Tach not working after engine swap - PeachParts Mercedes-Benz Forum

vonsmog · #1 06-30-2017, 04:37 PM

OK the tach on the 84' 280CE is not working after replacing the M110 with OM617a diesel. There was not a tach amp in the car and and there is no EGR box or OV relay on the pass side footwell. So did the car get its signal from the ignition? or some place else. There is a pickup on the front of the motor going to where the tach amp should go. And I have the same pickup on the diesel motor. I installed a amp, but no luck. What should I look for, and what should I test? Everything else is working, even the AC.

BillGrissom · #2 06-30-2017, 04:50 PM

Most gasoline engines get a tach signal from the spark ignition (at least thru 1995 in small block Chevy), but I don't know M-B gas. A 1984-85 OM617 diesel gets the tach signal from a pickup on the transmission bell-housing (driver's side). If a 1984 gas is the same and you kept the transmission, it "should work". These OM617 still had the front pickup on the crank damper but the cable dead-ends at the "diagnostic port" w/ no "tach amp" needed like earlier cars. Perhaps dealer mechanics hooked that to an engine monitor. It isn't needed for the car to work fine.

mach4 · #3 06-30-2017, 06:10 PM

Your gas tach signal comes from the ignition, which means your tach gauge in the dash will not work.

What I did on my swap (617->107) is swap in the EGR controller from the diesel. Mine was an '85, so it has a VR sensor on the bell housing that reads the teeth on the flywheel. The earlier engines had a tach pickup on the vibration damper and used a tach amp. In either case the trick is to swap in the tach gauge module into the cluster - mine bolted right in. Now your calibration will be completely off, so what I did was glue the diesel tach gauge face over the gas one and voila everything works. Now you technically don't need the EGR controller - there are threads describing how to hook the tach pickup directly to the tach module....just do a quick search.

One other thing - you may find that your transmission kickdown switch won't work. If your system is the same as mine, the fuel pump relay is required for power to the transmission solenoid (go figure!) The fuel pump relay gets a signal from the tach to turn on the fuel pump - no signal no power to the fuel pump (the idea is that if the engine is off the pump won't continue to pump, as in an accident which might sever a fuel line) You can jumper the relay to permanently power the transmission's kickdown.

If your speedometer doesn't work, there are work arounds for that too

benzguy300 · #4 06-30-2017, 06:14 PM

Vonsmog I also did a swap like yours my was in a W124 300CE and the tachometer is not working I did a search and the first thing I learned is tach on a gas car is not same on a diesel there is member here that made tachometer the works in a w123 that didn't have a tachometer I just need to remember his name

vonsmog · #5 06-30-2017, 09:56 PM

OK thanks guys, guess I have to find a diesel tach to stick into my gauge cluster. At least I now know. Will check to see if there are any new cars in the Pick-n-pull or buy one on ebay.

mach4 · #6 06-30-2017, 10:19 PM

Quote:

Originally Posted by vonsmog

OK thanks guys, guess I have to find a diesel tach to stick into my gauge cluster. At least I now know. Will check to see if there are any new cars in the Pick-n-pull or buy one on ebay.

Make sure you get the one that matches your engine's pickup - the flywheel pickup is completely different than the vibration damper/tach amp pickup....and get the tach face too.

If you need something in the interim to just get you by you could get something like this - https://www.banggood.com/4-Digital-LED-Tachometer-RPM-Speed-Meter-Proximity-Switch-Sensor-NPN-p-1045439.html. I have one on my lathe and it works great.

There are also guys that have tapped into the alternator for a signal and done a calibration based on pulley sizes.

Post 235 and the next several posts on the following page will give some additional information on the tach issue - 380SL Diesel Conversion Project