‘85 Maxim x tack repair help?

Good morning all,

I’ve been reading about disassembling the electronic tach to free the needle but I can’t figure out how to get it apart without breaking it. Would anyone have directions? With pictures?

Once I get the tach fixed I’ll move on to the “bogging issue “ at higher rpm under hard acceleration!

Thanks

 

AFAIK you have to remove the chrome surrounding. I've tried that with an extra speedometer I have to change the scale. No success... I've found it impossible to remove the chrome ring without damaging it too much. Maybe someone else knows a good way to do this.

 

Courtesy of @hogfiddles :
http://www.xj4ever.com/repairing your 700 gauges.pdf

 

He did a nice job in this writeup. Probably I wasn't patience enough and because of that damaged the chrome surrounding more than necessary.

 

Thanks Gents,
I’ll give it a whirl tomorrow and let you know how I make out.
Cheers

 

So I managed to remove the chrome ring and get the tack apart. Needle is moving freely and rebounds back to zero.

pluged it in and still no luck. Tried a second tack and same thing.

it’s looking like an electrical issue.( not my strong suit!)

anyone able to explain electrical tachs? What voltage/ amperage I should be seeing at the plug?

thanks

 

They are really pretty simple and use a frequency to voltage ASIC that drives the DC meter. The trigger signal is derived from the TCI ignition coil primary. I believe the orange wire is used on the XJ700.

So, you need the 12V and return to power the electronics within the tach, and the ignition coil primary signal to convert to DC to drive the meter. You can measure the 12 VDC at the tach to be sure it is OK, and then check the continuity from the ignition coil primary up to the tach.

Coil primary drive signal courtesy of XJCD.org:



Some tachs (not sure about the Maxim X) have terminals that are easily removable from the meter movement and with those it is very easy to test the meter movement with a DMM or 1.5 volt battery to see if the meter movement is functioning. If you could, please post a couple of pictures of the internals of the Maxim X tach for posterity.

 

Well, I finally got around to running these tests. 12.6 volts on both the brown and orange wires at the tack plug and good continuity on the orange from the coil. Still no luck.

rooster, I don’t fully understand how to test the gauge with a multimeter or 1.5 v battery?

here are the internal pictures as requested.
Cheers

 

So just to be sure when you checked the 12V did you use the black wire connected to the tach as a return? If not do so as it would seem unlikely that you have two bad meters.

To test the meter movement - this does not test the internal electronics in the tach:

The red and black wire on the circuit board connect to the meter movement. If you have and old style analog meter and put the red probe on the red wire and the black probe on the black wire the tach needle should move. You can do the same with the 1.5V battery but if the wires are left connected to the board then be careful with polarity as the battery will not be current limited like the analog meter.

The 1.5V from the battery or a typical analog meter will move the needle to about 7K, but each meter movement is slightly different and you may get a different number. What the test is really accomplishing is to verify the most delicate part of the tach is still functioning and the needle responds normally to voltage applied and voltage removed.