Tach questions - PeachParts Mercedes-Benz Forum

rcounts · #1 08-03-2008, 12:39 AM

I seem to be having the worst luck imaginable with tach sensors. I tested mine according to the procedure in the manual. It tests fine for resistance - 1.8k Ohms +/- .2k Ohms, but the voltage measurement is basically zero at idle - whereas the manual says it should be putting out about 4 volts when idling at 750 RPMs.

So I bought a used one from another member and it tests exactly the same. Reistance is fine, voltage is only a couple of millivolts.

So I bought yet a THIRD used one from another member and damned if it doesn't test out the same too.

Any body got any theories on what's going on here? It just doesn't seem likely that all THREE of them are bad - unless they are just a really weak and failure prone part - and my luck really sucks!

The only other theory I can come up with is that the newer flywheel for use with the bellhousing mounted sensor has some kind of special configuration (imbedded magnets or something) and someone replaced mine with an earlier style flywheel at some point in the past...

One other question. On the 84 & 85 with the bellhousing mounted tach sensor, does the cruise control need a tach signal? My cruise control seemed to go whonky about the same time as my tach. When you engage the cruise it just floors the pedal and accelerates non-stop. That kind of makes me wonder if the lack of voltage signal from the tach is the problem...

my123ca · #2 08-03-2008, 06:57 AM

Did you check you OVP relay? There is a fuse on top of the relay. My tach wont work if the fuse is not there. I believe the elec. connector can be put backwards too.

bgkast · #3 08-03-2008, 01:00 PM

Quote:

Originally Posted by rcounts

I
The only other theory I can come up with is that the newer flywheel for use with the bellhousing mounted sensor has some kind of special configuration (imbedded magnets or something) and someone replaced mine with an earlier style flywheel at some point in the past...

That very well could be.

I don't think the cruise is tied to the tach system.

I second checking out the OVP. My '85 had a busted tach too. The OVP fuse looked fine, so I replaced the whole relay and it fixed the tach.

Jeremy5848 · #4 08-03-2008, 01:28 PM

The OVP being bad doesn't explain why the tach sensor tests bad. I suggest the following possibilities: (a) wrong setting on multimeter -- should be measuring AC volts, not DC, (b) bad voltmeter circuit in multimeter, (c) mounting bracket for tach sensor bent, holds sensor too far from flywheel.

You should be able to wave a screwdriver or other metal object close to the sensor and see a reaction on a voltmeter.

The cruise control gets its speed information from a sensor mounted on the back of the speedometer. It's part of the instrument cluster, has its own connector. Your cruise problem could be either an electronic failure in the brain box (under the dash, driver's side) or something stuck in the actuator (under the hood, near the power steering pump/reservoir).

Jeremy

rcounts · #5 08-03-2008, 03:41 PM

Quote:

Originally Posted by Jeremy5848

The OVP being bad doesn't explain why the tach sensor tests bad.

Exactly....

Quote:

Originally Posted by Jeremy5848

I suggest the following possibilities: (a) wrong setting on multimeter -- should be measuring AC volts, not DC, (b) bad voltmeter circuit in multimeter, (c) mounting bracket for tach sensor bent, holds sensor too far from flywheel.

(a) I thought about the volt meter thing and plan on checking it with another meter today. The one I'm using is auto ranging AND automatically switches between AC & DC depending on what it is measuring. I actually have two identical ones with those features. (b) using the other volt meter should eliminate that as a possiblility (c) there is no bracket - the sensor slips through a hole in the bellhousing and a bolt through a tab on the side of the sensor threads into the bellhousing to secure it in place. I don't see where there is any way to adjust it's distance from the flywheel.

Quote:

Originally Posted by Jeremy5848

You should be able to wave a screwdriver or other metal object close to the sensor and see a reaction on a voltmeter.

Another good test to try...

Quote:

Originally Posted by Jeremy5848

The cruise control gets its speed information from a sensor mounted on the back of the speedometer. It's part of the instrument cluster, has its own connector. Your cruise problem could be either an electronic failure in the brain box (under the dash, driver's side) or something stuck in the actuator (under the hood, near the power steering pump/reservoir).

Wish there were some way to test that while I have it out (just did the odo repair). I presume it is the little black box screwed onto the back of the speedo?

I can also check the actuator. Though I kind of doubt that one because of the way it acts. When you hit the cruise switch it will start accelerating and continue to do so until you either flick the switch off or tap the brake. Seems like it is lacking a feedback reference signal to tell it when it is at the set speed.

rcounts · #6 08-03-2008, 05:22 PM

OK, I tried the other meter and VIOLA' - AC voltage at the connector. So, now I know I have a good sensor. Good suggestion Jeremy, I would never have suspected the other one of being bad. Sometimes its the obvious things...

Next question; What is the OVP and where is it located? I'm in the process of trying to put the cluster back in and put everything back together after having installed my new dash overlay, so a quick reply would be MOST appreciated....

JimmyL · #7 08-03-2008, 06:32 PM

Quote:

Originally Posted by rcounts

Next question; What is the OVP and where is it located? I'm in the process of trying to put the cluster back in and put everything back together after having installed my new dash overlay, so a quick reply would be MOST appreciated....

This isn't a real quick reply, but it is under dash, just to the right of the glove box area, kind of behind the passenger AC vent.......

rcounts · #8 08-03-2008, 06:57 PM

Quote:

Originally Posted by JimmyL

This isn't a real quick reply, but it is under dash, just to the right of the glove box area, kind of behind the passenger AC vent.......

"Real quick" is relative - the important thing is your reply was quick enough

I read it before I put the passenger side under-dash panel back in. Using your description and some pictures I found when I searched for threads containing "OVP", I was able to find it easily, and sure enough, the 10 amp fuse on top was blown. Replaced it and VIOLA' I have a tach again!

You and I have had some pretty ...er....animatated disagreements a couple of times, but I have to hand it to ya' - you came through in spades on this one for me buddy. THANKS!