XJXJ that statement when I say it comes from my personal experience. I replaced the TPS on my 88 XJ using the Multimeter method. I did an spread sheet so I would just look up output value paired with the "source" value. I then take it to the dealer for a problem that turned out to be a stickey EGR. The mechanic told me my TPS was out of adjustment and set it right for me. I only paid him for the EGR, no charge for the TPS adjustement.
I then buy the snap on tool MT2500 scanner since I read on here it the only tool beside the dealer tool which can read the computer on a Renix. I plan to keep the XJ and was worried the dealer tool may 'disappear".
I have the tool and my buddies with older vehicles I offered to hook up the MT2500 to see what going on with their vehicles. I hooked it up to a 85 Vette, and the following with the GM 3.8/3800 V6: 87 buick Century, 88 buick Le Saber, 88 buick park Ave, 88 Olds 98, 90 buick le sabre, and 91 buick le sabre.
Three different people owned those vehicles, each had installed a new TPS and used the multimeter method to set the TPS and were happy with the result. I hooked up the MT2500 and discovered the value read at the computer was lower than the published spec. The 85 Vette was to be set at 0.6 v, and the GM 3.8/3800 was to be 0.4 volts but across the board they were all 0.2 volts low. The Vette was 0.4 volts and the 3800s were like 0.2 or 0.22 volts.
The guy with the Vette had been out showing me how it ran, we came back to the house and checked and adjusted the TPS, then less than 5 minutes later we went back out to see what the difference. With the owner driving we pulled up to a stop light and pulled away when it was green. He said "Oh yes, it did make a difference!!!"
The 4 of us talked to why the voltages were lower and we concluded the Multimeter method does not account for the voltage drop from the TPS to ground. This would expain the difference between the DVM method vs the MT2500 scan tool. With the computer reading a lower voltage from the TPS the computer will not give the full amount of fuel for best power. Remember by the TPS measurement the computer determines how "open" the throttle body is. If the computer thinks the throttle is less open than you will have less than optimal performance.
By all means using the mulitmeter method is the way to go it you don't have access to a scan tool. Just passing on the lesson I learned.