We have a 97 Eldorado Touring Coupe and it's got a real head-scratcher of a problem. We have the Factory Service Manual for the car, and we do a lot of our own work, which saves LOTS of money. We're pretty good at diagnosing its problems if I do say so myself, but this one's got us stumped.
Let me say right up front that the car is not throwing any codes. It was, initially, for other issues, and we repaired those, and the codes now remain clear. These remaining problems have always been present (owned the car for 5 months now).
It appears to have two different main problems, but maybe they're related somehow, and maybe I'm stuck with taking it into the dealer for repair (GULP!)
The first issue is that both the Speedo and the Tach (analog gauges) show consistently low readings. The Speedo is a constant 9 MPH low across the board, and the Tach is about 700 RPM low. There is a button-sequence (thru the AC/Heater Control Panel) that lets me access the PCM and get direct digital numeric readings from the PCM as to what it thinks the speed and RPM really are. Those readings are correct, but the gauges always display way too low. I've checked for opens and shorts between the PCM and the dash cluster, and the wiring and grounds all check out good.
The other issue is the ignition. I have one of those wireless tachometers that determine engine RPM from the electrical noise that it emits. It has a second function where I can " hook" its antenna to an ignition wire to read the high-voltage. I've only looked at the front four cylinders so far, but I got 3 readings at about 30K volts, and the fourth at 38K volts. What should the normal reading be as I can't find it in my manuals?
A. MPG is only 15 when it should be 22-23 (I have another 99 ETC so I know what it SHOULD be), and I assume this is a side-effect of the ignition issue.
B. SES light does not come on with the initial ignition-on like it's supposed to, and the MPH indicator on the Speedo is never lit, although the KPH lights when configured for that. The gauge cluster is fine otherwise.
Could all of this be due to a bad PCM? The wiring diagram verifies that it's really the only thing that directly drives the speedo and tach gauges and the ignition module, and also the SES light. I would hate to replace the Ignition Modules only to find that it didn't change anything.