Ammeter recently showed discharge when running....... 1992 W250 with 300,000 kilometres. Assumed it was a faulty alternator... it was the original and is allowed to wear out. Replaced alternatror and problem remains. I know the voltage regulator is a possibility and that its tied in with the computer but I'm electronically challenged. Any ideas on how to troubleshoot and/or repair/ replace the regulator?????

 

A common problem on the '92 and '93 trucks is a bad CPS (crank speed sensor). If the CPS is not sending a signal to the PCM, the PCM does not know the engine is running so it does not run the charging system. There are a couple easy things you can check to rule this out. If the truck is air conditioned, the compressor will not cycle when this happens. Also automatic trans trucks will lose OD.

The other easy thing to check is the ASD relay, that is the relay the PCM uses to send current to the field in the alternator. There are three relays on the driver's side fender. The front one is the ASD relay, the middle one is the starter relay, and the rear one is the A/C relay. Swap the ASD and A/C relays and see if it starts charging. If so, buy a new relay and you are all set.

 

thanks for the advice. Tried swapping relays , no improvement . Air conditioning seems OK ( altho its doing its best to snow hare so its tuff to tell for sure). The CPS sensor seems to be in place OK and the wiring in general is neat and tidy. The wait to start light does not come on like it used to , there is 12.5 volts at the large terminal at the back of the alternator but zero voltage across the two small terminals at the back of the alternator. ( with ignition turned on , engine not running) The guy in the alternator shop... who confirmed that the new one is OK ... said there should be 7-9 volts across these terminals. Any more ideas???

 

Did you watch the compressor clutch to see that it engages?

If the A/C compressor cycles, then the regulator circuit in the PCM is probably dead. There are a few recent threads on changing over to the old-style external regulator, that's a lot cheaper than a PCM. There's also some posts from 59Ford about making the Nippondenso alternator into a 1-wire internally regulated alternator.

 

When this happened to mine, I bought a new ECM because the regulater is inside that thing. After I removed the windshield washer bottle to get at the ECM, I found a broken wire laying on the fenderwell. I spliced that wire back together and the alternator has charged fine ever since. Hope your problem can be as easy to fix as mine was.

chaikwa.

 

I have the same problem my tec tell me it is a short in the pcm I'm in the process of replaceing it will let you know how it goes

 

For closure purposes...... tried all the above with no luck , took it to a recommended auto electrical specialist. They diagnosed a complete open circuit.... broken wire on the firewall, no obvious reson , just old age apparently. Two hours labour and shes running fine again . Thanks for all the sugestions.