DMM - high internal impedance ??

steptoe · Post by **steptoe** » Mon Sep 13, 2010 11:22 am

I am going to get around to test my L Series knock control sensor circuit - the instructions say that when measuring the voltage at the check terminal " be sure to use a circuit tester with a high internal impedance. If the intrernal impedance of the tester is low, the voltage indicated on the tester is somewhat lower than the actual voltage. If adjustment is made using such a tester both the advance angle and retard angle will become somewhat small"

Can someone explain in "DMMs for Dummies" style what impedance is and how do I find what my DMM internal impedance is and what is regarded as high impedance? How low is too low?

I was not aware any 'adjustment' could be made to this control system. There are no adjustment instructions to follow just repair or replace components type details in a flow chart.

thanks

Gannon · Post by **Gannon** » Mon Sep 13, 2010 7:04 pm

All digital multimeters should have a very high input impedance. In the millions of ohms i believe.

Analogue multimeters have a lower internal resistance, which will place excessive load on a signal circuit causing low readings or even malfunction.

Impedance basically is the internal electrical resistance of the multimeter, so the lower the resistance, the higher the load on the device being tested, and if the device only produces a small voltage or current, it will pull the voltage down.
Kinda like when you increase load on the alternator when the car is idling and not producing enough current, the lights dim

steptoe · Post by **steptoe** » Mon Sep 13, 2010 11:33 pm

OK, sort of got it now, thankyou Gannon

Your explanation and the date or the manuals written I'd say DMM's were not around at the time ca 1986 or many in the hands of the average Joe. Written for analogue MM's Id guess. So when I have a go at checking knock sensor my readings should be spot on. Priced a newie today just in case. $195 not sure if ex Japan or NLA

Gannon · Post by **Gannon** » Tue Sep 14, 2010 6:23 am

Just for the knock sensor?

Wow!

YouTube · Post by **El_Freddo** » Tue Sep 14, 2010 1:05 pm

That sounds about right - I only say this because I've just bought some new sensors for the EJ. FROG sorted me out and I can't recommend him enough. One of the sensors - the TPS - was about $400.00 new!

I think the EJ knock sensor was about $160.00. You can get them on ebay for $120.00 but I don't know how good these units would be...

Cheers

Bennie

steptoe · Post by **steptoe** » Tue Sep 14, 2010 10:10 pm

$195 was not the first price we found for EA82T - the AWD Vortex I think it was had a price around $600 !

2Grumpy · Post by **2Grumpy** » Tue Sep 14, 2010 10:55 pm

If you don't mind importing parts,check the pricing http://www.rockauto.com I save a heap of $ with a gasket kit and timing belt kit. You want to do it while the $ is up like it is now.