Hi All

I'm new to the forum, I've just got hold of a B250 V8 '93 and the previous owner said he'd had issues with the dash voltmeter occasionally going sky high (18v) and then fluctuating with lights (dash, headlights, interior) all going brighter and dimmer. When the voltage went high he'd switch off, wait for a bit and then re-start and it would be ok for a bit.

Just before I bought it, he'd taken it to an alternator specialist who fitted what I think is an external regulator - I'll post more info about this (photo etc). This seems to have stopped the 18v readings on the dash voltmeter but lights still go brighter and dimmer when driving. When doing 60mph, voltmeter is reading about 15-16v. I believe this is a bit high? What should it be reading?

What's the best advise for diagnosing this sort of thing (I've got a multi-meter ?

many thanks, looks like you guys know a thing or two about these beasts.!

Wilbo

 

13.8v - 14.2v is your optimal charging range. The 15v-16v you're getting will burn up your battery, bulbs, and headlights in no time flat. I would eliminate the micky mouse external regulator setup and put everything back the way it should be. Then you'll have to replace the PCM since the voltage regulator is built into it.

 

Is the PCM also called the ECU? I note from an old service sheet that it had one of these fitted a couple of years ago. Is there any way to absolutely confirm it's the PCM? I've read it could be bad grounding somewhere? Or is high fluctuating voltage only caused by a bad PCM?

thanks for the advice!

 

Yes.
Only by swapping the PCM out with another one.

A bad body to engine ground will wreak havoc with a vehicle's electrical system, making it do all sorts of weird things including a lack of voltage regulation for the alternator. Since the previous owner brought it to a "specialist", I was giving the benefit of the doubt that simple things like grounds and wiring were checked and ruled out. Also, considering an external regulator was added, this was telling me the "specialist" confirmed the PCM (ECU) was not regulating the voltage from the alternator. Of course all of my assumptions could be wrong and the "specialist" just did the mickey mouse job to help out a friend dump this vehicle off on some chump. (No offense.)

 

yeah, I'll do some tests on this next. Any advice on the most common problematic grounding sites other than main -ve from batter?

heh, he did say 'he couldn't work it out' so I'm not sure

[edit - price]
, none taken - to be fair this was 'explained' to me but if you know how I can get a replacement PCM for under $400 in the UK that would be cool !

happy days

 

Right rear of the engine, there you'll find an engine to body ground strap. This one is just as critical than the one coming off the negative terminal of the battery going to the body.

Post the number off of your PCM and I'll see about finding one.

 

I'll check this connection on the weekend and also get the PCM info - which is on the firewall on the engine side or under the hood ?

Really appreciate your help here alloro, and here's me thinking the rust was the only problem

 

I'm not sure if it was was moved by 93 or not, but look under the hood right above the engine. If you see a grayish box (about 9x6x2) with 3 large connectors going to it, then that is it.

 

Here is a pic of my PCM to help show you where it is at, it is behind the wiring electrical connectors on the firewall above the engine and just below the wiper motor.

 

So this is mine (red arrow pointing) As I said earlier, the ECU was replaced (part no on invoice I just found is R6027848)



and this is what I think is the external voltage regulator which has wires going to the alternator (will add pic of this tomorrow)



Am I right in thinking the VR is 'part of' the PCM, can it be replaced by itself or is it a case of replacing the whole thing only?

thanks guys for your continued interest!

Wilbo