So confused. 87 300d ovp n8 no tach. - PeachParts Mercedes-Benz Forum

winmutt · #1 01-29-2013, 12:27 AM

Ovp is good. OVP gets power (verified by clicking when plugging in).

The good book says pin #9 on n8 should be a red wire and hot, coming from the OVp. #11 is supposed to be brown and go to W10 ground. n8 is the little klima relay next to the ovp right? I've pulled it all apart and #11 is not brown its white blue and does not go to ground.

I've also got a fuse blowing on turn signals of all things. They run in a common harness anywhere?

winmutt · #2 01-29-2013, 12:35 AM

Dur I see now that box next to the OVP is n6. Where is n8? Google dont fail me now!

Jeremy5848 · #3 01-29-2013, 02:00 AM

N6 is Klima. My schematics don't have an N8 for the '87 300D. For N6 Klima, pin 11 (white/blue) and pin 9 (red/blue) connect to the compressor speed sensor. The hot wire goes to pin 5 and the ground is pin 1.

Klima is powered from Fuse 7 not OVP and the turn signals from Fuse 6. They are unrelated.

Jeremy

winmutt · #4 01-29-2013, 08:07 AM

So my tach is non op and so is my ELR (although when I was testing the tach sensor when I plugged it in the engine would rev). The test procedures all point to verifying power on 9 and 11 of n8 which I dont have. I have verified the tach sensor is generating 4v and that the test terminal is generating 22v ac between 1 and 2 of the test jack. I just tore apart half the electronics last night thinking I didnt have power to n8 (which I now know doesnt even exist).

What to check next? Put it all back together and see if my spare klima relay kicks it up? Try my spare EDS?

I guess the turn signal fuse is just unrelated.

Any change that B11/1 being dead or short might be related? A while back my fan started running all the time so I unplugged it. I have a spare as well.

Jeremy5848 · #5 01-29-2013, 01:47 PM

Klima is a "consumer" of the tach signal; it doesn't modify it or send it anywhere else. If something in Klima is shorted and is "absorbing" the tach signal then the signal might be restored if you unplug Klima. Unplugging Klima stops the A/C compressor from working but won't affect the tach, so try unplugging Klima. If that doesn't help then Klima is not the problem.

If ELR doesn't work then the EDS controller N39 (behind the battery) might be bad although that is not a common problem. The RPM sensor on the ring gear feeds to EDS which generates the tach signal for Klima, the tachometer gauge, and maybe other "consumers" of the tach signal.

Does your A/C work? Klima REQUIRES a good tach signal or it will not engage the compressor. If A/C works then the tach signal is there somewhere. That might mean that the tach itself or its wiring is bad.

Note that the tach has its own 2-pin plug (signal, +12V) and ground through the case, same ground used by the clock. [Please confirm that in your car, I no longer have an '87.] The ground passes along a metal strip on the underside of the speedo, thence to the instrument group on the left side of the cluster, thence to the ground wire in the big 15-pin round plug. That strip often corrodes or a screw is missing. No ground, no tach. See the picture.

Jeremy

jay_bob · #6 01-29-2013, 07:34 PM

Yes the 87 tach relies on the ground strip as you described.

winmutt · #7 01-29-2013, 08:53 PM

Well after staying up till 2am chasing geists im mein ol machine I feel sickly. I will put her back together later this week and get the cluster out since both the turn signals and tach are in there. Luckily I have a spare EDS and klima handy (thank you pullapart sundays).

winmutt · #8 02-02-2013, 01:01 PM

So, ELR is working, verified by the first test of the system (that I subsequently skipped thinking I had other issues). Oddly I get 20 v ac at #1 and #2 of the test harness (manual says 4v). I pulled the cluster, no V AC into the tach gauge.

Jeremy5848 · #9 02-02-2013, 08:26 PM

Quote:

Originally Posted by winmutt

So, ELR is working, verified by the first test of the system (that I subsequently skipped thinking I had other issues). Oddly I get 20 v ac at #1 and #2 of the test harness (manual says 4v). I pulled the cluster, no V AC into the tach gauge.

Bench tests of a spare tach show that the absolute minimum AC voltage is 2.7 V RMS and 3.0 is probably the practical minimum so 4V is a good number per the manual. Your 20V reading sounds high but it's hard to tell without an osilloscope, there may be noise on the signal. I'll have to check my '95 E300, see what it reads. They should all be the same or at least similar, with the gasoline engine tachs having a higher range.

The tach is essentially an analog frequency meter. It responds to an AC signal (from a function generator) at a ratio of 20 RPM per Hertz, so 100 Hz gives 2000 RPM, 200 Hz gives 4000 RPM, and 300 Hz gives 6000 RPM. If you don't have a signal generator don't take chances and destroy the tach.

Jeremy

winmutt · #10 02-03-2013, 05:56 AM

Yes, I've been working on a project to replace the tach with a LCD. Maybe its time to make that happen.