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#1
08-02-2005, 07:43 AM
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Need tachometer help on 85 300 D PLEASE!
I posted this in "Diesel discussion" with no avail
Ok got an 85 300 D with a non functioning tach. Here is what I have done so far: 1) I have tried another gauge with the same results. 2) Checked the EGR relay fuse and its good 3) Replaced the tach pick up in the bellhousing with a new one 4) The 85 Models DON'T have the tach amp on the fender, so that does not apply 5) Looked for anything obviously wrong with wiring, nothing. 6) All other gauges work just fine On this year model if the tach does not work, then the klima relay won't engage the A/C compressor, which of course it does not. What am I missing??
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1981 300D 147k 1998 VW Jetta Tdi 320k 2001 Dodge Ram 2500 141k 1979 300D 234k (sold) 1984 300D "Astor" 262k(sold) Mercedes How-To and Repair Pictorials I love the smell of diesel smoke in my hair 
Last edited by Diesel Giant; 08-04-2005 at 08:35 PM. Reason: additional info 
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#2
08-02-2005, 09:26 PM
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Anyone?
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1981 300D 147k 1998 VW Jetta Tdi 320k 2001 Dodge Ram 2500 141k 1979 300D 234k (sold) 1984 300D "Astor" 262k(sold) Mercedes How-To and Repair Pictorials I love the smell of diesel smoke in my hair
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#3
08-03-2005, 08:30 AM
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The computer has to have voltage & ground to transfer the a-c voltage signal from the flywheel pick-up into a dcV signal. THE power comes from the overvoltage relay that is next to the right A/C vent.
ON the ECU check pins 1 (power) & 14 (ground). ALSO the crank sensor should make 4-6V a-c at idle, measured at pins 16 & 20.
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MERCEDES Benz Master Guild Technician (6 TIMES) ASE Master Technician Mercedes Benz Star Technician (2 times) 44 years foreign automotive repair 27 Years M.B. Shop foreman (dealer) MB technical information Specialist (15 years) 190E 2.3 16V ITS SCCA race car (sold) 1986 190E 2.3 16V 2.5 (sold) Retired Moderator
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#4
08-03-2005, 11:17 AM
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MB DOC: I really appreciate your response
 I checked all the pins they are not numbered at the connector or the computer. No voltage at any with car running. Relay fuse is not blown. Since I am not sure of the pin numbering I cant check the pickup either. Do you know which way the pins are numbered? Since no voltage at ANY I am assuming the relay is at least bad, would you agree? UPDATE: I just had someone test the relay and it is no good. Took it apart found the coil not closing the switch. I manually closed the switch with a piece of non conducting plastic to see what happened if it was working. The EGR now gets a signal and works, there is voltage at what I think is pin 1 on the computer, but still no tach.  I can't really test the other pins because they are not numbered.This is getting really frustrating. I am goining to get a new relay tommorow. Could the Klima relay being defective cause the tach not to work??
__________________
1981 300D 147k 1998 VW Jetta Tdi 320k 2001 Dodge Ram 2500 141k 1979 300D 234k (sold) 1984 300D "Astor" 262k(sold) Mercedes How-To and Repair Pictorials I love the smell of diesel smoke in my hair
Last edited by Diesel Giant; 08-03-2005 at 06:29 PM. Reason: add pics
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#5
08-30-2005, 08:44 PM
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Replace the egr relay and the evaporator temp sensor and all is well.
__________________
1981 300D 147k 1998 VW Jetta Tdi 320k 2001 Dodge Ram 2500 141k 1979 300D 234k (sold) 1984 300D "Astor" 262k(sold) Mercedes How-To and Repair Pictorials I love the smell of diesel smoke in my hair
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