Audio Systems: auto electrical problem, oil pressure guage, buzzer circuit

Question
Sir,
I know this is not an audio question, but it is auto electrical, and I would really appreciate any advice at all.

I put a (12 volt DC) VDO oil pressure sender unit (switch) into my Toyota Landcruiser. It works just like all oil senders - when there's no oil pressure, the oil warning light on the dash is on, once oil pressure builds, it switches the warning light off. Instead of a light, I connected the VDO sender to a reverse warning buzzer, (the type which is usually connected to the reverse light circuit of trucks). So it beeps when there's no oil pressure, and stops beeping when the pressure builds. Problem: as the buzzer beeps, my rev counter needle bounces back and forth. The negative/ground wire of the buzzer is connected to the battery positive terminal, and the negative wire is connected to the sender unit on the engine block. I would like to know why this is happening, and how to fix it. How about putting a diode in the buzzer circuit?
Any advice would be greatly appreciated
thank you
Alan Cobban
South Africa

Answer
Buzzer is causing fluctuations in the tacho signal. In all honesty, BIN IT! By the time the oil light (buzzer) comes on it is normanly too darn late and damage is done. fit an oil pressure guage and keep an eye on that.
Neg wire on buzzer to switch on block, poss to output side of ignition fed fuse in fuse box would work though.
Peter