High revving tach, inop temp gauge.

[Automan2164]

I know that the best way to get some action on a problem is to give it the full on spot light here. I've been having some trouble with the tachometer and the temp gauge on the diesel. I tossed out some info on the thread, but I want to reach out here to touch some people who might not be following along. I would like to consider myself pretty electrical friendly, and am pretty good at mapping out paths and routes in my head. I can make the circuits in my head. Not being able to figure this out isn't from a lack of trying.

 

What's happening.

 

The tach has been on the high side ever since I bought the truck. It wasn't charging at the time, so I assumed that it was related to the tach, where it gets it's signal. This was also confirmed by opsled, who has the other diesel MJ. He said that whenever his alternator started going out, one of the first identifiers would be that the tach would start acting goofy. Easy enough, or so I thought. I changed out the alternator, but to the same effect. It charges now, but the tach is still doing the same thing it did before. I would estimate that it is around 2-3X faster than what it should be, or as best as I can guess by ear. A solid idle is about 2300 on the gauge. It will rev all the way past the end point (6K) if I snap the throttle.

 

Also, the temp guage. It never, never moves. I've replaced the sensor to no avail. The connector is in good shape on the unit, and all seems well.

 

What I've done so far.

 

First, I wanted to see what I could figure out with the tach. I know that it goes way overboard, so I wanted to see if it might have been getting some excess voltage from somewhere. I took the signal wire off of the alternator, and fired it up. Stone dead tach. Not a move at all. Next I wanted to make sure that the alternator had a good ground, so I checked it with the DVOM, it was good. Just to be sure, I also put a jumper from the B- post the the neg battery cable. Fired it up, no change. Still high. The wires seem good, and in good condition, so I moved my operations into the cab. I checked the pin on the cluster coming from the alternator. It started out at about 6V, and climbed up to 7 with some throttle. Grounds on the connector all tested good. I tested from the respective points at the back of the tach to their pins at the connector, also seemed good.

 

The temp sensor is aggravating also. I've tested the wire out to the sensor, from the connector to the sensor, there is almost no resistance. The connection is good. I've also pinned out the back of the gauge to the respective pins on the connector, all seem good. I can get the gauge to move when touch straight ground to the sensing pin at the back of the gauge. I just touch it though, not hold it there.

 

I want to say that the tach and the temp problem are related in some sick way. The gauges are close together, and I am betting that they share something that's causing this issue. There has to be something that I am overlooking. This is why I am coming to you guys. I took some pics of the pages in the wiring diagram that are of interest, and also I have one of the back of the cluster to show the routes of the pins/contacts.

 

 

 

 

 

I am open to any suggestions, and will try any test, even if I have already done it.

 

Rob

[jimoshel]

My guess would be a bad tach amplifier or faulty ground in the instrument cluster. Can you swap tachs with your friend and see what happens? You can test the amp by feeding a 6-12VAC signal into it and monitor the output. Some AC signal generators will generate a strong enough signal for this. One thing to remeber when using a AC signal. Automotive tachs are calibrated with a 60cps signal. Normal house hold AC supply. The tach on your diesel is taking a AC signal from the alternator which I believe is 400some cycles AC at idle.

[cruiser54]

Might want to try this just for fun. Can't hurt and might help.

 

Improving the Instrument Panel Ground

 

The ground point for the complete instrument cluster on your XJ or MJ is located up under the driver’s side dash. If you lay on your back and look up under there with a flashlight, without wearing a hat, you will see a black wire attached to a shiny piece of metal almost directly above the hood release knob. The screw will have either a ¼” or 5/16” head on it.

 

This ground point is responsible for handling the ground circuit for the following items: Dome lamps, Seatbelt and key warning, trans comfort switch, wiper switch, headlamp switch and delay module, fog lamp switch, cargo lamp switch, all instrument panel grounds and illumination, power windows and door locks, cruise control dump valve, and a few more things.

 

The problem is that where the ground point is located does not have a good contact with the chassis where the ground should be. The solution is simple.

 

Make up a jumper wire with #10 gauge wire about 10” long. On one end, crimp on a ¼” round wire terminal. On the other end, crimp on a 3/8” round wire terminal.

 

Remove the screw from the existing ground wire and attach the small terminal of your jumper so that the original wire and your new jumper share the same attaching point, one over the other.

 

Look above the driver’s side plastic kick panel just forward of the top of the hood release knob. You will see an 8mm stud there. Attach the large terminal end there with a washer and nut over it tightened securely.

 

**Special note for Comanche owners: Make your jumper wire 12” long and attach it on the driver’s side kick panel close to the fusebox on the 8mm stud.**

 

 

Revised 11-29-2011

[Automan2164]

I've spent some more time playing with it this week. Driving me batty for sure. I took the grounding advice, and tried that. With all my wires, jumpers, and insanity trying to track this down, the truck looked like spaghetti.

 

 

 

 

Just in the testing phase, I wanted to ensure the ground on the dash was good. I took the jumper cables I had, clipped them to the dash ground and the other end straight to the neg. I also had a reference wire to the valve cover, the stud of the sensor, chassis ground off the block... I still had nothing. Then I took apart the cluster to inspect the back of the gauges in question.

 

 

That's when I remembered that I had a few 3/4 clusters of what I thought were about the same vintage.

 

 

For those who don't know, on the older style clusters, once you have the bezels off the gauges you can just pull them out without pulling the whole cluster. The 3/4 gauges unfortunately have a paired fuel light and temp gauge, so I had to pull the fuel gauge out to try it out. I started the truck, and kept my foot about 1/4 of the way in it, wanting to see some movement. Then it happened. Movement.

 

 

Wondering if the old gauge would register, I then tossed that back in.

 

 

Success!!!

 

While I don't have the tach fixed yet, this is the one I was really worried about. At least at this point, I can start driving it around. I wasn't about to drive a truck around with an engine made of soft metal without knowing what the temp is... But anywho... The bigger of my problems seems to be corrected for now.

 

Rob