey all - just don't want to do this wrong and kill the XJ! I need to check for signal at pin 43 of the PCM on my 95. The FSM says "with engine running, check for signal at pin 43." Seems to me that with the PCM unplugged it probably wouldn't even start, and I'm a little afraid to even try it. Is it the right thing to do to start the motor and then unplug the PCM and check? Then shut the motor off before I plug it back?

Thanks a lot!

BruceB

I believe they are referring to using a straightened safety pin and pierce the wire of pin 43 from the back of the plug while still plugged into the PCM then attach or touch the safety pin with your tester. Be sure to use nail polish of some type of sealer on the pin hole when finished.

lawsoncl

Pin 43 on a 95 PCM (grey wire with light blue stripe) is the tachometer output, so I guess you're trying to troubleshoot the tach not working?

Don't unplug it. They're expecting you to "backprobe" it by either piercing the wire or sliding your probe into the back of the connector.

Thetastycat

Thanks for the great replies! Yes, it's for the tach - it suddenly stays at zero all the time, but all the other gauges are working fine. Figured I'd follow the FSM troubleshooting section and the first thing they said is to check that wire.

Could you tell me more about sliding a probe into the back of the connector? I'd rather do that than pierce the wire if I can help it!! But if I must, I must. I didn't know nail polish could seal it up again!

Sure appreciate all the help.

lawsoncl

Intermittent is probably not caused by a bad signal from the PCM. That tends to be an all or nothing kinda failure. I'd suspect a bad wire, bad ground connection or the tach itself first.

https://e-z-hook.com/blog-guide-to-b...be-connectors/
https://www.agcoauto.com/content/news/p2_articleid/244

Being the PCM connector, you'd have to take the back of the connector apart to get to the wires. It's a bit easier on small connectors where you can slide a thin probe into the back next to the wire.

** Related thread - https://www.naxja.org/forum/showthread.php?t=93271

Thetastycat

Actually it's not intermittent - it just suddenly went dead. But it's just the tach - which is why it's just hard to understand - speedo and everything else working fine!

Great resources, Lawson - I'm going to try to back-probe it for sure!

cruiser54

Thetastycat

Makes perfect sense - thanks Cruiser!

awg

I have backprobes made with insulated sewing or safety pins,

You can gently slide them into the female pin cavity and make contact without piercing anything

In addition, try rapping hard directly over the tacho as they sometimes freeze, mine used to do this, until I lubed it

Thetastycat

Hey all - sorry for the delay! Life gets way too busy and the fun stuff has to wait.

Finally got me a multimeter that reads Hz so I can follow the FSM troubleshooting! I got a small safety pin and put it as far into pin 43:



The FSM says to first check for tachometer signal at pin 43 with the engine running. It should be 66.7 Hz at 2000 RPM and 166.7 at 5000. So I've got my pin stuck in there and my multimeter on Hz, but I'm too stupid to know how to actually test this - do I just put the black probe on ground? I don't get any signal doing that, and no voltage either - figure there has to be some voltage in the signal. AWG, you mentioned using an insulated pin - I have no idea if this pin is insulated or not. There wasn't room to shove the whole multimeter probe in there. Maybe I should just pierce the wire and then "fix" it with nail polish?

OR - I do have an entire spare gauge cluster (had to get a whole one just to get a temp gauge) - should I just jump straight to replacing the tach and see if that works?

Sure appreciate all the help!

awg

touch the red probe to the pin. and the black probe should be on ground, which can be battery negative, or any grounded point on the vehicle

I have not read the FSM on this topic, but Hz is a measure of frequency, which suggests AC voltage not DC

My meter can read frequency, yours may not, if no, set to AC volts, see if it goes up when you rev engine

In my opinion you should perform the needed tests before replacing any parts

I would also remove the clear perspex, and lube the needle before replacing anything, as I have had a frozen needle on 2 x XJ

Thetastycat

Yeah, I tried that and didn't get anything :/ Fortunately my multimeter does read Hz, but I got nothing. Maybe I should try a longer/larger pin? Or a different type?

No kidding about the frozen needle - I'll definitely try it before replacing the tach, but at that point I already have to have everything taken off so figure I might as well follow the troubleshooting steps to get me there!

awg

just wiggle the pin around a bit to see if you can get any reading of DC, AC, or HZ

I have not read by what mechanism the PCM senses and generates the tach signal

Thetastycat

Well AWG, you saved the day, my man! I wiggled the pin and sure enough, I got some current to show up, so I figured the PCM wasn't the issue since the tach wouldn't budge off zero - some current, even if it was the wrong amount, would make it do something!

I got the dash bezel off (for the 200th time it seems, lol) and unscrewed the plastic - I went to diddle the needle, and it suddenly moved effortlessly. Somehow, the needle had gotten stuck ON TOP OF the peg that stops it from going lower than zero RPM - as soon as I got it back into its usual range it worked 100%. So there didn't seem to be anything I could do to stop that from happening again, freak accident though it might have been, but at least I know what it was!

Huge thanks to everyone who walked me through this process - my XJ would be even more of a hunk of garbage without you all!! I truly appreciate it.

awg

Excellent,

while you have the plastic off, I suggest a drop of machine oil on the spindle of all instruments after first blowing them out with an air duster

I have had other sticky needles on XJ instruments, after 25yrs, dust and lack of lube seems to gum them up a bit, my tach is sometimes a bit reluctant sometimes first thing, I oiled it 5yrs ago

getting the plastic off without dropping the steering column is tricky (in my '96sers at least)