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  #16  
Old 06-23-2012, 10:18 PM
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Quote:
Originally Posted by Scottr View Post
Mach4Is the OVP located in the fuse box under the hood? I have one relay that has a red spade type fuse built onto the top. It's good. I also found a set of fuses on the fender wall next to the blue vac amp.
The OVP is located in the same general area as the EGR computer. It's kind of tucked up under the dash against the side of the pax compartment.

Here's what you're looking for.



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Tach not working-ovp.jpg  
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  #17  
Old 06-24-2012, 12:45 AM
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You can get to the OVP by removing the glove and it's on the pass side wall. My 10 amp fuse on my 85 coupe was intact but I changed it anyway and the tach worked for a bit and then it didn't. Now it works sometime but not all the time. I also wired an additional ground wire for the instrument cluster. Not sure where the bad connection is but I would love to get it to work all the time. Does the OVP ever go bad, how do you test these relays?
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85 300CD Signal Red/Tan sold
83 300D Manganese Brown 109K
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95 E320 Green Wagon 125K, sold
94 E320 White 127K, sold
85 300SD 156K Grey (Annie), sold
84 300D Lapis Blue 170K (Judy), sold
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  #18  
Old 06-24-2012, 12:46 AM
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Thanks, that's what I was hoping it was, fuse checks out good, no power on pin one with the key on in first position. Will continue looking
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  #19  
Old 06-24-2012, 02:24 AM
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Originally Posted by Scottr View Post
Thanks, that's what I was hoping it was, fuse checks out good, no power on pin one with the key on in first position. Will continue looking
The tach definitely won't run w/o power. You could make a jumper and bring power directly to pin 1 and see what you get. Don't do this for long, as running without the OVP is putting your EGR controller at risk. Better yet, get a small battery and run power to the EGR computer from that. If it works, then just track down your power problem, if not, it's most likely the computer itself, but finish the other troubleshooting first to make sure.
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  #20  
Old 06-24-2012, 10:45 AM
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From member rcounts: "On this style the tach gets its signal from a different sensor mounted at the BACK of the engine in the bellhousing of the transmission - just behind and below the bottom of the oil filter assembly. The sensor is held in place by an allen-head screw and it takes a 5mm allen key to remove it.

My sensor is bad and needs to be replaced. You can test it by unplugging the 2-wire cable connector at the center of the hood opening just in front of the cowl. If you follow the wire from the sensor up to the connector it is easy to find.

With the car off, connect an Ohm meter to the two make pins of the cable connector. It should measure 1.9k Ohms +/- .2k ohms (that is 1900 Ohms +/- 200 ohms). If that measures OK, then start the car and use a volt meter to measure the voltage across those same tow pins. At normal idle speed of 750-800 RPMs it shoud measure ~4V +/- .1V.

If EITHER of these measurements is outside these value ranges, it needs to be replaced. "
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  #21  
Old 06-24-2012, 10:47 AM
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My sensor is good, does anyone have any idea why my '85 tach only works sometimes?
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85 300CD Signal Red/Tan sold
83 300D Manganese Brown 109K
97 E420 Midnight Blue 197K sold
98 BMW 328i Vert White 100K, sold
95 BMW 525i White 125K, sold
93 BMW 525iT Red 193K, sold
95 E320 Green Wagon 125K, sold
94 E320 White 127K, sold
85 300SD 156K Grey (Annie), sold
84 300D Lapis Blue 170K (Judy), sold
99 ML 320 Black (lease), 1998 C230 White (lease)
00 Honda S2000 Red (lease)
86 Mercedes 300E (sold)
84 Porsche 911 Red (sold), 1965 Porsche 911 White (sold)
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  #22  
Old 06-24-2012, 12:02 PM
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I need to back up and make sure I'm on track here. I found what I think is my OvP relay mounted inside the fuse box under the hood on the drivers side fender. It looks exactly like the picture posted ( silver relay with red fuse holder on top) I have pulled the glove box, lower panel, ect off the passengers side (inside ) and under the kick pannle is the EGR computer, however I cannot find another relay anywhere in that area.
Is it possible the OVP relay was moved at some point in the manufacturing process? I want to make sure I have eliminated this as a problem before I continue ( making a jumper, etc. )
On a side note, removing all this has given me acess to a lot of vac lines to check
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  #23  
Old 06-24-2012, 12:05 PM
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Edge
Thanks for the post, these values are great to have for checking the pickup sensor
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  #24  
Old 06-24-2012, 12:34 PM
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Not sure about the '85 300SD but the '85 300D and 300CD has the OVP on the pass side of glove.
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85 300CD Signal Red/Tan sold
83 300D Manganese Brown 109K
97 E420 Midnight Blue 197K sold
98 BMW 328i Vert White 100K, sold
95 BMW 525i White 125K, sold
93 BMW 525iT Red 193K, sold
95 E320 Green Wagon 125K, sold
94 E320 White 127K, sold
85 300SD 156K Grey (Annie), sold
84 300D Lapis Blue 170K (Judy), sold
99 ML 320 Black (lease), 1998 C230 White (lease)
00 Honda S2000 Red (lease)
86 Mercedes 300E (sold)
84 Porsche 911 Red (sold), 1965 Porsche 911 White (sold)
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  #25  
Old 06-24-2012, 01:03 PM
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FWIW, this is what happened to me last year - my '85 tach was working only sometimes. My OVP was new and I replaced the sensor on the flywheel.

It so happened that I took the car in for A/C service - wasn't working at all. Mechanic found something amiss under the hood, said it was part of the electronics - and the A/C was working again. Lo and behold so was the tach. If I had known that was going to happen, I would have asked more questions on what he did. But if your A/C isn't working, it could be related.?.
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  #26  
Old 06-24-2012, 01:22 PM
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Well that's interesting. I assume this car is a 85. The data plate on the door shows 1984. My observations have always been the listed sale year is one year after manufacturing date. ??
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  #27  
Old 06-24-2012, 01:57 PM
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Quote:
Originally Posted by VW1300 View Post
FWIW, this is what happened to me last year - my '85 tach was working only sometimes. My OVP was new and I replaced the sensor on the flywheel.

It so happened that I took the car in for A/C service - wasn't working at all. Mechanic found something amiss under the hood, said it was part of the electronics - and the A/C was working again. Lo and behold so was the tach. If I had known that was going to happen, I would have asked more questions on what he did. But if your A/C isn't working, it could be related.?.
Does anyone know if the '85 tach has anything to do with the KLIMA relay because I was wiggling that also...
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  #28  
Old 06-24-2012, 08:12 PM
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Originally Posted by edge View Post
Does anyone know if the '85 tach has anything to do with the KLIMA relay because I was wiggling that also...
No, two separate systems - see Tach not working
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  #29  
Old 06-25-2012, 12:30 PM
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tach not working

Scott just because the fuse on top of the ovp relay is good does not mean the relay is good that was the case on my 85 300 SD the fuse on top of the relay located in fuse box under hood on drivers side was the first thing me and the M.B. guru looked at since the fuse was good went on to look at other things for approx. 9 months after that and some reading I replaced the relay which was very cheap $15 or something the tach took off. Currently I do have a problem with it fluttering and it is strange that if I so much as just push on the plastic covering the instrument cluster it works fine for awhile then starts fluttering again telling me I have a lose connection or poor ground somewhere back there which I really have not had time to look closely at yet. Hope this helps you.
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  #30  
Old 06-25-2012, 01:17 PM
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Thanks Yuke, this helps. I now know I have the correct relay. Your the first one to varify that my OVP relay was under the hood. I have voltage to the relay and I can feel it pull in the contacts when I push it into the socket. Sounds like its worth replacing just to make sure. I also checked the ohms on the pickup at the tranny per the post above. It shows 1.86k ohms. So I beleve that's correct also. The hunt continues.
Anyone k ow if these relays can be purchased at a local parts store? My nearest MB dealer is 365 miles away

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