1990 520h tachometer not working....

[sdgmc05 0]

Picked up a 1990 wheelhorse 520h recently and noticed the tach wasn't working at all. I checked all obvious problems( broken damaged gauge and wires) but everything looks fine.all other gauges are working properly also. Common problem? Easy fix? Thanks in advance.

[Save Old Iron 1,573]

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Several possibilities. One is the sensor wire to the gauge has beome disconnected from the rectifier / regulator. The tach gauge "counts" the AC pulses per minute coming off the charging stator and represents them as engine RPM. One of the terminals on the RR unit will have 2 leads, one to the stator and the other to the gauge. A corroded terminal could be the issue.

Another issue is incorrect placement of the terminals on the RR unit. One of the AC terminals on the RR unit is directly connected (internally) to the B+ terminal of the RR. If the tach input is connected to this terminal, the tach willnot function properly. I think I helped someone with this very same issue not so long ago. A search for 520 and tach should reveal the thread.

Third, the gauge could be fried as seen in autopsy pictures below. Usually, removing the illumination bulb from the guage housing and giving the gauge the "sniff test" will reveal the smell of burned electronics.

[sdgmc05 0]

Thanks for the info, turned out to be corrosion on the connections. I cleaned them up and it works perfect.

[BMW Phil 0]

On 8/2/2012 at 12:06 AM, Save Old Iron said:

“Another issue is incorrect placement of the terminals on the RR unit. One of the AC terminals on the RR unit is directly connected (internally) to the B+ terminal of the RR. If the tach input is connected to this terminal, the tach willnot function properly. I think I helped someone with this very same issue not so long ago. A search for 520 and tach should reveal the thread.”

 

I thought this unlikely- I thought that BOTH AC terminals would be isolated from B+.

i just finished replacing the 9-pin connector and the fuse block on my ‘96 520H, due to corrosion, and upon restarting, no tachometer.

i tried swapping the AC leads around, and viola! Tach is back!

On my machine, the grey AC wire with the black tachometer signal wire attached must be connected to the RR terminal towards the back, or it won’t work.

Always learning......

 

On 8/2/2012 at 12:06 AM, Save Old Iron said: