Real Time Engineering board Tach, help

[walt1800]

Also found this on another site...might be a problem.


My '70 has the buzzer. I liked it but it developed a fault. I believe the relay has a diode to prevent backfeed of current to the rest of the electrical system with key off. Anyhow, it failed and it was passing a small current back to the EFI ECU. The ECU didn't know if it was off or on and went to fault mode. No start. Once I figured this out the buzzer was disconnected. Starting is more important than the buzzer. On an OE system, a fault like that would probably drain the battery.

[Cuda Cody]

Boy, that would make sense if it was something to do with the Key In buzzer switch.  That goes off when you go to the RUN position.  The buzzer is located over by the passenger side door just under the dash.  Lots of people disconnect it (if it's still working).  Many stopped working a long time ago.  See if you car has the buzzer hooked up (even if it's not working).

[walt1800]

Yes its hooked up and working. I also could be something to do with the door jam switches. They have been replaced and they are not correct for a Challenger. Does anyone have pictures of what is correct for both sides. I hear the drivers side is a three prong and the pass side is a two prong...any ideas? These could be feeding back through the system at a lower voltage. 

[walt1800]

Update...I disconnected the key buzzer and the problem is gone. The tach doesn't move when in the ACC position. Still don't know about the door jam switches and if they are causing a problem...but one down!

[Cuda Cody]

@walt1800  I'm thinking it's a ground thing.  The door buzzer on the underside of the dash on the passenger side needs to be grounded to work.  So when you disconnect the wire you separate that ground and for some reason it makes the tach work.  There are some wires on the back of the tach that needs to be separated with a small fiber washer.  Take a look at these photos....

https://forum.e-bodies.org/electrical-and-audio/11/engine-wont-fire-when-tach-wire-hooked-to-coil/589/

Just trying to help find the reason why it's doing it.   

Update...I disconnected the key buzzer and the problem is gone. The tach doesn't move when in the ACC position. Still don't know about the door jam switches and if they are causing a problem...but one down!

[HP_Cuda]

You will use the grey wire from the MSD unit, if you have any issues with it you may have to get the MSD adapter for older tachs.

As a side note.... I have not wired in the MSD unit yet. This problem is with the standard wiring. I don't know if it will go away with the MSD unit wired in.

[lulurocks(Ken)]

Im running this tach kit in my car, on the tach signal from my msd ,  if you want to compare  between working and not working PM me i check whatever you need ,  mines in my car but im pretty flexable for a fat guy...

[Cuda Cody]

 

... but im pretty flexable for a fat guy...

[walt1800]

Hi thanks. I haven't installed the MSD box yet but I did check the two posts on the tach and I seem to have continuity to ground on both posts. This seems like it isn't right. It is a bit difficult getting to them under the dash though.

[Aar1064]

I installed the M5 board and don't have these issues. It stays at zero in both ACC and on position. I also have the original buzzer that works.

Let me know if I can look at anything for ya. Not sure if this is relevant but I also installed their voltage limiter.

Edit: oh and I'm running stock dual point

[lulurocks(Ken)]

Hi thanks. I haven't installed the MSD box yet but I did check the two posts on the tach and I seem to have continuity to ground on both posts. This seems like it isn't right. It is a bit difficult getting to them under the dash though.

Well the fat guy  got the following info:

like aar1064 said my tach stays on zero thru acc and on

My tach , with wires off , did not ! have continuity to ground  on tach power tab  (pink in my pic) or tach coil signal tab  (white in my pic). I did have  a small amount of resitance  but not a dead short.. I couldnt read the meter  upsidedown lol.  But it did not tone for dead short.


And your 5vdc limiter is not part of the circuit its for the other gauges (fuel,oil,temp)

Have you run a 12vdc + wire unswitched from the battery to the tach 12vdc+  tab  to segregate any. Back feed from other devices ?    So. Just 12vdc+ feed    and tach coil wire.   

 

[walt1800]

Thanks for checking that out. I went out and did some rooting around and I have a reading of 0 on the power terminal with the key off and almost 0 on the coil side. I will be running a wire directly from the MSD box to the tach when it arrives so if there is any interference or back feed on the tach wire that should take care of that. As for the buzzer thing Greg from RTE is going to get back with me with some ideas about what might fix that problem since the upgrade they did some time ago to fix that problem doesn't seem to work on my car.

Thanks for all the help with this...some Challengers fight a little harder coming back from the dead...lol. I will let you know what I find out as soon as I get the MSD box installed.

[lulurocks(Ken)]

Sweet ,

My concern is not so much tach signal feed wire , but the 12 vdc acc feed .  My concern is the acc feed has a device connected to it with a spoiled ground and it seeking ground causeing feedback on tach. 

another concern is the tach holding 5k...  When you calibrte the tach with their  kit there is a calibration mode  were you use specific resistor packs to set a specific reading .   Just a long shot is it still in calibration mode ...?  Idk.   Just throwing out an idea..

See page 7 pic 20.

http://rt-eng.com/rte/images/c/ce/M5tachboard_manual.pdf