Author Topic: 1979 300 SD Irregular Tachometer  (Read 169 times)

Jjutzy

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1979 300 SD Irregular Tachometer
« on: 12 December 2019, 01:35 PM »
I just got the 300 SD up and running again and I've been having an awful time with some sort of short.

When I start the vehicle, my tach will either race up to about 4,000 RPM or remain dead. When I touch a window switch, turn signal, or just about anything else electronic in the vehicle, the tach will start bouncing around. Once the tach comes online, when I accelerate, the tach will drop in RPM's. if I use the turn signals, the vehicle loses power, in cadence with the tach and turn signals.

Has anyone ever experienced anything like this? Seems like I can't be the only one who's ever had this happen. It is also strange to me because it doesn't seem like electrical issues should cause the OM617 to surge. Under acceleration, I can feel the vehicle surge with the turn signal. When the light is on, loss of power. As soon as it cycles off, power returns. The car actually lurches a bit with the use of turn signals upon acceleration. Turn on the defroster blower, same result; loss of power until I turn it back off.

I'm at a loss here. Maybe you all have some ideas?

Thanks,
John
1979 300 SD 348,000+ Miles

Harv

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Re: 1979 300 SD Irregular Tachometer
« Reply #1 on: 12 December 2019, 05:50 PM »
Check the voltage regulator on the back of the alternator. That is a wear part that should be replaced when they wear down.

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1980 300SD
1983 240D
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Jjutzy

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Re: 1979 300 SD Irregular Tachometer
« Reply #2 on: 14 December 2019, 03:29 AM »
Thanks Harv,

I'll pop that out on Sunday and see what it looks like.

Has anyone ever mapped the ground points on the W116 300 SD? I think it's time to get busy looking for bad grounds as well. I have bright lights, they are not flickering (supposedly one of the signs you're regulator is on the fritz). One other thing is that my radio will not power up. It is an aftermarket Sony the PO installed. Worked when I bought it. Now, I can get the CD player to load/unload, but the stereo itself is not getting power. Checked the plug wires with my voltage meter and the only wire with any juice is the yellow constant wire. everything else is dead. With the lights dimming and loss of power with the turn signals, it seems to me that at the very least I need to do a once over on all the grounding points? Maybe new battery cables, check engine to chassis ground, and if I can get the gauge pod out, check the grounds behind the gauges?

Thanks,
John
1979 300 SD 348,000+ Miles

Jjutzy

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Re: 1979 300 SD Irregular Tachometer
« Reply #3 on: 14 December 2019, 02:42 PM »
Quick update:
I found the Negative battery cable was loose. I started it up to move it today, and a few seconds later, the engine died. Hopped in to start it again and got a click when I turned the key. Checked fuses, all looked good. Remember laced amplifier with my spare. Got my battery cleaner out and sprayed the posts. Went to wipe them clean and the ground cable swiveled with one finger. Tightened battery ground and had to leave for work. Was able to start the vehicle again, but have no movement n the tach, will swap out Amplifiers. To be continued, tomorrow...
1979 300 SD 348,000+ Miles

Squiggle Dog

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Re: 1979 300 SD Irregular Tachometer
« Reply #4 on: 14 December 2019, 05:18 PM »
My tachometer has the same problem. I have a fairly new battery and alternator. I think I cleaned the grounds okay. I even tried the trick where you take off the cap, pull out the piece inside. and put a couple pennies underneath it, but that didn't work, either.
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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Heated Seats, 350,000+