I kinda think I need help. I have had an 82 300D turbo, now has 240,000 miles and generally runs good. My current problem is a crazy tachometer.

Recently, I replaced the OEM trash can tachometer amplifier with the one with the blinking lights. It worked very well for a while, and from reading other posts I don't think it is at fault.

My problem is the tachometer needle jumps in time to the turn signal indicator and pegs all the way when the horn is sounded. Then sometimes it will fall to the bottom (0 rpm) and stay there. At the same time, I will lose air conditioning (blower and compressor?) as if the number 8 (or possibly 6) fuse has blown. When I pull the fuse, it is fine and usually everything works after I put the same fuse back in the fuse box.

The other instruments in the cluster work fine. The hazard flasher does not cause the tach needle to jump. All turn signals, headlights, radio, etc seem to be doing what they are supposed to be doing.

I have seen lots of info on the horn issue but nothing on the turn signal, and some on the A/C issue but nothing I can point to and say "that is what mine is doing".

I have tried putting an old style tach amp in and still get the same needle jumps with the turn signal. I have tried replacing the socket the tach amp plugs into with no improvement. I have tried replacing the multi-switch with one from my parts box, no change. I have added another ground wire from the back of the instrument cluster to the frame; no change.

Since the car is an 83, I don't think I have an OVP fuse.

I have lived with it for a while, but the A/C issue is new and it still gets hot in west Texas in September.

Any helpful suggestions would be appreciated as I have no good ideas left about where to look to track this down.

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Very likely a ground issue. Add another ground wire to the cluster and I'll bet the condition resolves. Make sure it's a good ground - I did mine and spent another few hours troubleshooting because I didn't get a solid ground! This should be required PM for most any vintage MB.

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Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

i agree, sounds to me most likely to be a ground issue. check the dieselgiant (?) photos of adding a supplemental ground to the cluster.

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'77 240D, 504H, OM617.952, etc.

Thanks for your suggestions. I will try them this morning and let you know how it works.

I have already put a ground from the back of the instrument cluster to the same dash frame that has all of the other ground wires attached. I will try to add one from one of the screws that hold the tach onto the instrument cluster.

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Better, Faster, Cheaper, pick two.

Well, I tried it with a good ground to the instrument cluster or tach housing, no ground, or poor ground. All without any change in weirdness. I tried disconnecting left horn, right horn, both horns. Left and right produced no change, both disconnected and the horn didn't work (you think?) or have any effect on the tach.

I also put in an OE tach amplifier, and the tach needle would never settle down. It just jumped around all over the gauge. One thing I did notice when I put the new amplifier from the tech guy (I can't remember his name) was that the needle used to make a smooth swing when the key was turned to heat the glow plugs, and now it swings to 2500 rpm, stalls for a second, then swings fast to peg and then back with a slight pause around 2500 on the way back. I don't remember the tach needle doing this before and makes me wonder about the new amplifier.

Re-checking my fuse box cheat-sheet, I find the tach and turn signal lamps are on circuit 12 while the horn is on circuit 14. The air conditioning compressor and blower motor are on circuit 8. With three circuits sort of involved, I agree that the problem probably a grounding issue. But other that the steering column, I cannot figure where they all come together. Grounding the instrument cluster doesn't seem to have any effect.

I'm back to asking for suggestions!

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Better, Faster, Cheaper, pick two.

There is a ground cable on the drivers side under the vehicle. I bolts to one of the engine to Transmission bolts and then to the body. this could have some corrosion, loose or too much oil leaked on it. remove it and clean the area.

Look down below the Oil Filter Housing and you will see it.

This gave me some problems with the gauges one time.

Charlie

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there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

Thanks Charmalu, I will look for & try it. I have a dental appointment in a little while so it may be until tomorrow before I can send any results. It sounds like a real possibility since I do need to replace the oil filter housing gasket (again!) because it leaks.

I will let you know if (1) I can find it and (2) it works to solve my problem.

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Better, Faster, Cheaper, pick two.

Well, I climbed under the car this morning, found the cable going from the frame to the transmission, removed the bolt and cleaned it to shinny new (also the cable end) and put it back in. Did mostly the same to the transmission end but didn't remove the entire bolt.

Result when I started the car? No change in tachometer operation. (insert sad face here.)

Oh well, If you or anyone else has a suggestion, I would be glad to hear it as I have no idea where to go next.

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Better, Faster, Cheaper, pick two.

Tach amp with a blinking light? More info please.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Funola, my tachometer needle jumps erratically when the turn signal is on and will spike all the way to the right side when the horn is honked. I have tried replacing the multi-switch, the tach amp, the tach amp socket, added ground wire to back of instrument cluster, checked all light bulbs (OK), fuses, and anything else I could think of. Now I am stumped!!

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Better, Faster, Cheaper, pick two.

Funola, I didn't read your entire response/question. The tack amp with the blinking lights is the one sold by techguy512 as a replacement for the one provided by Mercedes. Here is a post about it.

http://www.peachparts.com/shopforum/diesel-discussion/219949-om617a-tach-amp.html

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Better, Faster, Cheaper, pick two.

Have you taken apart and cleaned ground point G102 that is at RH F fender behind headlight?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Not yet. I will go out to the car and see if I can find what you are talking about.

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Better, Faster, Cheaper, pick two.

Wire brush everything, reassemble with grease. See if it works.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I found the ground point (it has four brown wires connected to a small bolt to the firewall). I don't think I can get the bolt out without removing the turn signal lens but it does turn and I was able to move the wire connecter. I think it is grounding properly but will try again to remove the bolt and make it a shinny metal connection.

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Better, Faster, Cheaper, pick two.