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240 Lazy tachometer.

G

Grommit55

New member
Joined
Mar 20, 2018
I just bought an '84 240 Turbo (321,000 Kms) which has been fully restored. It runs great but I notice that the tachometer drops to the bottom of the gauge at idle but still increases with revs. I figure it is about -1000 rpm off accurate. Any ideas to fix?

Cheers.
 
Probably the simplest solution is to replace it.

But . . . the tach operates off of a ground pulse from the spark coil. Make sure that you have good continuity.

If you perceive that it is off 1000 rpm at all times it could be that the needle just needs to moved a bit on its shaft like adjusting the hands on a clock.
 
Mine did this, it ended up being the cable going from the speedo sensor up into the cabin had a break in one of the two wires that was causing it to short out and act all wobbly.
 
^Tachometer, not speedo.

Anyway, if the needle is sitting at the bottom with the key off, you can take it out and readjust the position of the needle. You will likely find that it maxes out at 6500 when you rotate the needle. Just spin it on the shaft until it points to ~7500, then let it drop. Should be sitting at zero or just slightly above zero when you're done. I've fixed a few that have had the needle sitting way too low when the car's off.

-J
 
D'oh, my bad. It was late last night lol.

If you take your foot off the gas, does it just collapse down to 0, even at highway speed? Or does it hang steady, just at an RPM too low?

I've once or twice had the issue where the tachometer would just collapse quickly to 0, and spike up when I gave it gas, but it's working now.
 
Yes. The tach needle stays steady at higher rpm/speed but drops to the bottom while the car is at low idle or off. It is just like it is not positioned properly.
 
John242Ti said:
^Tachometer, not speedo.

Anyway, if the needle is sitting at the bottom with the key off, you can take it out and readjust the position of the needle. You will likely find that it maxes out at 6500 when you rotate the needle. Just spin it on the shaft until it points to ~7500, then let it drop. Should be sitting at zero or just slightly above zero when you're done. I've fixed a few that have had the needle sitting way too low when the car's off.

-J
Click to expand...

^ This
 
Ok, so, now that we know what's going on, here's how to gain access to it...

Remove clock bezel and bezel below clock
Remove rheostat knob and headlamp switch knob (you might have to use a slotted screwdriver inserted between the bezel and the knobs to pop them loose)
Remove headlamp switch bezel
Remove the four screws holding the cluster in place
Remove the two screws holding the top of the steering column trim in place and lift trim out - set on top of dash and leave screws in their holes, so you'll find them
Next, remove the three plastic clips that hold the front edge of the dash insulation piece in place and, also, remove the rubber donut around the floor vent
Drop insulation panel
Crawl under dash and reach up behind the speedometer, turn collar on speedometer cable counter-clockwise until it disengages the speedometer
Crawl out from under the dash, then pull cluster toward you and up onto the steering wheel trim piece
Remove the tach wire, clock wiring harness, the round plug, the half-moon plug, and, if your car has either cruise and/or an autotragic, remove the wires for the speed sensor and/or the overdrive wire from their terminals - If you do have working cruise, make note of which terminal the blue and gray wires attach to, so you don't get them backward and fry the speed sensor when you reattach it
Now, remove the cluster
Place it on a flat surface and remove the four screws that hold the tachometer to the circuit board
Use a slotted screwdriver to pop the tachometer out

Next, do what I mentioned in my previous post.

Reassemble cluster, and reinstall it. Fire up the car and take it for a test drive to see if everything works normally.

-J
 
That sounds like fun but a good plan. Hopefully we'll have some warm weather next week so I can tackle the job. It's a new car so I'm still sorting out the quirks. Like if the ignition key sits too far clockwise when running the stereo and w/w wipers don't work. Thought I had an electrical problem until I discovered moving the key back a degree. All well now.

Cheers.
 
John242Ti said:
Ok, so, now that we know what's going on, here's how to gain access to it...

Remove clock bezel and bezel below clock
Remove rheostat knob and headlamp switch knob (you might have to use a slotted screwdriver inserted between the bezel and the knobs to pop them loose)
Remove headlamp switch bezel
Remove the four screws holding the cluster in place
Remove the two screws holding the top of the steering column trim in place and lift trim out - set on top of dash and leave screws in their holes, so you'll find them
Next, remove the three plastic clips that hold the front edge of the dash insulation piece in place and, also, remove the rubber donut around the floor vent
Drop insulation panel
Crawl under dash and reach up behind the speedometer, turn collar on speedometer cable counter-clockwise until it disengages the speedometer
Crawl out from under the dash, then pull cluster toward you and up onto the steering wheel trim piece
Remove the tach wire, clock wiring harness, the round plug, the half-moon plug, and, if your car has either cruise and/or an autotragic, remove the wires for the speed sensor and/or the overdrive wire from their terminals - If you do have working cruise, make note of which terminal the blue and gray wires attach to, so you don't get them backward and fry the speed sensor when you reattach it
Now, remove the cluster
Place it on a flat surface and remove the four screws that hold the tachometer to the circuit board
Use a slotted screwdriver to pop the tachometer out

Next, do what I mentioned in my previous post.

Reassemble cluster, and reinstall it. Fire up the car and take it for a test drive to see if everything works normally.

-J
Click to expand...

My speedometer cable has a plastic shield around it that prevents me from moving the cable. Whats the best way to break it?
 
I have not experienced the anti tamper seal but there is a fragile plastic 90 degree twist lock connector holding the cable onto the back of the tach. Break that and you will have to create a way to hold the cable onto the tach.

P1231415.jpg
 
If the collar on the cable breaks, the cable will usually stay in place, since it's a press-fit into the back of the speedometer.

-J
 
John242Ti said:
Insert it from behind, then twist the screwdriver slightly, and it'll snap into two or more pieces.

-J
Click to expand...

I cant get it to come off. I tried using plyers as well. No luck.

Can I disconnect it from the transmission and with the extra slack just pull out the cluster so it's easier to work on?
 
I know it is a 90 degree twist release. You might have to push in a bit to get it to turn.

Like you suggested, pulling a bit of slack by removing it from the transmission will make it a lot easier, coming out and going back in.
 
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