Sears sells a very affordable and competent digital multimeter for $30 WITH duty cycle. You need to measure the mA coming from the EHA if you find that your duty cycle is fluctuating wildly (beyond the allowable 10-15%).
If after you check your duty cycle and it fluctuates wildly you'll then need to check the mA coming from the EHA using a MB harness tool -- which I have and you're welcome to borrow it. (please email me and I'll be happy to mail it out to you)
If the EHA is not giving out the proper mA, this still doesn't mean your EHA is bad .... it could be due to improper lower chamber pressure in the fuel regulator, or a short circuit somewhere between the ECU and your EHA.
Good luck with your diagnosis...