Working an engine change on a DC9-33F, and having issues with the EGT system. We are replacing a -15 with a -15A, the new engine has the Boost Circuit System installed, where as the old one did not. When running the engine, after start we got no roll back, the engine stabilized at idle at approx 500*c then slowly continued to creep up, we shut it down at about 625*c. We checked the probes and got 6 probes reading 2.5 ohms, 1 at 4.0 ohms, and the last one at 1.0 ohm (possible bad probes). Unfortunately our manuals do not discuss the boost circuit, and they way it is hooked up electrically to the junction box (JB) is slightly confusing to myself and my counter part avionics buddy. The way it is hooked up, probe #6 has its chromel wire hooked up separately to spot 6 on the JB, which is then hooked directly in line with the chromel wire to the indicator. Probe #1 chromel wire is hooked to spot 1 on the JB, and is not connected to anything else. The alumel wires for probes 1 and 6 connect to spot J on the JB, which is bussed together with all chromel wires from all the other probes. The alumel wire from the the other probes (not 1 & 6) all connect together at spot AL on the JB.
While doing some research we have noted that in some installations there is are additional jumpers on the JB which connect 3-5 and 1-6, which then all connect to the chromel side of the system, going to the indicator. Our installation does not have this set-up. Also talking with a friend that has about 30 years on 9's he mentioned placarding that was required for the boost system installation, but again our manuals don't show the system.
My questions are:
1. What could cause the EGT to not roll back, and continue to rise after start?
2. Given the description above is there a possible probe/harness fault? (The lack of extra buss bars on our harness leads us to believe there is a problem here)
3. Can a non-boost circuit system be used on a -15A (All research done by us shows the boost circuit as one of the -15A differences among many others, so far)?
4. Is there any placarding needed?
5. Finally does anyone have a good description of how this system works?
I hate playing behind the power curve, and not having all the information at hand. Thanks in advance for all of your guy's help!!!