Service Engine Light w/ no speedometer

My '89 lesabre keeps popping up a service engine light and then the speedometer immediately drops off and it feels like the car shifts out of overdrive - Please help me troubleshoot this problem, I'm not sure where to start. A couple of other things also: my temperature guage intermittently pegs wide open and my a/c has just quit working - worked great yesterday and now nothing, it doesn't feel like the compressor is kicking in, the fuse is still good. I'm not sure if this is all related to a single problem or if I have multiple problems.

Please help - thanks in advance!

If I remember right, the speedometer receives its reading from the ECM, which gets a signal from the VSS on the transmission. You may be losing the VSS signal.
You can check the a/c voltage output of the sensor with a voltmeter Backprobe the signal wire at the ecm to ground. With the wheels turning you should have over .5v a/c (if over .5 v a/c most likely cause is a bad ECM...if less then .5v...bad sensor or wiring)

Thanks for the reply - I have since found that my positive terminal was severely corroded, once cleaned, everything seemed to work again. However, after I shut the engine off, I can no longer get it to start. It will crank and the fuel pump seems to be working, but I think I've got no spark. Could I have taken out a fuseable link or something? Where and what should I start looking at? Please help - I'm pulling my hair out over this thing. It was running so good one day and now it seems all of the electrical sensors are failing? Thanks again.

It is not likely that everything is failing at once. The common denominator here is the battery. Check the battery cables carefully, check for voltage drops, etc. After checking the battery with no cables connected, you should have 12.4 volts minimum. If not, it is either not being charged, or is defective. You cvan check for charge with a charger. If the battery draws current with the charger connected, let it charge, and check the vehicle's charging system. Then repeat the test. Helter-skelter testing only leads to confusion. Let me know... Ray

Thanks Ray - I have since taken the car into the shop, where it spent 3 full weeks trying to get fixed. They ended up replacing the cam sensor, the computer and the ignition coil seat - I probably got hosed on some of this stuff, but at least the car is now running. I'm not sure that it is back to 100%, and so I'll take your suggestion and check the battery out again and look over the cables and wiring. Thanks for caring - Aaron

Yeah, you got hosed on the sensor and computer. It was ONLY the ignition coil. Maybe the seat, but nothing else. Makes you mad, doesn't it? That's why I fix thigs myself...wish I had seen your post sooner.

Had a 1991 LeSabre doing almost the exact same thing - intermittent loss of speedometer, check engine light on, and would tend to accelerate without foot on gas esp. when engine cold; codes set included VSS, temp sensor, and electronic ignition timing.

Replacing the ECM solved the problem....