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byronnash 02-23-2010 11:38 PM

Sending Unit for tachometer
1 Attachment(s)
I read some old posts about how to troubleshoot a dead tachometer. I found the cylindrical connector with the screw top. I put a tiny piece of foam under the cap to increase pressure. It didn't seem to make a difference. How you do get that thing out to check the bottom of it? See photo.

Also, which part is the magnet on? Someone said the flywheel, where is that on the engine? Forgive me for that newbie question, I'm just getting started.


4x4_Welder 02-24-2010 12:16 AM

Yours should be on the front of the engine. Look in between the power steering pump and the a/c compressor, on the front balancer.

charmalu 02-24-2010 04:24 AM

You don`t have the year of your car listed.

84 and 85 have a sensor on the adaptor plate between the engine and transmission. mounted on the drivers side, picks up a signal off the flywheel.

83 and earlier get the signal off the front crank pully. follow the cable from the bottom of the plug you show in your picture, and it will lead you to the sensor at the pully.


MS Fowler 02-24-2010 07:35 AM

I have a dead tach on my '82 300SD. The pickup in on the front pulley below the IP and A/C.
Is there a way to verify the condition of the piuckup, and that it is providing a signal? I "assunme" some sort of resistance ot voltage check would be in order, but do not know what to look for.

byronnash 02-24-2010 08:19 AM

Sorry, it's a 1984 300TD. I'll try following that wire down. Should I bother trying to pull that sending unit thing out of the housing? Am I looking at the right object in that I took a photo of?

blackestate 02-24-2010 08:37 AM

When I was doing an engine change, mine was fixed by cleaning and reseating the plug on the bottom of that amp. has worked since that day.

techguy512 02-24-2010 09:11 AM


Originally Posted by MS Fowler (Post 2411991)
Is there a way to verify the condition of the piuckup, and that it is providing a signal? I "assunme" some sort of resistance ot voltage check would be in order, but do not know what to look for.

Here's a link to some diagnostic information, including resistance measurements....

marfa300td 02-24-2010 10:06 AM

My tach amp was broken. I contacted techguy512. He loaned me a device to test the tach system. Then I bought a new tach amp from him that he designed. It works like a charm. My favorite part is that the tach needle slowly sweeps up to 5000 and then back down when the key is turned on. He's a good guy to deal with.

byronnash 02-24-2010 10:20 AM

After looking over that PDF, I do not have a tach amp. My cap is the smaller one with no contacts inside it. I'll trace the wire down to the magnet and check that and see if I see anything strange. If I don't have a tach amp, what can be replaced to get the tach working?

toomany MBZ 02-24-2010 01:02 PM

^It gets the signal from the transmission. Check the fuse.

charmalu 02-24-2010 01:43 PM

That plug you have is only for diagostic testing at the dealer. you have no tach amp that plugs into it. just a cap to keep dirt out of the connectors.

Behind the passenger floor kick panel on the wall, is a black box computer thing. large cable connector with a bunch of wires going into it. EGR, rack position sensor, tack pick up etc... send signals to it. not sure what else it does.
The tack pick up is off the flywheel, on the drivers side. look down behind the oilfilter housing, there should be a plug with a small bolt holding it into the engine adaptor plate. has a large black cable attached to it. could be a broken connection there at the plug. above the valve cover on the firewall is a large round connector it plugs into. from there it runs through the firewall to the black computer box.

there is also the OVP (over voltage protection) relay, it is a silver box with a red spade fuse in the top. it could be bad. I think it is under the dash behind the glove box. 40300DT++%2F+L5%2D2998cc+3%2E0L+DIESEL+FI++Turbo%40%40300DT&catid=Electrical%40%40Electrical&subcati d=Overload+Protection+Relay@@Overload+Protection+Relay&mode=PA 10000135@@300D&catid=241959@@Body+Electrical&subcatid=242058@@RPM+Sensor&applicationid=W0133-1615020&mode=PA


byronnash 02-24-2010 01:45 PM

Thanks, I'll check those out.

charmalu 02-24-2010 01:57 PM

The OVP is actually cheaper on Fast Lane. Though we don`t know that is your problem, I was checking prices, $20 difference :eek:.

I listed the tack pickup also. so you can see what it is.


yuke 02-25-2010 01:27 PM

sending unit for tach
My problem with my 85 SD was the OVP relay even though the fuse on top of the relay was good the relay was bad replaced relay problem solved.Took me 9 months to figure out why the tach did'nt work.

byronnash 03-06-2010 10:23 PM

1 Attachment(s)
I poked around under the car today trying to spot the pickup and magnets. I took a photo of what I think the flywheel is. I can't see any magnets on it, but I see some round holes where a magnet might go? I'm going to look at the fuses tomorrow and see if there is anything funky there. How can I test the OVP relay?

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