Whacko Tacho

[CycloSteve]

1996 FZJ80, 170k+ miles. Modded most everything.

The issue is that on startup the tachometer blips slightly, then goes dead. Moving the truck into drive blips it again, and then after a minute or so it comes back to life. For the first few times rowing through the gears, it seems to go dead in R, but back to life in N and D. Once warm this does not seem to happen, and everything works fine. All the time the engine runs smooth, and no audible issues. I have checked all of the grounds at the batteries, engine, t-case. Voltage is perfect.

My next test is to hook up my OBDII scanner to see if there is a tacho signal coming through the port. Should that not have a signal, then I think it points towards the igniter, or something in that direction (ground wire?). If the OBD reads a signal, but the tacho does not, then I will pull the cluster to see if that is the issue, as it could simply be one of the plugs. No water in the truck, or any other electrical gremlins.

Any other areas to check?



Steve

 

[landtank]

I believe that signal comes from the distributor. It uses the can sensor.

 

[Kernal]

The tach on my truck acted up only once on a cold day; it would drop out (lurch) around 1800-2000 rpms and at idle, then come back. The tach went back to normal once the truck warmed up in 20-30 minutes. I pulled and cleaned the connectors to the MAF, the Ignition Control Module by the ABS unit, and the connector to the distributor (while I was at it cleaned the TPS and IAC connectors also). Sprayed them all very well with CRC quick dry electronics cleaner, some Q-tips to clean the contacts then a smear of dielectric grease on the seals, popped them back in. Problem has not come back. The connector to the ICM was tight, first time it ever was pulled off I'm sure, needed a small screwdriver to help back it out. A few other people have experienced the same thing in the past.

Here's the link:

https://forum.ih8mud.com/80-series-tech/143684-jumpy-tachometer-any-fixes.html

 

[CycloSteve]

I did see that thread, but mine is either on or off, not jumpy.

I will pull, clean and lube all of the electrical related plugs and see if anything looks odd in the dizzy such as moisture. Just read through the EWD and FSM, and looks like there is just one wire to the tacho for the signal (11B @ cluster, 10D @ ECU), so should be not too difficult to figure out if the clean and lube does not work. Would rather not pull the cluster, as it can be a PITA.

It has been exceptionally wet here with over twelve inches of rain since Thursday...though the truck has for the most part been parked.



Steve

 

[FloridaFJ80]

Please keep us updated on this fix, mine is doing this sporadically, currently it shows an idle of 200rpm but the ultragauge shows 660 a normal idle.

 

[CycloSteve]

Minor update.

Checked the wiring loom plugs at the dizzy, coil, and MAF. All looked fine, and added some dielectric for good measure. Opened the dizzy itself, and it looks 100% normal and is dry. Cap and rotor have minimal miles and are all OEM.

Confirmed that the signal at the OBDII is fine at all times. Engine is running normal otherwise. The tacho can be at zero, yet the OBDII scanner will show correct RPM. When the gauge goes whacko, the OBDII is rock-steady.

Here is a quick video for those wondering what this looks like.

http://youtu.be/aG1m09wh4PI

Will pull the cluster next, as I suspect a ground fault with one of the plugs at the rear.



Steve

 

[Kernal]

Everytime you put the transmission into Reverse the tach went dead (one lurch between D and 2), would that be a ground to the cluster related to the R light, the printed circuit board,---?? Interesting.

 

[CycloSteve]

OK, a bit more debugging but not really any success. Pulled the cluster, and checked at the ECM. At all times the OBDII continues to give the correct RPM reading.

• Tacho feed wire between ECM and cluster tests fine
• All ground wires at cluster test fine
• No breaks in traces on the flex-board on back of cluster
• When engine is running the TM post on the back of the cluster reads between 6.4 and 6.8 volts, regardless of RPM.

So, electrical guru's, what say ye? I am not finding any information of the type of signal which should be at the TM post as it is sent from the ECM. At this point I am leaning towards either a bad ECM or bad Tacho. Neither is cheap.

Anyone have a spare cluster in the SF Bay Area I can use to see if that is my issue?



Steve

 

[CycloSteve]

Now that I have the cluster on the bench, does anyone know of any "bench" tests that can be done before replacement?

On a similar vane, does anyone know the proper signal which should be at the TM post on the back of the cluster?



Steve

 

[CycloSteve]

Received my "replacement" cluster from AJIK Offroad, and confirmed that the tach itself was the culprit.

Moved the "new" tacho over to my cluster, as I want to keep my speedo/odo, and RavenTai mod temp gauge. The only issue that I ran into was that the needle on the new one was a bit more faded, so I moved my more orange one over...which took a few tries to get the tach speed to 100% match the OBDII signal.

Good to go now, but waiting a brand-spanking new plastic "cover" from my local dealer, a whopping $28. Money well spent, as mine had some odd spatter from some PO cleaning with something too caustic for the material.



Steve

 

[Kernal]

Can you describe how you moved the needle, got photos? Any postmortem on what actually broke inside the tach?

 

[djsixbillion]

Good to go now, but waiting a brand-spanking new plastic "cover" from my local dealer, a whopping $28. Money well spent, as mine had some odd spatter from some PO cleaning with something too caustic for the material.



Steve

Got a part number? Mine is "hazy" but I've been reluctant to do anything about it since I figured I'd have to buy the whole cluster to get a new plastic cover.

 

[CycloSteve]

Can you describe how you moved the needle, got photos? Any postmortem on what actually broke inside the tach?

You can pull the needle from the gauge...just takes a little tug. I basically placed it on the spindle, started the rig, and saw how far "off" it was. Getting it right meant placing it below the stop-pin very lightly on the spindle, then moving it up over the pin and seating fully. It is now dead-on at idle, and also at 3k rpm.

Why did it die? I can smell a very faint burnt component stench. One of those smells which sticks in your head, no matter how long ago you worked with PC boards, and burn-in cabinets. Essentially a component fried, which I do not know. I will likely mess around with it at some point when I am really bored to see if I can figure out what fried. Plenty of standard bits, but there is a little chip, which could very well be the dead component.



Steve

 

[CycloSteve]

Got a part number? Mine is "hazy" but I've been reluctant to do anything about it since I figured I'd have to buy the whole cluster to get a new plastic cover.

COVER, COMBINATION METER, NO.1
83134 60100

There were two in the warehouse in LA as of yesterday...now just one. $28 at my local dealer with TLCA discount.

If you want my old one, it is not too bad. No haze or discoloration, just some odd little spots. Pay the freight, and it is yours. I am just anal about fixing these niggly things while I have them out.



Steve

 

[Kernal]

Had you performed any work on the truck just before this occurred? Please come back if you figure out what got fried/died.

 

[djsixbillion]

COVER, COMBINATION METER, NO.1
83134 60100

There were two in the warehouse in LA as of yesterday...now just one. $28 at my local dealer with TLCA discount.

If you want my old one, it is not too bad. No haze or discoloration, just some odd little spots. Pay the freight, and it is yours. I am just anal about fixing these niggly things while I have them out.



Steve

Thanks Steve, think I'll take you up on that. Sending you a PM.

 

[CycloSteve]

Had you performed any work on the truck just before this occurred? Please come back if you figure out what got fried/died.

Last month I changed the oil and filter, and lubed everything. So nothing out of the usual, and no major changes or things taken apart. I wish I could figure out if there were any causal issues, as I do not really want to fry another tach. Basically the tach just did not come on one morning, then I noticed it was intermittent, and went on and off with manual gear shifts. Then it went completely dead.



Steve

 

[Kernal]

Is there a harness that could be bumped by the shifter assembly ie: if there were a short in a harness that somehow sent a ? power spike to a component in the tach?? Sounds impossible but in the video everytime you shifted into Reverse the tach dropped out, curious what could connect those two things??

 

[bsmart82]

I guess I over looked this thread. The ewd shows the cam signal goes to the ecm, and then the ecm outputs a tach signal to the combination meter. If there is a problem with the cam sensor, it will either throw a code, or have some type of driveability problem, like no start, miss, or stall. If the ecm is dropping the signal, then usually the tach won't work, but you will also not have tach at the obd port. So in your case, no problems except tach, and you have rpm signal at obd, I would suspect a fault in the combination meter. But you already know that, so I posted this info for future searches.

 

[Kernal]

Any ideas why the tach would drop out when he shifted into Reverse?