Tach signal wire/fuse - PeachParts Mercedes-Benz Forum

koya1893 · #1 06-29-2010, 09:42 PM

My apologies for these question. As I mentioned I am new to owning a MB, its a 1985 300SD which I am using to experiment with Hydrogen. The tach in the car does not work. Can someone direct me where to start troubleshoot to fix it. thank you.

Aquaticedge · #2 06-29-2010, 09:48 PM

the tach amp is a place to start, iirc it's on the fender and looks like a rounded off vertical tube. black about 2-4 inches tall they do go funny. also if your SRS light is on and the tach is dead, check the OVP relay near the battery

Jeremy5848 · #3 06-29-2010, 09:50 PM

The 1985 300SD has an early "EGR computer" through which the tach signal passes. The EGR brain is usually very reliable but it gets its power from an "over-voltage protection relay" which has a built-in 10 Amp fuse. Either the fuse or the entire relay may have failed. I believe the OVP relay in your car is in the relay box with the fuses, under the hood. [Source: Mercedes- Benz 1985 "Introduction into service" manual.]

This car does not have a "tach amp" on the inner fender.

There is also an RPM sensor and various wires and connectors. I would check the OVP and its fuse first.

Jeremy

Aquaticedge · #4 06-29-2010, 10:07 PM

oops, my bad on the amp suggestion!

koya1893 · #5 06-30-2010, 06:24 AM

Thank you for the input. On another note, the trip odometer does not work. My old "Bimmers" the gears inside where dried up and can be replaced. Is it posible to do on this car? How does one remove the gauge cluster I don't see any hidden screws that holds it inplace. Thank you.

toomany MBZ · #6 06-30-2010, 07:45 AM

The cluster is a friction fit affair, remove the lower kick panel via a few metal screws visible in front and a plastic 90 degree screw high on the transmission hump. Once removed, simply push out from the behind. All connections are electric, not sure how to remove the speedo cable tho, here are a couple of pics of the back side.

koya1893 · #7 06-30-2010, 09:59 AM

YOU GUYS ROCK. While I was finishing up the hydrogen system, then let it run for a few minutes I check the fuse box and notice a "relay" looking box with a fuse on top. The fuse was blown, replace it not the tach works. Also, the antilock brake was on, did all four brake yesterday now the light off. Thank you. I thought I enjoyed working on keeping this vintage car alive. This one is going to me an hybrid.

TX76513 · #8 06-30-2010, 10:04 AM

Quote:

Originally Posted by koya1893

YOU GUYS ROCK. While I was finishing up the hydrogen system, then let it run for a few minutes I check the fuse box and notice a "relay" looking box with a fuse on top. The fuse was blown, replace it not the tach works. Also, the antilock brake was on, did all four brake yesterday now the light off. Thank you. I thought I enjoyed working on keeping this vintage car alive. This one is going to me an hybrid.

Show us some pictures of what you are doing -that would be very interesting.

koya1893 · #9 06-30-2010, 10:16 PM

Those of you that are familiar with the Electrolysis process that produce HHO, the system I've installed consist of 7 plate, 6 cell system that produce 1.5 lpm. the ideal induction is .5 lpm per 1 liter of engine displacement. I've installed the same system on several domestic truck with diesel ranging from 5.9-7.3 turbo. Thus far, the result: diesel engine lover HHO. I've never driver a Benz, (diesel) before so I have littler to compare aside the short time I had this car. But from what I feel with the HHO onboard this car's throttle response is similar to a gas engine (instant throttle response).

I've enclosed a photo of the car. I am having a hard time finding the owners manual and maintenance manual for this car, hence that's why I've been asking a lot of basic questions. This one for example, I drained and replace the trans filter. According to the local parts store the trans holds 6.6 quarts of ATF. I filled it with 6 quart and check the level after several minutes of idling. The dip stick registered overfilled.

My question is: is the tranny fluid checked while the engine is running in park like domestic cars or car is turn off in park.

Also has anyone used the SeaFoam trans additives to condition the trans? I've used it on several tired Ford and it work wonders (smooth shifting).

Jeremy5848 · #10 06-30-2010, 11:39 PM

You have to drain the torque converter as well as the transmission. You will get a total of about 6-1/2 quarts out. Put the same amount in after changing the filter.

You have to rotate the engine by hand (27mm socket on the bottom pulley) while watching the torque converter through the hole in the bottom cover; eventually a little drain plug (5 mm Allen) will come around. Remove that and you will get another quart or two out.

koya1893 · #11 07-01-2010, 02:23 PM

Jeremy,

Thank you for the detailed help I started a new post for this trans fluid change that is kicking my "B****". Boy I sure could use a service manual for this car.

toomany MBZ · #12 07-01-2010, 02:51 PM

Manuals are available here: http://www.mercedesmanuals.com/Mercedes_Benz_W_126_Manual_p/w126.htm
Don't feel bad, a transmission shop did the same thing, the mechanic, I've never seen a transmission that only holds a couple of quarts. This is after I told them they need to drain the torque convertor, they said "we'll get it, we'll get it".