My 90 Eldorado, 86,000 miles, drives great, but the service engine light is on and I am told there is a "hard code" indicating transmission problems. A trans shop said the solenoids are going bad and it needs to be overhauled. I asked if changing the fluid would help (it hasn't been changed in a very long time and I live in hot Phoenix, AZ) and they refused to do it, saying it would make the problem worse. Can you tell me if changing the fluid might help? The value of the car makes the overhaul not worth it.

I no nothing of your trans shop, but in any case, I would get other trans shop opions and base the next move on that, and a flush job, I don't see how it would make the problem worse,btw does the vehicle have an extra oil cooler?

What codes are you getting?..You said drives great.no slipping,noises ,shift problems at all?
Post codes if you got 'em.
I agree with Dave,get a 2nd opinion.

Autozone has a deal with AAMCO transmissions...if your an Autozone customer,they'll do initial diagnostics for free.
you need to make an appt/first.
Just tell 'em Autozone sent ya...

your trans shop is right it will make it worse whatever problem you have will show up with a trans service. when you change the fluid it washes away all the gunk thats holding it together if you serviced the trans regularly you would be ok but if not done dont do it if you do it will wash out loose clutch material and start slipping or it wont have enough pressure with the thinner fluid or many other things I suggest drive it till it dies then get it rebuilt either way you will be without a car fer a few days